School of Convict Surgeonfish at the Waiopae Tidepools Dr.Grabowski at Honaunau Bay,Hawaii Green Sea Turtle at Onekahakaha, Hawaii Robust Redhorse staging for spawning in the Savannah River, Georgia-South Carolina Reef herbivores at Kealakekua Bay, Hawaii Atlantic Cod in production facility in GRindavik, Iceland Texas Logperch Achilles Surgeonfish and Yellow Tang Dr. Grabowski relaxes after setting up hydrophone array at Kollafjordur, Iceland Yellow Tangs at Kealakekua Bay, Hawaii Dr.Grabowski examines a Spotted Bass on the Brazos River, Texas Green Sea Turtle at the Waiopae Tidepools, Hawaii Electrofishing in the South Llano River Rio Grande at Santa Elena Canyon Dr. Grabowski listening to Burbot calling in Moyie Lake, British Columbia Ornate Butterflyfish at Kealakekua Bay, Hawaii Sunset from a jet-powered kayak on Lake Livingston
Journal articles | Book chapters | Technical reports | Theses and dissertations

Journal articles

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Pecos assiminea in its natural habitat

Roesler EL, & TB Grabowski. In press. Estimating factors influencing the detection probability of semi-aquatic freshwater snails. Hydrobiologia.

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Developing effective monitoring methods for elusive, rare, or patchily distributed species requires extra considerations, such as imperfect detection. Although detection is frequently modeled, the opportunity to assess it empirically is rare, particularly for imperiled species. As part of an assessment of survey methodologies for Pecos assiminea (Assiminea pecos), an endangered semi-aquatic snail inhabiting the riparian zones of springs and sinkholes, we conducted an assessment of the detection and accuracy of quadrat searches. Quadrats (9 x 20 cm metal frame; n = 12) were placed along spring banks in suitable Pecos assiminea habitat and a randomly assigned to a treatment level, defined as the number of empty snail shells (n = 0, 3, 6, or 9) placed in the quadrat. Ten observers rotated through each quadrat, conducting 5-minute visual searches for snail shells. The probability of detecting a snail shell when present was 72.2%, but decreased with increasing litter depth and lower numbers of shells present. The mean (± SE) observer accuracy was 43.6 ± 17.1%. Observer accuracy was positively correlated to the number of snail shells in the quadrat and negatively correlated to the number of times a quadrat was searched and the number of quadrats an observer searched prior to a given observation. Our results indicate that surveys for Pecos assiminea underrepresent true abundance, but likely accurately determine presence or absence. Understanding the detection probability of numerous behaviorally or environmentally elusive, small, rare, threatened and/or endangered species improves density estimates and aids in conservation efforts.


Section from a Blue Sucker pectoral fin ray

Acre MR, C Alejandrez, J East, WA Massure, S Miyazono, JE Pease, EL Roesler, HM Williams, & TB Grabowski. 2017. Comparison of the precision of age estimates generated from fin rays, scales, and otoliths of Blue Sucker. Southeastern Naturalist 16:215-224. doi: 10.1656/058/058.016.0208

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Evaluating the precision of age estimates generated by different readers and different calcified structures is an important part of generating reliable estimations of growth, recruitment, and mortality for fish populations. Understanding the potential loss of precision associated with using structures harvested without sacrificing individuals, such as scales or fin rays, is particularly important when working with imperiled species, such as Blue Sucker Cycleptus elongatus. We collected otoliths (lapilli), scales, and the first fin rays of the dorsal, anal, pelvic, and pectoral fins of nine Blue Sucker. Age estimates were generated from each structure by both experienced (n=5) and novice (n=4) readers. We found that independent of the structure used to generate the age estimates, the mean coefficient of variation (CV) of experienced readers was approximately 29% lower than that of novice readers. Further, the mean CV of age estimates generated from pectoral fin rays, pelvic fin rays, and scales were statistically indistinguishable and less than those of dorsal fin rays, anal fin rays, and otoliths. Anal, dorsal, and pelvic fin rays and scales underestimated age compared to otoliths, but age estimates from pectoral fin rays were comparable to those from otoliths. Skill level, structure, and fish total length were factors influencing reader precision between subsequent reads of the same aging structure from a particular fish. Using structures that can be harvested non-lethally to estimate the age of Blue Sucker can provide reliable and reproducible results, similar to those that would be expected from using of otoliths. Therefore, we recommend the use of pectoral fin rays as a non-lethal method to obtain age estimates for Blue Suckers.


A newly hatched Arkansas River Shiner

Mueller JS, TB Grabowski, SK Brewer, & TA Worthington. 2017. Effects of temperature, dissolved solids, and suspended solids on the early life history stages of Arkansas River Shiner. Journal of Fish and Wildlife Management 8:79-88. doi: 10.3996/112015-JFWM-111

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Decreases in the abundance and diversity of stream fishes in the North American Great Plains have been attributed to habitat fragmentation, altered hydrological and temperature regimes, and elevated levels of total dissolved solids and total suspended solids. Pelagic-broadcast spawning cyprinids, such as the Arkansas River Shiner Notropis girardi, may be particularly vulnerable to these changing conditions due to their reproductive strategy. Our objectives were to assess the effects of temperature, total dissolved solids, and total suspended solids on the developmental and survival rates of Arkansas River Shiner larvae. Results suggest temperature had the greatest influence on the developmental rate of Arkansas River Shiner larvae. However, embryos exposed to the higher levels of total dissolved solids and total suspended solids reached developmental stages earlier than counterparts at equivalent temperatures. Although this rapid development may be beneficial in fragmented waters, our data suggest it may be associated with lower survival rates. Furthermore, those embryos incubating at high temperatures, or in high levels of total dissolved solids and total suspended solids resulted in less viable embryos and larvae than those incubating in all other temperature, total dissolved solid, and total suspended solid treatment groups. As the Great Plains ecoregion continues to change, these results may assist in understanding reasons for past extirpations and future extirpation threats as well as predict stream reaches capable of sustaining Arkansas River Shiners and other species with similar early life-history strategies.


Section of a Zebrafish thyroid from a 45-dpf individual

Sharma P, TB Grabowski, & R Patiño. 2016. Thyroid endocrine disruption and external body morphology of Zebrafish. General and Comparative Endocrinology 226:42-49. doi: 10.1016/j.ygcen.2015.12.023

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This study examined the effects thyroid-active compounds during early development on body morphology of Zebrafish (Danio rerio). Three-day postfertilization (dpf) larvae were exposed to goitrogen [methimazole (MZ, 0.15 mM)], combination of MZ (0.15 mM) and thyroxine (T4, 2 nM), T4 (2 nM), or control (reconstituted water) treatments until 33 dpf and subsequently maintained in reconstituted water until 45 dpf. Samples were taken at 33 and 45 dpf for multivariate analysis of geometric distances between selected homologous landmarks placed on digital images of fish, and for histological assessment of thyrocytes. Body mass, standard length, and pectoral fin length were separately measured on remaining fish at 45 dpf. Histological analysis confirmed the hypothyroid effect (increased thyrocyte height) of MZ and rescue effect of T4 co-administration. Geometric distance analysis showed that pectoral and pelvic fins shifted backward along the rostrocaudal axis under hypothyroid conditions at 45 dpf and that T4 co-treatment prevented this shift. Pectoral fin length at 45 dpf was reduced by exposure to MZ and rescued by co-administration of T4, but it was not associated with standard length. Methimazole caused a reduction in body mass and length at 45 dpf that could not be rescued by T4 co-administration, and non-thyroidal effects of MZ on body shape were also recognized at 33 and 45 dpf. Alterations in the length and position of paired fins caused by exposure to thyroid-disrupting chemicals during early development, as shown here for Zebrafish, could affect physical aspects of locomotion and consequently other important organismal functions such as foraging, predator avoidance, and ultimately survival and recruitment into the adult population. Results of this study also suggest the need to include rescue treatments in endocrine disruption studies that rely on goitrogens as reference for thyroid-mediated effects.


