Life-History and Ecology of the Brown Stingray
Dasyatis lata
Closey related to sharks, rays are a unique and diverse group of dorso-ventrally flattened elasmobranch fishes. Like sharks, many rays tend to exhibit life-history characteristics such as slow growth, late age at maturity and low reproductive output which make them succeptable to overfishing. However, there is relatively little known about the general biology and ecology of rays. We do know that rays can be ecologically important benthic predators in marine ecosystems. Because of this and in light of increasing levels of exploitation, it is becoming especially important to evaluate their roles in the ecosystems in which they occur in order to understand the consequences of their large scale removal.
Fishing for stingrays in Kane‘ohe Bay
Stingrays are a group of rays generally characterized by one or more venomous barbs located towards the base of their tail. They are found world-wide in tropical and subtropical seas. Although mainly found in marine systems, many species can occupy a wide range of salinities and some, such as the river stingrays, are found only in freshwater. Dasyatidae is the largest family of stingrays including 5 genera and over 60 species and is the dominant ray family on the continental shelves of tropical and subtropical regions of the world.
Night fishing in Kane‘ohe Bay
The brown stingray is the only elasmobranch endemic to the Hawaiian Islands and is one of 19 species of Dasyatis (the largest genus of Dasyatidae with 35 species) occuring in the North Pacific. Common in in-shore waters, it is also found at depths greater than 700 ft. Research at the Hawai‘i Institute of Marine Biology Shark Lab focuses on the ecology and life-history of the brown stingray in Kane‘ohe Bay, O‘ahu. We are primarily interested in the diet, age and growth, bioenergetics, nursery use and long-term movement patterns of these stingrays. The goal is to gain a better understanding of their habitat use and ecological role as benthic predators in near-shore marine ecosystems.
130 cm wide female brown stingray
Longlines are used to capture stingrays in Kane‘ohe Bay. Stingrays are predominantly caught in deep muddy portions of the bay but are occasionally captured in shallow water. Little is known about the biology of this species and what is known is based on the observations of only a few individuals. The smallest stingray captured to date was 35 cm (13 inches) and the largest was 130 cm (51 inches) weighing 56 kg (124 lbs). Although a wide range of sizes have been captured, the majority are immature stingrays between 50 and 70 cm, suggesting Kane‘ohe Bay is a nursery habitat for this species.
Brown stingray buried in sand
Analysis of stomach contents indicates they feed primarily on crustaceans, with teleosts and polycheate worms making smaller contributions to the diet. Dietary comparisons with the scalloped hammerhead shark, Sphyrna lewini, which also utilize Kane‘ohe Bay as nursery habitat suggest resource partitioning between these two species. Ongoing research includes determining the age and growth rates of brown stingrays through vertebral analysis, estimating their population size and movement patterns in the bay through tagging studies, evaluating their use of the bay as a nursery habitat through stable isotope analyses and estimating their metabolic rates in order to evaluate their role in the flow of energy in the Kane‘ohe Bay ecosystem.