Ecological Changes Following an Alewife Introduction in an Oligotrophic Reservoir: A Case History

Alewife (Alosa pseudoharengus) was introduced into oligotrophic Mayo Reservoir, North Carolina, during 1992 or 1993. The species established a self-sustaining population and increased from <1% of total fish biomass in 1993 to 31% in 2000. Size-selective planktivory by the species, a well-documented phenomenon in other alewife introductions, was implicated in observed changes in the reservoir zooplankton community. Large- and mid-sized zooplankton (> 0.7 mm) (Onchyodiaptomus birgei, calanoid copepodites, Daphnia spp., Diaphanosoma brachyurum, and Holopedium gibberum) decreased in density and biomass within a year after the alewife introduction. Total cladoceran densities and biomass and total copepod biomass also exhibited the same pattern. Conversely, smaller or more evasive zooplankton (e.g., Bosmina longirostris, Mesocyclops edax, Tropocyclops prasinus, cyclopoid copepodites, and rotifers) either increased or did not change in abundance following introduction. Subsequent linkages between changes in zooplankton composition and key water quality variables were not apparent in this study. Chlorophyll a concentrations remained low, indicative of oligotrophy, and there were no apparent long-term blooms or detectable changes in water clarity after the alewife introduction. Total nitrogen and phosphorus concentrations were highly variable; only total nitrogen exhibited an increasing trend after the alewife introduction. Changes in the fish community were minimal, which likely reflected the slow buildup of the alewife population during the study period. Alewife supplanted gizzard shad as the dominant clupeid within four to seven years after introduction. Chain pickerel and redear sunfish abundance increased after alewife introduction, which corresponded with increased aquatic vegetation in the reservoir littoral zone. Decreases in green sunfish and pumpkinseed were noted, but most likely were related to littoral zone competitive interactions with bluegill, warmouth, and redear sunfish populations or predation from increased chain pickerel abundance. Early life stages of the dominant centrarchids—bluegill, warmouth, largemouth bass, and crappie—may have spatially segregated zooplankton and other invertebrate prey resources and utilized those resources within nearshore, vegetated zones to minimize interspecific competition with alewife.

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