Brandon Cheek and Jillian Goreschel using side scan sonar to map substrates in the South Llano River near Junction, Texas

Cheek BD, TB Grabowski, PT Bean, JR Groeschel, & SJ Magnelia. 2016. Evaluating habitat associations of a fish assemblage at multiple spatial scales in a minimally disturbed stream using low-cost remote sensing. Aquatic Conservation: Marine and Freshwater Ecosystems 26:20-34. doi: 10.1002/aqc2569

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Habitat heterogeneity at multiple scales is a major factor affecting fish assemblage structure. However, assessments that examine these relationships at multiple scales concurrently are lacking. The lack of assessments at these scales is a critical gap in understanding as conservation and restoration efforts typically work at these levels. A combination of low-cost side-scan sonar surveys, aerial imagery using an unmanned aerial vehicle, and fish collections were used to evaluate the relationship between physicochemical and landscape variables at various spatial scales (e.g. micro-mesohabitat, mesohabitat, channel unit, stream reach) and stream–fish assemblage structure and habitat associations in the South Llano River, a spring-fed second-order stream on the Edwards Plateau in central Texas during 2012–2013. Low-cost side-scan sonar surveys have not typically been used to generate data for riverscape assessments of assemblage structure, thus the secondary objective was to assess the efficacy of this approach. The finest spatial scale (micro-mesohabitat) and the intermediate scale (channel unit) had the greatest explanatory power for variation in fish assemblage structure. Many of the fish endemic to the Edwards Plateau showed similar associations with physicochemical and landscape variables suggesting that conservation and restoration actions targeting a single endemic species may provide benefits to a large proportion of the endemic species in this system. Low-cost side-scan sonar proved to be a cost-effective means of acquiring information on the habitat availability of the entire river length and allowed the assessment of how a full suite of riverscape-level variables influenced local fish assemblage structure.


Photograph of paired pushnets mounted between jet-powered kayaks

Acre M & TB Grabowski. 2015. Deployment of paired pushnets from jet-propelled kayaks to sample ichthyoplankton. North American Journal of Fisheries Management 35:925-929. doi: 10.1080/02755947.2015.1069426

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Accessing and effectively sampling the off-channel habitats that are considered crucial for early life stages of freshwater fishes constitute a difficult challenge when common ichthyoplankton survey methods, such as push nets, are used. We describe a new method of deploying push nets from jet-propelled kayaks to enable the sampling of previously inaccessible off-channel habitats. The described rig is also functional in more open and accessible habitats, such as the main channel of rivers or reservoirs. Although further evaluation is necessary to ensure that results are comparable across studies, the described push-net system offers a statistically rigorous methodology that generates replicate samples from a wide range of freshwater habitats that were previously inaccessible to this gear type.


Habitat map of the South Llano River near Junction, Texas integrating aerial and side-scan sonar data

Birdsong TW, M Bean, TB Hardy, TB Grabowski, T Heard, D Holdstock, K Kollaus, S Magnelia, & K Tolman. 2015. Application and utility of a low-cost unmanned aerial system to manage and conserve aquatic resources in four Texas rivers. Journal of the Southeastern Association of Fish and Wildlife Agencies 2:80-85.

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Low-cost unmanned aerial systems (UAS) have recently gained increasing attention in natural resources management due to their versatility and demonstrated utility in collection of high-resolution, temporally-specific geospatial data. This study applied low-cost UAS to support the geospatial data needs of aquatic resources management projects in four Texas rivers. Specifically, a UAS was used to (1) map invasive salt cedar (multiple species in the genus Tamarix) that have degraded instream habitat conditions in the Pease River, (2) map instream meso-habitats and structural habitat features (e.g., boulders, woody debris) in the South Llano River as a baseline prior to watershed-scale habitat improvements, (3) map enduring pools in the Blanco River during drought conditions to guide smallmouth bass removal efforts, and (4) quantify river use by anglers in the Guadalupe River. These four case studies represent an initial step toward assessing the full range of UAS applications in aquatic resources management, including their ability to offer potential cost savings, time efficiencies, and higher quality data over traditional survey methods.


Full moon rising over Esja near Reykjavik, Iceland

Grabowski TB, BJ McAdam, V Thorsteinsson, & G Marteinsdottir. 2015. Evidence from data storage tags for the presence of lunar and semilunar behavioral cycles in spawning Atlantic Cod. Environmental Biology of Fishes 98:1767-1776. doi: 10.1080/19425120.2014.920744

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Understanding the environmental processes determining the timing and success of reproduction is of critical importance to developing effective management strategies of marine fishes. Unfortunately it has proven difficult to comprehensively study the reproductive behavior of broadcast-spawning fishes. The use of electronic data storage tags (DSTs) has the potential to provide insights into the behavior of fishes. These tags allow for data collection over relatively large spatial and temporal scales that can be correlated to predicted environmental conditions and ultimately be used to refine predictions of year class strength. In this paper we present data retrieved from DSTs demonstrating that events putatively identified as Atlantic Cod spawning behavior is tied to a lunar cycle with a pronounced semi-lunar cycle within it. Peak activity occurs around the full and new moon with no evidence of relationship with day/night cycles.


A Common Snook captured from the Rio Grande

Huber CG, TB Grabowski, KL Pope, & R Patiño. 2014. Distribution and habitat associations of juvenile Common Snook in the lower Rio Grande. Marine and Coastal Fisheries: Dynamics, Management, and Ecosystem Science 6:170-180. doi: 10.1080/19425120.2014.920744

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Common Snook Centropomus undecimalis were once abundant off the Texas coast, but these populations are now characterized by low abundance and erratic recruitment. Most research concerning Common Snook in North America has been conducted in Florida and very little is known about the specific biology and habitat needs of Common Snook in Texas. The primary objective of this study was to describe the habitat use patterns of juvenile Common Snook and their role in the fish assemblage in the lower portion of the Rio Grande, Texas. Secondarily, we documented the relationship between age and juvenile reproductive development. Fish were collected during January–March 2006 from the lower 51.5 km of the Rio Grande using a bottom trawl and boat-mounted electrofisher. Measurements of water quality and other habitat traits were recorded at each sampling site. We captured 225 Common Snook exclusively in freshwater habitats above river kilometer 12.9. The distribution of juvenile Common Snook was not random, but influenced primarily by turbidity and dissolved oxygen. Sex differentiation and gonadal development based on histological examination of gonads established that age-1 and age-2 Common Snook were juvenile, prepubertal males. There was no difference between the age groups in their overall distribution in the river. However, age-2 Common Snook were associated with deeper areas with faster currents, higher conductivity, and steeper banks. Overall, Common Snook in the lower Rio Grande show substantial differences in habitat use than their counterparts in other parts of the range of the species, but it is unclear whether this is due to differences in habitat availability, behavioral plasticity, or some combination thereof.


Portion of Atlantic Cod spawning aggregation from northern Iceland

Grabowski TB, V Thorsteinsson, & G Marteinsdóttir. 2014. Spawning behavior in Atlantic Cod: analysis by use of data storage tags. Marine Ecology Progress Series 506:279-290. doi:10.3354/meps10787

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Electronic data storage tags (DSTs) were implanted into Atlantic Cod captured in Icelandic waters from 2002 to 2007 and the depth profiles recovered from these tags (females: n = 31, males: n = 27) were used to identify patterns consistent with published descriptions of cod courtship and spawning behavior. The individual periods of time that males spent exhibiting behavior consistent with being present in a spawning aggregation—i.e. periods consisting of a clear tidal signature in the DST depth profile associated with an individual remaining on or near the substrate—were longer than those of females. Over the course of a spawning season, male cod spent approximately twice the amount of time in spawning aggregations than females, but female cod visited more aggregations per unit time. On average, males participated in approximately 57% more putative spawning events, i.e. vertical ascents potentially corresponding to gamete release, than did females. However, males <85 cm total length participated in the same number of putative spawning events as females of comparable size. In both sexes, larger individuals and/or individuals that spent a longer period of time within an aggregation participated in a larger number of putative spawning events. Although further validation and refinement is necessary, particularly in the identification of spawning events, the ability offered by DSTs to quantify cod spawning behavior may aid in the development of management and conservation plans.

