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SEASONAL CYCLES, DISTRIBUTION, AND BIOMASS OF CRUSTACEAN ZOOPLANKTON, AND FEEDING AND GROWTH OF YOUNG AMERICAN SHAD (ALOSA SAPIDISSIMA) IN THE HOLYOKE POOL, CONNECTICUT RIVER
Abstract
Interactions between young American shad (Alosa sapidissima) and their food (crustacean zooplankton and aquatic insect drift), and seasonal cycles and distribution of crustacean zooplankton were studied in the Holyoke Pool of the Connecticut River, Massachusetts from June 1977 through 1979. Aquatic insect drift was highest during darkness and in the mid-channel where flow was greatest. Erratic flow can increase drifting of some benthic invertebrates and enhance potential food available to fish. Automatic zooplankton samplers sampled four times each day throughout the study and were effective in capturing most species and size classes of crustacean zooplankton equally as well as a commonly used towed-net plankton sampler. Thirty-six species of Cladocera and 20 species of Copepoda were encountered; Bosmina longirostris and Chydorus sphaericus were the most abundant zooplankton. Total density and size and species composition of zooplankton entering the Holypoke Pool were similar to that which was discharged from the Pool, except during early July through September when losses of up to 1,000 kg dry weight of zooplankton occurred within the Pool. During summer, zooplankton was most abundant in the mid-channel where flow was greatest, and density decreased rapidly as zooplankton flowed downstream. The size distribution of zooplankton in the upper end of the Pool was similar in the near-shore and mid-channel, except during mid-July through September when the mid-channel contained higher frequencies of large organisms. The size distribution shifted toward the smallest size classes in a downstream manner during summer. The species and size composition of the zooplankton assemblage which entered the Holyoke Pool was typical of communities subjected to low levels of vertebrate predation, while that which was discharged from the Pool was typical of zooplankton communities subjected to intense predation by fish. Intrinsic factors of mortality, turbulence, sedimentation, filtration through vegetation, and predation by invertebrates and resident vertebrates seemed not to affect zooplankton abundance as strongly as predation by juvenile Alosa spp. Shad in the Holyoke Pool were progressively smaller downstream, except during the larval period. Mean lengths of shad at emigration were the smallest ever reported for the Pool and among the smallest reported elsewhere. Shad occasionally fed throughout the 24-hour period, although peak feeding generally occurred at dusk. Stomachs emptied within 8 to 10 hours. Shad selected an optimal diet based on food items available in the water column. Fish in upstream areas were able to feed in a more energetically profitable manner than those downstream. The abundance, size distribution, and spatial distribution of zooplankton was rapidly altered by feeding shad. Fish in upstream areas fed heavily on zooplankton of all sizes, but generally selected larger individuals; insects were also eaten. In the middle of the Pool, fish ate primarily insects but also utilized rare large zooplankton and the more abundant small zooplankton. Fish in the lower section of the Holyoke Pool utilized the remaining small zooplankton and some insects. The number of food items in stomachs progressively decreased downstream. Differential growth of shad within the Holyoke Pool was food based. The mean length of juveniles at emigration was inversely related to the number of adult shad lifted into the Pool. American shad may over-exploit their rearing grounds. It appears reproductively advantageous for Connecticut River shad to spawn as far upstream as possible. Wide variations in the size of American shad stocks may result partially from the relation between juvenile density and foods available in the nursery areas.
Subject Area
Freshwater ecology
Recommended Citation
ROSEN, RUDOLPH ALBERT, "SEASONAL CYCLES, DISTRIBUTION, AND BIOMASS OF CRUSTACEAN ZOOPLANKTON, AND FEEDING AND GROWTH OF YOUNG AMERICAN SHAD (ALOSA SAPIDISSIMA) IN THE HOLYOKE POOL, CONNECTICUT RIVER" (1981). Doctoral Dissertations Available from Proquest. AAI8118040.
https://scholarworks.umass.edu/dissertations/AAI8118040