Feeding habits and trophic ecology of the dominant ray species of Almejas Bay (Baja California Sur, Mexico)
Feeding habits and trophic ecology of the dominant ray species of Almejas Bay (Baja California Sur, Mexico) | |
Joseph J. Bizzarro |
Elasmobranchs (sharks, skates, and rays) have played important roles in marine ecosystems throughout their history, often occupying the highest trophic levels as apex predators. Despite the central importance of feeding relationships in understanding food web dynamics and energy transfer, little is known about the feeding biology of the extant species of elasmobranchs. Additionally, due to recent global declines in traditional teleostean fish stocks, elasmobranch fisheries are expanding in size and importance. Elasmobranchs have life history characteristics (slow growth, late ages at maturity, low fecundity) which make them especially vulnerable to overfishing. Fisheries targeting elasmobranchs can therefore cause severe and often rapid depletion of stocks, endangering their resource and ecological value over broad regions.
Mexican elasmobranch fisheries have expanded to become the largest in the Americas yielding almost 30,000 tons/year. Almejas Bay (111°40'N, 24°30'W), a lagoon system located in southeastern Magdelena Bay (Baja California Sur, Mexico), is home to an established ray fishery. Rays are upper-level carnivores which prey primarily on benthic invertebrates and fishes. In Almejas Bay, rays are abundant and most likely occupy elevated trophic positions. Therefore, their removal may have profound and unpredictable cascading effects on the benthic community structure.
Through an analysis of gut contents from fishery derived specimens, the feeding habits and relative importance of prey items will be determined for the four most abundant species in the fishery: the shovelnose guitarfish (Rhinobatos productus), California butterfly ray (Gymnura marmorata), diamond stingray (Dasyatis brevis), and lesser electric ray (Narcine brasiliensis). The infaunal and epifaunal benthic invertebrate community will be characterized to provide baseline information against which future changes can be assessed and to determine electivity of rays for the suite of available prey items. Trophic ecology will be determined based upon the dietary composition of prey items utilized by each species and their proposed role in the Almejas Bay food web. Specific aspects of feeding ecology (temporal feeding periodicity, ontogenetic differences in feeding, dietary overlap between species, spatial differences in feeding ecology due to habitat variability) will be assessed. This study addresses the need for quantitative information on the feeding ecology of rays in Baja California. To date, no published data of this kind exist. Determination of feeding habits for the dominant species of rays in Almejas Bay will elucidate the ecological role of these species and provide a better understanding of benthic community structure and stability. If ray abundance declines in this area due to overfishing, resulting effects on the benthic community can be predicted from the results of this study.