Typical flow conditions in an altered Great Plains river

Worthington TA, SK Brewer, N Farless, TB Grabowski, & MS Gregory. 2014. Factors affecting the transport time of semibuoyant fish eggs in large, altered river systems. PLoS One 9(5):e96599. doi: 10.1371/journal.pone.0096599

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Habitat fragmentation and flow regulation are significant factors related to the decline and extinction of freshwater biota. Pelagic-broadcast spawning cyprinids require moving water and some length of unfragmented stream to complete their life cycle. However, it is unknown how discharge and habitat features interact at multiple spatial scales to alter the transport of semi-buoyant fish eggs. Our objective was to assess the relationship between downstream drift of semi-buoyant egg surrogates (gellan beads) and discharge and habitat complexity. We quantified transport time of a known quantity of beads using 2–3 sampling devices at each of seven locations on the North Canadian and Canadian rivers. Transport time was assessed based on median capture time (time at which 50% of beads were captured) and sampling period (time period when 2.5% and 97.5% of beads were captured). Habitat complexity was assessed by calculating width∶depth ratios at each site, and several habitat metrics determined using analyses of aerial photographs. Median time of egg capture was negatively correlated to site discharge. The temporal extent of the sampling period at each site was negatively correlated to both site discharge and habitat-patch dispersion. Our results highlight the role of discharge in driving transport times, but also indicate that higher dispersion of habitat patches relates to increased retention of beads within the river. These results could be used to target restoration activities or prioritize water use to create and maintain habitat complexity within large, fragmented river systems.


An Arkansas River Shiner in a seine

Worthington, TA, SK Brewer, TB Grabowski, & J Mueller. 2014. Backcasting the decline of a vulnerable Great Plains reproductive ecotype: Identifying threats and conservation priorities. Global Change Biology 20:89-102. doi: 10.1111/gcb.12329

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Conservation efforts for threatened or endangered species are challenging because the multi-scale factors that relate to their decline or inhibit their recovery are often unknown. To further exacerbate matters, the perceptions associated with the mechanisms of species decline are often viewed myopically rather than across the entire species range. We used over 80 years of fish presence data collected from the Great Plains and associated ecoregions of the United States, to investigate the relative influence of changing environmental factors on the historic and current truncated distributions of the Arkansas River Shiner Notropis girardi. Arkansas River shiner represent a threatened reproductive ecotype considered especially well adapted to the harsh environmental extremes of the Great Plains. Historic (n = 163 records) and current (n = 47 records) species distribution models were constructed using a vector-based approach in MaxEnt by splitting the available data at a time when Arkansas River Shiner dramatically declined. Discharge and stream order were significant predictors in both models; however, the shape of the relationship between the predictors and species presence varied between time periods. Drift distance (river fragment length available for ichthyoplankton downstream drift before meeting a barrier) was a more important predictor in the current model and indicated river segments 375–780 km had the highest probability of species presence. Performance for the historic and current models was high (area under the curve; AUC > 0.95); however, forecasting and backcasting to alternative time periods suggested less predictive power. Our results identify fragments that could be considered refuges for endemic plains fish species and we highlight significant environmental factors (e.g., discharge) that could be manipulated to aid recovery.


Arkansas River Shiner eggs being held in suspension

Mueller JS, BD Cheek, Q Chen, J Groeschel, SK Brewer, & TB Grabowski. 2013. A simple device for measuring the minimum current velocity to maintain semi-buoyant fish eggs in suspension. The Prairie Naturalist 45:84-89.

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Pelagic broadcast spawning cyprinids are common to Great Plains rivers and streams. This reproductive guild produces non-adhesive semi-buoyant eggs that require sufficient current velocity to remain in suspension during development. Although studies have shown that there may be a minimum velocity needed to keep the eggs in suspension, this velocity has not been estimated directly nor has the influence on egg buoyancy caused by physicochemical factors been determined. We developed a simple, inexpensive flow chamber that allowed for evaluation of minimum current velocity needed to keep semi-buoyant eggs in suspension at any time frame during egg development. The device described here has the capability of testing the minimum current velocity needed to keep semi-buoyant eggs in suspension at a wide range of physicochemical conditions. We used gellan beads soaked in freshwater for 0, 24, and 48 hrs as egg surrogates and evaluated minimum current velocities necessary to keep them in suspension at different combinations of temperature (20.0±1.0°C, 25.0±1.0°C, and 28.0±1.0°C) and total dissolved solids (TDS; 1,000±300 mg L-1, 3,000±300 mg L-1, and 6,000±300 mg L-1). We found that our methodology generated consistent, repeatable results within treatment groups. Current velocities ranging from 0.001-0.026 needed to keep the gellan beads in suspension were negatively correlated to soak times and TDS and positively correlated with temperature. The flow chamber seems to be a viable approach for evaluating minimum current velocities needed to keep the eggs of pelagic broadcast spawning cyprinids in suspension during development.


Portion of figure showing the influence of temperature and dissolved oxygen on Nile Perch depth distributions

Taabu-Munyaho A, RJ Kayanda, I Eversons, TB Grabowski, & G Marteinsdóttir. 2013. Distribution and exploitation of Nile Perch Lates niloticus in relation to stratification in Lake Victoria, East Africa. Journal of Great Lakes Research 39:466-475 doi: 10.1016/j.jglr.2013.06.009

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Stratification restricts habitable areas forcing fish to balance between favourable temperature and minimum dissolved oxygen requirements. Acoustic surveys conducted during the stratified and isothermal periods on tropical Lake Victoria indicated that stratification of temperature and dissolved oxygen (DO) affected vertical distribution of Nile Perch. There was higher mean temperature (25.6 ± 0.5 °C) and lower DO (6.4 ± 1.8 mg/l) during stratified period compared to the isothermal period (mean temperature 24.9 ± 0.3 °C; mean DO 7.3 ± 0.6 mg/l). Higher mean densities of Nile perch were recorded in the coastal (0.44 ± 0.03) and deep (0.27 ± 0.02 g/m3) strata during the stratified compared to the isothermal season (coastal: 0.24 ± 0.01; deep: 0.12 ± 0.02 g/m3). In addition, Nile perch density in the upper 0–40 m depth layers in the coastal and deep strata increased by over 50% from the isothermal to the stratified season. Daily landings from 65 motorised fishing boats between October 2008 and September 2010 show higher mean catch (26.29 ± 0.17 kg/boat/day) during stratified compared to the isothermal (23.59 ± 0.15) season. Thermal stratification apparently compresses the habitat available to Nile Perch and can potentially result in higher exploitation. Managers should evaluate the potential benefits of instituting closed seasons during the stratified period, and stock assessment models should take into account the seasonal niche compression.


Tim Grabowski clears the screen of a Moore egg collector in the Rio Grande in Albuquerque, New Mexico

Worthington, TA, SK Brewer, TB Grabowski, & J Mueller. 2013. Sampling efficiency of the Moore egg collector. North American Journal of Fisheries Management 33:79-88. doi: 10.1080/02755947.2012.741557

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Quantitative studies focusing on the collection of semibuoyant fish eggs, which are associated with a pelagic broadcast-spawning reproductive strategy, are often conducted to evaluate reproductive success. Many of the fishes in this reproductive guild have suffered significant reductions in range and abundance. However, the efficiency of the sampling gear used to evaluate reproduction is often unknown and renders interpretation of the data from these studies difficult. Our objective was to assess the efficiency of a modified Moore egg collector (MEC) using field and laboratory trials. Gear efficiency was assessed by releasing a known quantity of gellan beads with a specific gravity similar to that of eggs from representatives of this reproductive guild (e.g., the Arkansas River Shiner Notropis girardi) into an outdoor flume and recording recaptures. We also used field trials to determine how discharge and release location influenced gear efficiency given current methodological approaches. The flume trials indicated that gear efficiency ranged between 0.0% and 9.5% (n = 57) in a simple 1.83-m-wide channel and was positively related to discharge. Efficiency in the field trials was lower, ranging between 0.0% and 3.6%, and was negatively related to bead release distance from the MEC and discharge. The flume trials indicated that the gellan beads were not distributed uniformly across the channel, although aggregation was reduced at higher discharges. This clustering of passively drifting particles should be considered when selecting placement sites for an MEC; further, the use of multiple devices may be warranted in channels with multiple areas of concentrated flow.


