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Relations between reservoir trophic state and gizzard shad population characteristics in Alabama reservoirs



Relations between reservoir trophic state and gizzard shad population characteristics in Alabama reservoirs



North American Journal of Fisheries Management 16(4): 888-895



We compared relative abundance, condition, length at age, and the age and size structure of gizzard shad Dorosoma cepedianum between eutrophic ( gtoreq 8 mg/m-3 chlorophyll a) and oligo-mesotrophic ( ltoreq 7 mg/m-3 chlorophyll a) Alabama reservoirs. We quantified the size structure of gizzard shad in each reservoir by calculating the index of vulnerability (IOV), defined as the percentage of the electrofishing catch that was 203 mm or less in total length. Fish were collected with experimental gill nets and electrofishing, and although gill nets tended to catch larger and older fish, the gears provided similar trends in trophic state response. Gizzard shad in eutrophic reservoirs were characterized by high relative abundance, slow growth after age 1, a low percentage of fish age 5 and older (7%), and poor condition. This resulted in a high IOV (70) for gizzard shad in eutrophic reservoirs. Conversely, in oligo-mesotrophic reservoirs, gizzard shad abundance was 80-86% lower than in eutrophic reservoirs. However, growth rates and condition were higher; 26% of the fish were age 5 and older; and a smaller proportion of the gizzard shad were vulnerable to predation (IOV = 26). These results suggested that differences in growth and condition were probably due to density-dependent mechanisms. Growth rates and the IOV responded to a continuum of algal biomass concentrations (chlorophyll-a range, 2-27 mg/m-3). Length at age for age-1 to age-6 fish were inversely correlated to chlorophyll-a levels (r = -0.49 to -0.90). The IOV increased, but reached asymptotes, with higher gizzard shad abundance and chlorophyll-a levels. Consequently, the potential use of gizzard shad as a forage fish was much greater in eutrophic reservoirs; not only were these fish more abundant, but populations in eutrophic systems were vulnerable to predation for a longer period of time and were skewed toward smaller fish.

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Accession: 009327454

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DOI: 10.1577/1548-8675(1996)016<0888:rbrtsa>2.3.co;2


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