Dissertation: Feeding Ecology of Skates and Rays in Delaware1-1-2013
Feeding Ecology of Skates and Rays in Delaware and Narragansett Bays
John Andrew Szczepanski Jr.
Skates and rays are an integral part of the trophic structure of many estuarine ecosystems. However, there are many aspects of the fisheries biology of these species that require further exploration. For example, few comparisons have been done of the feeding habits of sympatric species that potentially overlap in resource usage, diets over time, or food preferences between fish populations from different regions. Most of what is known of batoid elasmobranch (skate and ray) diets on the Western Atlantic coast is a compilation of data from the entire continental shelf with no distinction of diets for populations within different estuaries that vary in abiotic characteristics and trophic structure. My research objectives were to: (1) quantitatively characterize the diets of the major batoid elasmobranchs (skates and rays) in Delaware Bay and in Narragansett Bay, (2) describe seasonal variation in diet, if any, (3) examine diet overlap and food resource partitioning among species within each ecosystem, and (4) compare diets of selected species between the geographic locations (specifically populations of Little, Winter, and Clearnose skates; Leucoraja erinacea, L. ocellata, and Raja eglanteria, respectively,). Through gut content analysis, calculation of trophic level and overlap indices, and multivariate statistical techniques such as non-metric multidimensional scaling and ANOSIM, diets of 3 different batoids were characterized; 2 from Delaware Bay and 1 from Narragansett Bay. The diets of the bullnose ray Myliobatis freminvillii, the Clearnose skate Raja eglanteria, and the little skate Leucoraja erinacea were also evaluated for ontogenetic, spatial, and temporal differences within their respective species. Myliobatis freminvillii was considered a moderate gastropod specialist with shifts in diet over ontogeny. Raja eglanteria was characterized as a generalist with preferences for benthic crustaceans, exhibiting ontogentic differences between juveniles and adults, and strong dietary differences between sexes. Leucoraja erinacea was determined to be a broad-scale generalist preferring amphipods and sand shrimp and was shown to feed based on prey availability since there were temporal and spatial differences in diet that corresponded to variations in prey abundance. These data were used to contribute to estuary-specific community analyses of batoid trophic relationships in Delaware and Narragansett Bays and a comparison of the trophic dynamics between those two communities. The skate species of Narragasett Bay exhibited significantly different diets, but did not show ontogenetic differences when compared together. The skates and rays of Delaware Bay showed differences in diet by species and by size within species. It is proposed that a higher level of partitioning exists in Delaware Bay since there are more batoid species to compete for resources, but this would only be the case if resources were limiting and data were not collected that could confirm this. Since all the organisms studied were benthic secondary consumers based on available data, the diets of the batoids in each estuary were found not to be significantly different. Any observed differences were not due to the specific prey species that were found in the diet but to the proportions of the various prey that were consumed.
Open Access Dissertations. Paper 27.