DIDSON imagery of a putative Atlantic Cod spawning event

Grabowski TB, KM Boswell, BJ McAdam, RJD Wells & G Marteinsdóttir. 2012. Characterization of Atlantic Cod spawning habitat and behavior in Icelandic coastal waters. PLoS One 7(12):e51312. doi:10.1371/journal.pone.0051321

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The physical habitat used during spawning may potentially be an important factor affecting reproductive output of broadcast spawning marine fishes, particularly for species with complex, substrate-oriented mating systems and behaviors, such as Atlantic Cod Gadus morhua. We characterized the habitat use and behavior of spawning Atlantic Cod at two locations off the coast of southwestern Iceland during a 2-d research cruise (15–16 April 2009). We simultaneously operated two different active hydroacoustic gear types, a split beam echosounder and a dual frequency imaging sonar (DIDSON), as well as a remotely operated underwater vehicle (ROV). A total of five fish species were identified through ROV surveys: including Cusk Brosme brosme, Atlantic Cod, Haddock Melanogrammus aeglefinus, Lemon Sole Microstomus kitt, and Atlantic redfish Sebastes spp. Of the three habitats identified in the acoustic surveys, the transitional habitat between boulder/lava field and sand habitats was characterized by greater fish density and acoustic target strength compared to that of sand or boulder/lava field habitats independently. Atlantic Cod were observed behaving in a manner consistent with published descriptions of spawning. Individuals were observed ascending 1–5 m into the water column from the bottom at an average vertical swimming speed of 0.20–0.25 m s−1 and maintained an average spacing of 1.0–1.4 m between individuals. Our results suggest that cod do not choose spawning locations indiscriminately despite the fact that it is a broadcast spawning fish with planktonic eggs that are released well above the seafloor.


A male Apalachicola Redhorse

Grabowski TB, Young SP, JJ Isely, & PC Ely. 2012. Age, growth, and reproductive biology of catostomid species from the Apalachicola River, Florida. Journal of Fish and Wildlife Management 3(2):223-237. doi: 10.3996/012012-JFWM-008.

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Riverine catostomids can show a wide range of interspecific variation in life-history characteristics. Understanding these differences is an important consideration in evaluating the sensitivity of these fishes to disturbance and in formulating effective conservation strategies, particularly when dealing with an assemblage consisting of multiple species within a watershed. We collected Apalachicola Redhorse Moxostoma n. sp. cf. poecilurum (n  =  125), Spotted Sucker Minytrema melanops (n  =  94), and Quillback Carpiodes cyprinus (n  =  94) to determine age, growth, and reproductive biology of spawning catostomids in the Apalachicola River, Florida, during 2007. Quillback was the smallest in total length at age; longest-lived; most fecund; and produced the smallest eggs. Apalachicola Redhorse was the largest in body size; had an intermediate life span; and produced the fewest yet largest eggs. Spotted Sucker was more similar to Apalachicola Redhorse in most characteristics. Growth during ages 1–3 in all three species seemed to be negatively related to the proportion of observations of extreme flow, both high (Q90) and low (Q10), per year and a positive response in growth rate to high flows (>Q75 but < Q90). However, Apalachicola Redhorse and Spotted Sucker growth was more sensitive to flow conditions than that of Quillback. Our results suggest the life histories and ecological response of Apalachicola River catostomids to flow regulation are important components for developing strategies that incorporate the needs of these fishery resources into an ecosystem-based management approach.


Portrait of an Atlantic Cod being photographed for morphometric analysis

McAdam BJ, TB Grabowski & G Marteinsdóttir. 2012. Identification of stock components using morphological markers. Journal of Fish Biology 81:1447-1462. doi: 10.1111/j.1095-8649.2012.03384.x

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This study investigated the development of a quantitative method for distinguishing stock components of Icelandic cod Gadus morhua based on visual examination of morphology. The stock is known to be structured into genetically distinct geographic components (north and south of Iceland) and behavioural types that spawn sympatrically. Differences in morphology were tested between locations, genotypes (a proxy for behaviour) and sexes. Results show morphological markers on the head, fins and body of G. morhua that are correlated with the sex, genotype of the fish at the pantophysin (pan-I) locus and the location at which the fish were caught. Females were found to have relatively deep bodies, and the pan-IBB genotype (associated with deep-water feeding behaviour) have greater gaps between their fins. Overall, morphology is more useful for distinguishing sympatric genotypes but less powerful at identifying genetically distinct geographic sub-populations, perhaps because counter-gradient evolution reduces phenotypic differences even with an underlying genetic cause.


Map of cetacean sightings

McAdam BJ, TB Grabowski & G Marteinsdóttir. 2012. Testing for differences in spatial distributions from telemetry data. Fisheries Research 127-128:148-153. doi:10.1016/j.fishres.2012.02.024

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Spatial distribution is increasingly studied using individual based telemetric methods in lieu of, or supplementing, surveys. Distributions may be in two- or three-dimensions, or in an abstract space such as depth-temperature space. One of the most basic questions one might address when analyzing these data is whether the distributions of two species, populations, genders, age classes, or other units are different. However due to the inherent differences between survey and telemetric approaches, it is difficult for practitioners to find a simple and easily applied approach to answer this question. In contrast to surveys, telemetry collects a large amount of data about a small number of individuals. Methods must therefore account for the random effects of individual variation in a way not otherwise necessary. We will demonstrate, for example, that tests suitable for detecting differences in distributions from survey data can give false positives (type I errors) when faced with telemetry data. This is essentially because the test treats the large number of data points as a very high n, but in fact the small number of individuals makes the n very small. As a solution, we present a test for differences in distribution based on an existing test for survey data based upon randomizing the data at the level of individuals rather than observations.


Thermal bathymetric niche of coastal cod around southern Iceland

Grabowski TB, V Thorsteinsson, BJ McAdam, & G Marteinsdóttir. 2011. Evidence of segregated spawning in a single marine fish stock: sympatric divergence of ecotypes in Icelandic cod? PLoS One 6(3):e17528. doi:10.1371/journal.pone.0017528

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There is increasing recognition of intraspecific diversity and population structure within marine fish species, yet there is little direct evidence of the isolating mechanisms that maintain it or documentation of its ecological extent. We analyzed depth and temperature histories collected by electronic data storage tags retrieved from 104 Atlantic Cod at liberty ≥1 year to evaluate a possible isolating mechanisms maintaining population structure within the Icelandic cod stock. This stock consists of two distinct behavioral types, resident coastal cod and migratory frontal cod, each occurring within two geographically distinct populations. Despite being captured together on the same spawning grounds, we show the behavioral types seem reproductively isolated by fine-scale differences in spawning habitat selection, primarily depth. Additionally, the different groups occupied distinct seasonal thermal and bathymetric niches that generally demonstrated low levels of overlap throughout the year. Our results indicate that isolating mechanisms, such as differential habitat selection during spawning, might contribute to maintaining diversity and fine-scale population structure in broadcast-spawning marine fishes.


