Mesopelagic boundary community

INTRODUCTION

	The mesopelagic boundary community is composed of small (less than 4 inch long) fishes, shrimps, and squids, the most abundant of which are the myctophids, or lanternfishes. Mesopelagic - midwater, animals don't reach the seafloor or the surface Boundary - island-associated, the species found near the islands are different from those found in the open ocean
	This community serves as an important food resource for many animals Spinner dolphins Bottomfish Tunas Billfish

METHODOLOGY

ECHOSOUNDER

TARGET STRENGTH

CALORIES

SURVEY SITES

ECHOSOUNDER

The animals in the mesopelagic boundary community are good at avoiding trawling nets, the traditional way to survey them. This makes it difficult to survey their abundance. More importantly, trawls can only look at the distribution of animals at a gross scale, not the scale that is likely most important to predators. Acoustic surveying techniques offer several distinct advantages:

Can be conducted close to shore, in shallow water

Rapid surveying of large spatial areas

High-spatial resolution

Survey entire water column down to 156 m at once

We modified a 200 kHz commercial echosounder to digitize and save the returning echoes directly into a laptop computer. This allows us to have quantitative data, not just the images seen on the screen. We used this system to conduct surveys from vessels such as the NOAA ship, Townsend Cromwell .

In order to use this data, however, we need to know something about the acoustic properties of the animals in the mesopelagic boundary community, in particular, their target strength .

TARGET STRENGTH

Knowing the target strength of animals allows us to calculate the density of animals in the boundary community from our survey data. This allows us to look at the distribution of animals as a function of location and depth.

Target strength - the amount of acoustic energy reflected by a target, in this case, a fish, shrimp, or squid

To measure the animal's target strength, we captured live animals in a midwater trawl (left) and kept them alive aboard the ship. We then anesthetized the animal and mounted it on a specially designed rack (right) inside a sewater tank so we could control the animal's position, similar to the techniques used in the bottomfish project .

For more information, check out the paper in JASA or its abstract .

CALORIES

While knowledge of the density of animals in the boundary community helps us understand their dynamics, the amount of available energy, or biomass, of the boundary community is more important to its predators. I related the acoustic scattering strength, or target strength , or animals from the boundary community to their caloric content. Interestingly, while many acoustic surveys attempt to convert acoustic energy to biomass, the relationship has not been previously tested. The results will allow us to convert our acoustic data into the units that are most relevant to predators of the boundary community, calories.

For more information, see a lay-language version of this paper or its abstract .

SURVEY SITES

So far, we've used the modified echosounder system to survey the coasts of three of the Hawaiian Islands. While we have found some differences in the density and distribution of animals between the islands, the general patterns in the mesopelagic boundary community are conserved between islands.

Oahu sites	Lanai sites	Big Island sites

MIGRATIONS

The mesopelagic boundary community undergoes diel, vertical migrations
- 400-700 m deep during the day
- Near-surface to 400 m deep at night

My work has now shown that mesopelagic boundary community

undergoes diel, horizontal migrations as well
Offshore during the day -
Within 1/2 km of shore at night -

To observe the migration patterns of the boundary layer with more temporal resolution than is possible with a single-platform survey, I utilized a series of bottom-mounted, sonar moorings. These moorings were used to profile the mesopelagic animals in the water column in five locations up the slope of the island in five locations at the same time.

Mooring electronics

The moorings I constructed and calibrated are comprised of an echosounder board, a microcontroller, and compact flash memory inside a pressure housing to which the sonar transducer is mounted.

See more information about horizontal migration in the MEPS paper or its abstract or the abstract on the mooring work.

PATCHES

The boundary community is not evenly distributed around the islands. This heterogeneity, called patchiness is evident off all the islands surveyed. Interestingly, the boundary community off the coasts of the Big Island and parts of Oahu is distributed in distinct, relatively low prey density patches. The boundary community off Lanai and other areas of Oahu is distributed in relatively continuous, high density layers. This is shown in the figures below, where the brightness of the color purple is proportional to the density of mesopelagic animals. These differences in prey distribution are likely to affect the tactics predators use to forage on the boundary community.

See abstract

SOUTH OAHU and HAWAII	NORTH OAHU and LANAI

FUTURE WORK

My plans for future work on the boundary community include:

Using the sonar moorings to detail lunar effects on the boundary communities migrations

Video recording from a drop camera

Simultaneous high-resolution sonar and video recordings from the HURL ROV (remotely operated vehicle)