Feeding Ecology of Coexisting Larvae of American Shad and Blueback Herring in the Connecticut River

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Abstract
Food‐habit, niche‐breadth, and diet‐overlap analyses were conducted for larvae of American shad Alosa sapidissima and blueback herring Alosa aestivalis collected from the Connecticut River from June 3 to July 15, 1981. River zooplankton densities (numbers˙m−3 were used to assess the influences of prey availability on niche‐breadth and diet‐overlap indices for the species. Niche‐breadth values were consistently higher for blueback herring than for American shad larvae. American shad fed mostly on the less‐abundant crustaceans and immature insects, whereas blueback herring larvae usually utilized the numerically dominant rotifers. The cladoceran Bosmina sp. was utilized extensively by American shad of all sizes and by 13–16‐mm blueback herring larvae, and was the only prey for which interspecific competition between the larval predators might occur. The mean mouth gape of American shad larvae was always larger than that of blueback herring larvae at a given length, and American shad larvae of all sizes generally selected the larger cyclopoid copepodites and chironomid larvae. The diet‐overlap indices varied over time, but were generally higher among length groups within species than between species, suggesting that intraspecific competition for food may be more severe than interspecific competition. Received June 11, 1982 Accepted May 9, 1983