Photo of first Highfin Carpsucker reported from the Apalachicola River, Florida

Young SP, TB Grabowski, PC Ely, & JJ Isely. 2010. First record of Highfin Carpsucker Carpiodes velifer in the Apalachicola River, Florida. Southeastern Naturalist 9:165-170. doi: 10.1656/058.009.0112

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We document the discovery of Highfin Carpsucker in the Apalachicola River, FL. Three specimens were captured between river kilometer 170–171 on the Apalachicola River in the vicinity of Jim Woodruff Lock and Dam during spring 2007, The specimens were captured within a reach 0.5–1.0 km downstream from the dam and just upstream from a major spawning area for several other catostomids. This is the first record of the species east of the Choctawhatchee River, FL—AL, and is a range extension of 185 km eastward into the Florida Panhandle region of the Northern Gulf of Mexico Coast.


Juvenile Atlantic Cod in a chamber respirometer

Grabowski TB, SP Young, LA Libungan, A Steinarsson & G Marteinsdóttir. 2009. Evidence of phentotypic plasticity and local adaption in metabolic rates between components of the Icelandic cod (Gadus morhua L.) stock. Environmental Biology of Fishes 86:361-370. doi: 10.1007/s10641-009-9534-z

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Phenotypic plasticity and local adaptations are important considerations in delineating population structure of marine fishes and critical to their conservation and management. We compared the weight-specific oxygen consumption rates (VO2/M) of juvenile cod from the northern and southern components of the Icelandic stock acclimated to 4.0°C, 8.5°C, and 12.6°C and their metabolic response to abrupt temperatures changes within this range. Southern individuals exhibited VO2/M up to 50% higher than their northern counterparts when tested at their acclimation temperature. However, northern fish generally experienced greater changes in VO2/M, three to six-fold increases, relative to that expected at acclimation when moved to higher temperatures. Southern cod showed a greater decrease in VO2/M when exposed to lower temperatures. Our results indicate physiological differences exist between the northern and southern components of the Icelandic cod stock and warrant considering them as two distinct populations.


Tyler Ferguson holds the only radio-tagged Robust Redhorse captured during the study

Grabowski TB, TD Ferguson, JT Peterson & CA Jennings. 2009. Detection probability and response of the Robust Redhorse, a cryptic riverine fish, to electrofishing. North American Journal of Fisheries Management 29:721-729. doi: 10.1577/M08-060.1

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Conservation and management of riverine species, such as the Robust Redhorse Moxostoma robustum, can be hindered by incomplete understanding of their population status. Behavioral responses to sampling methods and susceptibility to capture by a particular gear type have implications for the reliability of population assessments yet are rarely evaluated. Consequently, we used radio-tagged Robust Redhorses in the Ocmulgee River, Georgia, to estimate capture probability when sampled with standard boat electrofishing techniques and to assess behavioral responses of individuals to single and repeated exposures to electrofishing, the recommended sampling method for this species. Transects containing one to eight radio-tagged individuals were sampled. The positions of radio-tagged individuals were recorded before electrofishing and at 1-h, 24-h, and 3–5-d intervals after sampling. We estimated Robust Redhorse abundance and capture probability by treating each transect as analogous to a repeated sample in time. Only one radio-tagged individual and six untagged individuals were captured after 7.46 h of effort. The radio-tagged fish did not exhibit an immediate response to boat electrofishing, but some did move from their original position after electrofishing. Mean movement was 0.15 km after 1 h, 0.19 km after 24 h, and 0.23 km after 3–5 d. However, this movement was similar to that exhibited by fish located at similar intervals during a related telemetry study without exposure to electrofishing. A mean capture probability of 0.031 with a 95% Bayesian credibility interval of 0.002–0.111 was estimated from the best-approximating model. Electrofishing is not a particularly effective method for capturing this species; therefore, inferences about population size and distribution of this or other cryptic species need to account for low detection probabilities associated with electrofishing. Alternative approaches or multiple strategies should be considered when assessing the status of such species.


Finishing closing a Robust Redhorse after transmitter implantation

Grabowski TB & CA Jennings. 2009. Radio-tagged “guide fish”: a method for uncovering information about rare or cryptic fishes. Fisheries Management and Ecology 16:68-71. doi: 10.1577/M08-060.1


A transport tank full of radio-tagged Robust Redhorse just prior to release

Grabowski TB & CA Jennings. 2009. Post-release movements and habitat use of Robust Redhorse transplanted to the Ocmulgee River, Georgia. Aquatic Conservation: Marine and Freshwater Ecosystems 19:170-177. doi: 10.1002/aqc.980

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Robust Redhorse Moxostoma robustum is an imperiled, potadromous fish in the south-eastern USA. Initial recovery efforts have focused on supplementing existing populations and establishing refugial populations through extensive stocking programmes. However, assessment of the success of these programmes has not yet been conducted, and there are few reports evaluating the effectiveness of such programmes with other potadromous species. Radio telemetry was employed to assess the effectiveness of a stocking programme aimed at addressing whether stocked individuals would remain in an area free of introduced predators and ascertaining the ability of stocked fish to integrate into a resident population. Hatchery-reared Robust Redhorse were captured from refugial populations established in other river systems and were transferred to the Ocmulgee River, Georgia where a population of hatchery-reared individuals and an unknown number of wild fish reside. These transferred Robust Redhorse exhibited an exploratory phase for the first 3 months before adopting behaviour patterns, including spawning migrations, that were consistent with those reported for wild fish in other systems. However, some individuals seemed unable to locate suitable spawning habitat. Approximately half of the radio-tagged fish remained within the area free of introduced predators. At least some radio-tagged Robust Redhorse fully integrated into the resident population as evidenced by their presence in spawning aggregations with resident individuals. The effectiveness of a stocking programme is dependent upon the ability of stocked individuals to integrate into an existing population or replicate the behaviour and functionality of a resident population. Evaluations of stocking programmes should incorporate assessments of behaviour in addition to surveys to estimate abundance and survivorship and genetic assessments of augmentation of effective population sizes.


A pair of spawning Robust Redhorse in the Savannah River

Grabowski TB & JJ Isely. 2008. Size of spawning population, residence time, and territory shifts of individuals in the spawning aggregation of a riverine catostomid. Southeastern Naturalist 7:475-482. doi: 10.1656/1528-7092-7.3.475

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Little is known about the behavior of individual fish in a spawning aggregation, specifically how long an individual remains in an aggregation. We monitored Moxostoma robustum (Cope) (Robust Redhorse) in a Savannah River spawning aggregation during spring 2004 and 2005 to provide an estimate of the total number of adults and the number of males comprising the aggregation and to determine male residence time and movements within a spawning aggregation. Robust Redhorse were captured using prepostioned grid electrofishers, identified to sex, weighed, measured, and implanted with a passive integrated transponder. Spawning aggregation size was estimated using a multiple census mark-and-recapture procedure. The spawning aggregation seemed to consist of approximately the same number of individuals (82–85) and males (50–56) during both years of this study. Individual males were present for a mean of 3.6 ± 0.24 days (± SE) during the 12-day spawning period. The mean distance between successive recaptures of individual males was 15.9 ± 1.29 m (± SE). We conclude that males establish spawning territories on a daily basis and are present within the spawning aggregation for at least 3–4 days. The relatively short duration of the aggregation may be the result of an extremely small population of adults. However, the behavior of individuals has the potential to influence population estimates made while fish are aggregated for spawning.


Microfiche reader screen schowing a Notchlip Redhorse scale

Grabowski TB, NL Ratterman & JJ Isely. 2008. Demographics of the spawning aggregations of four catostomid species in the Savannah River, South Carolina and Georgia. Ecology of Freshwater Fish 17:318-327. doi: 10.1111/j.1600-0633.2007.00284.

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Differences in the life history strategies employed by otherwise ecologically similar species of a fish assemblage may be an important factor in the coexistence of these species and is an essential consideration in the conservation and management of these assemblages. We collected scales to determine age and growth of four species of the catostomid assemblage (Northern Hogsucker Hypentelium nigricans, Spotted Sucker Minytrema melanops, Notchlip Redhorse Moxostoma collapsum and Robust Redhorse Moxostoma robustum) of the Savannah River, Georgia–South Carolina in spring 2004 and 2005. Robust Redhorse was the largest species; reaching sexual maturity at an older age and growing faster as a juvenile than the other species. Spotted Sucker did not achieve the same size as Robust Redhorse, but reached sexual maturity at younger ages. Notchlip Redhorse was intermediate between the abovementioned two species in age at maturity and size. Northern Hogsucker was the smallest species of the assemblage and reached the sexual maturity at the age of three. Both Robust Redhorse and Spotted Sucker were sexually dimorphic in size-at-age. The range of life history strategies employed by Savannah River catostomids encompasses the range of life history strategies exhibited within the family as a whole.


A larval redhorse <i>Moxostoma</i> sp.

Grabowski TB, & JJ Isely. 2007. Effects of flow fluctuations on riverine fish spawning habitat. Southeastern Naturalist 6:471-478. doi: 10.1656/1528-7092

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Shallow-water, lithophilic spawning fishes are among the most vulnerable to anthropogenic fluctuations in water levels. We monitored water levels and environmental conditions at the nest sites of Moxostoma robustum (Robust Redhorse) on a main-channel gravel bar in the Savannah River, GA–SC. During the course of the 2005 spawning season, over 50% of the observed nest sites were either completely dewatered or left in near zero-flow conditions for several days. This occurred on two separate occasions, once early during the spawning season and then again near its conclusion. We hypothesize the habitat preferences of spawning Robust Redhorse leave them vulnerable to water-level fluctuations, and this phenomenon may be widespread in regulated river systems.


A triad of Robust Redhorse in the Savannah River

Grabowski TB & JJ Isely. 2007. Spatial and temporal segregation of spawning habitat by catostomids in the Savannah River, Georgia and South Carolina, U.S.A. Journal of Fish Biology 70:782-798. doi: 10.1111/j.1095-8649.2007.01341.x

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Spawning aggregations of five species of catostomids were observed on the two mid-channel gravel bars of the Savannah River, Georgia and South Carolina, in 2004 and 2005 to assess the degree of spatial and temporal overlap in the use of this habitat and determine the habitat preferences leading to segregation. Spawning catostomids showed a considerable amount of temporal overlap in their use of these mid-channel gravel bars. The observed temporal overlap was consistent between 2004 and 2005 and corresponded to temperatures at which species were present. The distribution of catostomids was not uniform at the upstream gravel bar. Carpsuckers Carpiodes sp., Spotted Sucker Minytrema melanops and Robust Redhorse Moxostoma robustum both demonstrated some spatial overlap with Notchlip Redhorse Moxostoma collapsum; however, their overall distributions were different from one another. Northern Hogsucker Hypentelium nigricans was present across the gravel bars, apparently as an egg predator. Spawning catostomids segregated based on flow, depth, slope and substratum size. Whether due to limited habitat availability or changes in the timing of reproduction due to altered cues, temporal and spatial overlap occurs between spawning catostomids despite the apparent partitioning of available spawning habitat. It is unclear, however, if this overlap results in excessive mortality in the early life-history stages of these species. Results suggest spatial overlap among catostomid species was minimized due to species spawning in areas within a narrow range of conditions. Intraspecific interactions such as nest site superimposition or disturbance may be a concern.


A Robust Redhorse in the Savannah River

Grabowski TB & JJ Isely. 2006. Seasonal and diel movement and habitat use of Robust Redhorses in the Savannah River, Georgia and South Carolina. Transactions of the American Fisheries Society 135:1145-1155. doi: 10.1577/T05-230.1

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The Robust Redhorse Moxostoma robustum is a large riverine catostomid whose distribution is restricted to three Atlantic Slope drainages. Once presumed extinct, this species was rediscovered in 1991. Despite being the focus of conservation and recovery efforts, the Robust Redhorse's movements and habitat use are virtually unknown. We surgically implanted pulse-coded radio transmitters into 17 wild adults (460–690 mm total length) below the downstream-most dam on the Savannah River and into 2 fish above this dam. Individuals were located every 2 weeks from June 2002 to September 2003 and monthly thereafter to May 2005. Additionally, we located 5–10 individuals every 2 h over a 48-h period during each season. Study fish moved at least 24.7 ± 8.4 river kilometers (rkm; mean ± SE) per season. This movement was generally downstream except during spring. Some individuals moved downstream by as much as 195 rkm from their release sites. Seasonal migrations were correlated to seasonal changes in water temperature. Robust Redhorses initiated spring upstream migrations when water temperature reached approximately 12°C. Our diel tracking suggests that Robust Redhorses occupy small reaches of river (∼1.0 rkm) and are mainly active diurnally. Robust Redhorses were consistently found in association with woody debris and gravel streambed sediments along the outer edge of river bends. Fish exhibited a high degree of fidelity to both overwintering and spawning areas. Our observations of long-distance seasonal migrations suggest that successful Robust Redhorse conservation efforts may require an ecosystem management approach.


A Longear Sunfish which is similar in appearance to a Dollar Sunfish

Paller MH, DE Fletcher, MM Standora, TB Grabowski, TA Jones, SA Dyer & JJ Isely. 2006. Emigration of fish from two South Carolina cooling reservoirs. North American Journal of Fisheries Management 26:976-982. doi: 10.1577/M05-168.1

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We assessed fish escapement from two South Carolina reservoirs, Par Pond and L Lake, from spring 2002 through summer 2003. Escapement was greatest in the spring and early summer, with Lake Chubsucker Erimyzon sucetta dominating the escapement catch in early spring and several sunfishes Lepomis spp. dominating in late spring and early summer. Most of the escaping centrarchids were Bluegill L. macrochirus, Warmouth L. gulosus, and Redbreast Sunfish L. auritus, in L Lake and Warmouths, Bluegills, Dollar Sunfish L. marginatus, and Spotted Sunfish L. punctatus in Par Pond. Escapement was enhanced by high reservoir water levels and surface discharge over the spillway. Escapement declined during periods of hypolimnetic release from bottom discharge gates. Location of the Par Pond discharge structure in the littoral zone rather than the pelagic zone as in L Lake contributed to greater escapement of littoral species in Par Pond. Species composition in the escapement catches and reservoirs was not significantly correlated. Relatively low escapement from L Lake and Par Pond compared with that in other reservoirs may have been related to the configuration of the discharge structures, low water levels during 2002, fish habitat preferences, species composition and abundance in the reservoirs, and low rates of discharge.


A prepositioned grid electrofisher

Grabowski TB & JJ Isely. 2005. Use of prepositioned grid electrofishers for the collection of Robust Redhorse broodstock. North American Journal of Aquaculture 67:89-92. doi: 10.1577/A04-031.1

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We investigated the potential of prepositioned grid electrofishers as a means of collecting broodstock for the rare Robust Redhorse Moxostoma robustum. We found that combined with visual observation, this technique allowed for the efficient capture of fish in breeding condition. We were able to harvest eggs in the field and bring only fertilized eggs into the hatchery. There was no need to induce spawning hormonally. Although their use is limited by water depth and clarity, prepositioned grid electrofishers used in conjunction with visual observation warrants further consideration as an effective tool for the collection of reproductively active broodstock for conservation purposes.


Heather Williams shows off a Flathead Catfish

Grabowski TB & JJ Isely, and R.R. Weller. 2004. Age and growth of Flathead Catfish, Pylodictis olivaris Rafinesque, in the Altamaha River, Georgia. Journal of Freshwater Ecology 19:411-417. doi: 10.1080/02705060.2004.9664914

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Flathead Catfish were introduced to the Altamaha River system, Georgia in the 1970's. We determined the length-weight relationship, Von Bertalanffy growth parameters, and back calculated lengths by examining the sagittal otoliths of 331 individuals captured from this population. We found that there were no sex related differences in length weight relationship or Von Bertalanffy growth parameters. Flathead Catfish in the Altamaha River system grow at about the same rate as individuals in other introduced populations.


Entrance to Wallhalla State Fish Hatchery where this work was conducted

Isely JJ, DG Trested & TB Grabowski. 2004. Tag retention and survivorship of hatchery Rainbow Trout marked with large format VIalpha tags. North American Journal of Aquaculture 66:73-74. doi: 10.1577/C03-018

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Large-format visible implant alphanumeric (VIalpha) tags were implanted into 15,400 Rainbow Trout Oncorhynchus mykiss,/i>. Tag retention after 25 d was 82.6%, and survivorship was 92.8%. The results of this study compare favorably with those of similar studies on other species and suggest that large-format VIalpha tags are an appropriate choice for studies requiring the individual identification of larger Rainbow Trout.


A normal and sneaker male Rainbow Trout

Isely JJ & TB Grabowski. 2004. Occurrence, size, and tag retention of sneaker male hatchery Rainbow Trout. North American Journal of Aquaculture 66:234-236. doi: 10.1577/A03-023.1

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One alternative reproductive tactic involving early-maturing, cryptic males is referred to as “sneaking.” Although sneakers tend to be easily detectable upon close inspection, little is known about the proportion of a fish population consisting of sneakers. We examined 15,400 age-1 Rainbow Trout Oncorhynchus mykiss in a hatchery. Total length (mm), wet weight (g), and sex (sneaker male or unknown) were recorded for each fish. We also individually tagged each sneaker male with soft visual implant alphanumeric (VIalpha) tags that were sequentially numbered and held the fish for 25 d before inspection. Sneakers constituted 2.8% of the hatchery rainbow trout population and were smaller in total length and weight than typical rainbow trout of the same age. Retention of the VIalpha tags in sneakers was 58.9%, significantly lower than has been reported under similar circumstances. We found that sneaker males may contribute substantially to hatchery populations. Reduced tag retention in sneakers may bias studies evaluating the effect of hatchery fish on wild populations. We believe that hatchery-produced sneaker males have the potential to contribute importantly to the genetic composition of wild populations.



Book chapters

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Notchlip Redhorse otolith - placeholder for book cover

Long JM, & TB Grabowski. In press. Use of otoliths to estimate fish age. Pages XX-XX in MC Quist & D Isermann, eds. Age and growth of fishes: principles and techniques. American Fisheries Society, Bethesda, Maryland.

book cover

Birdsong TW, MS Allen, JE Claussen, GP Garrett, TB Grabowski, & 15 additional authors. 2015. Native Black Bass Initiative: Implementing watershed-scale approaches to conservation of endemic black bass and other native fishes in the southern United States. Pages 363-378 in MD Tringali, MS Allen, TW Birdsong, and JM Long, eds. Black bass diversity: multidisciplinary science for conservation. American Fisheries Society, Symposium 82, Bethesda, Maryland. ISBN-13: 978-1-934874-40-0

book cover

Isely JJ & TB Grabowski. 2007. Age and growth. Pages 187-228 in ML Brown & CS Guy, eds. Analysis and interpretation of freshwater fisheries data. American Fisheries Society, Bethesda, Maryland. ISBN-13: 978-1-888569-77-3



Technical reports

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A papio cruises through a reef in west Hawaii.

Grabowski TB & EC Franklin. 2017. What can volunteer angler tagging data tell us about the status of the Giant Trevally (ulua aukea) Caranx ignobilis fishery in Hawaii: revisiting data collected during Hawaii’s Ulua and Papio Tagging Project 2000-2016. U.S. Department of Interior, Fish and Wildlife Service, Cooperator Science Series FWS/CSS-126-2017, Washington, D.C.

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Giant Trevally (ulua aukea) Caranx ignobilis is one of the most highly prized and frequently targeted nearshore species. However, there is very little information on its current status in Hawaiian waters. This study uses mark-recapture data collected as part of recreational angler tagging program conducted by the Hawaii Department of Land and Natural Resources-Division of Aquatic Resources during 2000-2012. Mark-recapture data were used to estimate von Bertalanffy growth curve parameters and survivorship. Growth curves generated from the mark recapture data suggested that Giant Trevally from the main Hawaiian Islands may be growing faster and reach a smaller maximum size than individuals in the Northwest Hawaiian Islands, but there are a number of issues rendering this conclusion uncertain. The survivorship of Giant Trevally was positively associated with age, in part due to ontogenetic habitat shifts that result in older fish moving to offshore habitats where they are less vulnerable to anglers. When compared to stock assessments performed using commercial landings data and fisheries-independent visual surveys, the mark-recapture data produced similar estimates for the average length of exploited fish, a metric highly negatively correlated to fishing mortality. These results emphasize the need for additional information on the biology of Giant Trevally in Hawaiian waters and suggest that the data collected from this recreational angler tagging program may be useful to generate reliable estimates of mortality for stock assessment purposes.


Comparison of land cover in the Onion Creek watershed, 1970-1980 vs today.

Pease JE, TB Grabowski, & AA Pease. 2017. Variation and plasticity and their interaction with urbanization in Guadalupe Bass populations on and off the Edwards Plateau. U.S. Department of Interior, Fish and Wildlife Service, Cooperator Science Series FWS/CSS-125-2017, Washington, D.C.

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The Colorado River Basin in Texas has experienced major alterations to its hydrologic regime due to changing land and water use patterns. These anthropogenic influences on hydrologic variability have had major implications for riparian and aquatic ecosystems and the species dependent upon them. However, impacts are often assessed at a limited temporal and spatial scale, tending to focus on relatively short and discrete periods or portions of a river basin. It is not clear how basin-wide alterations occurring over decades affect species. Guadalupe Bass Micropterus treculii are endemic to central Texas and are typically associated with shallow runs and riffles in small streams. However, Guadalupe Bass are found throughout the Colorado River Basin, including the mainstem portion of the lower river downstream of the city of Austin where they support a popular fishery. Because Guadalupe Bass exist across a wide range of stream orders within the basin, it is unclear whether populations respond similarly to anthropogenic disturbances or to conservation and restoration activities. Therefore, our objectives were to: 1) assess the effects of urbanization and hydrology on the population structure and dynamics of Guadalupe Bass; 2) evaluate the effects of environmental gradients on ecomorphological variation in Guadalupe Bass populations across multiple spatial scales; and 3) describe the life history, habitat use, and behavior of the Guadalupe Bass population in the lower Colorado River and compare it to populations in more “typical” habitats. Results contribute to an understanding of the response of Guadalupe Bass to anthropogenic disturbances, including increased urbanization in central Texas and further assist in the conservation of the species. The ability of the population to not only persist, but flourish downstream of a heavily populated urban area presented a unique opportunity to investigate a native species response to anthropogenic disturbance. This research revealed differences in Guadalupe Bass habitat associations and movements, contrasts in age and growth, and morphological variation across a gradient of disturbance throughout the Colorado River Basin. Results of this work provide information on the potential effects of human population growth and increased water withdrawals on Guadalupe Bass populations. Additionally, this work adds to an understanding of the unique Guadalupe Bass population found in the lower Colorado River and how it differs from upstream tributary populations. Gathering additional population-level information facilitates conservation actions critical to preserving preferred habitat and promoting growth rates for Guadalupe Bass in streams of different sizes and flow conditions while highlighting interpopulation differences that may warrant consideration for stocking programs and other management strategies.


A burbot spawning aggregation in Moyie Lake, British Columbia

Grabowski TB. 2016. Assessing the feasibility of using acoustic monitoring for Burbot conservation, management, and production. U.S. Department of Interior, Fish and Wildlife Service, Cooperator Science Series FWS/CSS-118-2016, Washington, D.C.

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Burbot Lota lota is the sole freshwater representative of the cod-like fishes and supports subsistence, commercial, and recreational fisheries worldwide above approximately 40° N. It is a difficult species to manage effectively due to its preference for deep-water habitats and spawning activity under the ice in winter. Like other gadiform fishes, Burbot use acoustic signaling as part of their mating system, and while the acoustic repertoire of the species has been characterized under artificial conditions (i.e., net pen suspended under ice in a natural lake), there has been no work to determine whether the species is as vocal in natural spawning aggregations. Our objective was to assess the feasibility of collecting and using acoustic data to characterize the spawning activity and locations of Burbot under field conditions. We recorded audio and video of Burbot spawning aggregations through holes drilled into the ice at known spawning grounds at Moyie Lake in British Columbia, Canada. Acoustic recordings (call counts and audiograms) were analyzed using Raven Pro v 1. 4 software. Acoustic behavior was also related to video data to determine how acoustic activity correlated to any observed spawning behavior. In general, wild Burbot spawning in Moyie Lake did not vocalize as frequently as counterparts spawning under artificial conditions. Further, Burbot vocalizations were not recorded in conjunction with spawning activity. While it may be feasible to use passive acoustic monitoring to locate Burbot spawning grounds and identify periods of activity, it does not seem to hold much promise for locating and quantifying spawning activity in real time.


A tagged Guadalupe Bass in the South Llano River, Texas

Bean PT & TB Grabowski. 2015. Evaluation and refinement of Guadalupe Bass conservation strategies to support adaptive management. Department of Interior, Fish and Wildlife Service, Cooperator Science Series FWS/CSS-114-2015, Washington, D.C.

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Guadalupe Bass has experienced declines in parts of its range due to habitat degradation and introgressive hybridization with Smallmouth Bass. Efforts to restore Guadalupe Bass have been ongoing in the Guadalupe River drainage for over twenty years, but have recently been expanded to the Llano River drainage. Conservation and restoration efforts in the South Llano River have included supplemental stocking of non-hybrid fingerlings as well as landscape-level restoration projects in riparian and upland areas to benefit the river system. To further this effort, we undertook a multifaceted approach to evaluate Guadalupe Bass population characteristics as well as habitat use patterns in order to refine conservation and restoration strategies. Catch per unit effort was highest in sites containing swift moving water and large woody debris while abundances were lower in pool habitats with bedrock substrates. Additionally, population estimates resulted in densities of Guadalupe Bass that were highest in the sub-reaches possessing greater amounts of riffle and run habitats. Evaluation of current introgression rates indicates that introgression has declined from 3.9% to 0.9% as stocking has occurred. Our results suggest that maintaining flows is critical to providing highly utilized habitat types and that supplemental stocking is an effective management tool for reducing introgression rates in Guadalupe Bass.


Side scan sonar image from the South Llano River, Texas

Cheek BD & TB Grabowski. 2014. Evaluating habitat associations of a fish assemblage at multiple scales in a minimally disturbed stream on the Edwards Plateau, central Texas. U.S. Department of Interior, Fish and Wildlife Service, Cooperator Science Series FWS/CSS-104-2014, Washington, D.C.

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Understanding how environmental factors operating at different spatial scales within a watershed structure instream habitat is essential for accurately quantifying fish habitat associations and developing effective means for assessing stream conservation and restoration activities. In this study, we used a combination of side scan sonar surveys, imagery collected by an unmanned aerial vehicle, and instream surveys of fishes and physicochemical conditions to evaluate the effect of physicochemical and habitat variables at various spatial scales, e.g., micro-mesohabitat, mesohabitat, riffle-run-pool complex, stream reach, on fish assemblage habitat associations in the South Llano River, a spring-fed second order stream on the Edwards Plateau in central Texas. We found that the micro-mesohabitat scale and the riffle-run-pool complex scale had the greatest explanatory power. Many of the fishes endemic to the streams of the Edwards Plateau, such as Guadalupe Bass Micropterus treculii and Texas Logperch Percina carbonaria, exhibited associations with similar physicochemical and landscape variables. Our results suggest that conservation and restoration efforts targeting single species, such as the Guadalupe Bass Restoration Initiative, can benefit a suite of species. However, our results did not demonstrate incontrovertibly that a single species, such as Guadalupe bass, can serve as an indicator of the status of the stream fish assemblage as a whole. These findings will help provide data on the habitat use patterns of a fish assemblage in a relatively undisturbed Edwards Plateau stream and potentially help prioritize future restoration efforts for other streams in the region.


Close up of a Robust Redhorse

Grabowski TB. 2014. Suckers (particularly Moxostoma spp.). Pages 56-63 in MM Davis & SK Brewer (eds.). Gulf Coast Prairie Landscape Conservation Cooperative Regional Hypotheses of Ecological Responses to Flow Alteration. A report by the GCP LCC Flow-Ecology Hypotheses Committee to the Southeast Aquatic Resources Partnership (SARP) for the GCP LCC Instream Flow Project. Wildlife Management Institute Grant Number GCP LCC 2012-003.

Close up of a Guadalupe Bass

Grabowski TB. 2014. Guadalupe Bass (Micropterus treculii). Pages 42-48 in MM Davis & SK Brewer (eds.). Gulf Coast Prairie Landscape Conservation Cooperative Regional Hypotheses of Ecological Responses to Flow Alteration. A report by the GCP LCC Flow-Ecology Hypotheses Committee to the Southeast Aquatic Resources Partnership (SARP) for the GCP LCC Instream Flow Project. Wildlife Management Institute Grant Number GCP LCC 2012-003.


A tank full of Arkansas River Shiner

Brewer SK & TB Grabowski. 2013. Evaluating the reproductive success of Arkansas River shiner by assessing early life-history stage dispersal and survival at the landscape level. Final report to the U.S. Fish and Wildlife Service, Great Plains Landscape Conservation Cooperative (FWS agreement number F11AP00112).



Theses and dissertations

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Wade Massure with a Smallmouth Buffalo

Massure WA. 2016. Evaluating the effects of drought and anthropogenic alterations on the growth of stream fishes on the Edwards Plateau. M.S. thesis. Texas Tech University, Lubbock, Texas, USA.


Elizabeth Roesler in the field

Roesler EL. 2016. Development of Pecos assiminea Assiminea pecos monitoring methods and effects of habitat restoration at Bitter Lake National Wildlife Refuge. M.S. thesis. Texas Tech University, Lubbock, Texas, USA.


MAtt Acre with his jet-powered kayaks

Acre MR. 2015. Do river-reservoir interface habitats serve as surrogate nursery habitats for floodplain-dependent riverine fishes? M.S. thesis. Texas Tech University, Lubbock, Texas, USA.


Qingman Chen in the field

Chen Q. 2014. Reproductive behavior and mating system of Spotted Seatrout Cynoscion nebulosus. M.S. thesis. Texas Tech University, Lubbock, Texas, USA.


Jillian Groeschel on the South Llano River

Groeschel JR. 2013. Evaluations of growth and habitat use by Guadalupe Bass at a riverscape scale in the South Llano River, Texas. M.S. thesis. Texas Tech University, Lubbock, Texas, USA.


Lovisa Guðmundsdóttir and her cod eggs

Guðmundsdóttir LÓ. 2013. Variation in egg buoyancy and composition associated with intra-stock diversity of Icelandic cod. M.S. thesis. University of Iceland, Reykjavik, Iceland.


Julia Mueller and her shiners

Mueller JS. 2013. Effects of temperature, salinity, and suspended solids on the early life history stages of Arkansas River Shiner. M.S. thesis. Texas Tech University, Lubbock, Texas, USA.


Lisa Libungan and her larval cod

Libungan LA. 2010. Local adaptation and variation in life history reaction norms within the Icelandic cod stock. M.S. thesis. University of Iceland, Reykjavik, Iceland.


Tim Grabowski and Robust Redhorse

Grabowski TB. 2006. Reproductive ecology and seasonal migrations of Robust Redhorse (Moxostoma robustum) in the Savannah River, South Carolina and Georgia. Ph.D. dissertation. Clemson University. Clemson, South Carolina, USA.


Molly Miller

Grabowski TB. 2002. Temporal and spatial variability of blenny (Perciformes: Labrisomidae and Blenniidae) assemblages on Texas jetties. M.S. Thesis. Texas A&M University. College Station, Texas, USA.