Foodweb interactions of the endangered June sucker.

KEYWORDS: June sucker; gizzard shad; Utah; Utah Lake; competition; predation; exotic species

ABSTRACT

The June sucker Chasmistes liorus is an endangered fish endemic to Utah Lake, Utah. Increased human water usage and habitat disturbance have played a role in its decline; however, evidence suggests that the introduction of white bass Morone chrysops is also of major concern. In addition, other exotic species such as the gizzard shad Dorosoma cepedianum are currently being considered for introduction into this already devastated lake system. For the past two years, we have been conducting experiments to determine the feeding ecology, reproductive biology and general foodweb interactions of June sucker. Feeding experiments conducted in limnocorrals indicated that June suckers show very strong preferences for Ceriodaphnia and Bosmina two of the smallest daphnids available. While direct competitive interactions with gizzard shad were not strong, the potential exists for shad to alter the foodweb by changing phytoplankton species composition and size structure and altering the size structure of the zooplankton community. If the zooplankton community becomes dominated by large-bodied daphnids, it is unclear whether June suckers could effectively forage. In addition to the competition experiments, predation experiments were performed to test the hypothesis that white bass predation has a significant impact on sucker recruitment. We also wanted to predict how the possible introduction of gizzard shad may affect the predator/prey dynamics by considering prey switching possibilities. These experiments were conducted in 800 gallon tanks and suggest that predation on juvenile suckers and gizzard shad by white bass was limited only by gape size of the predator. Using a "gape-limitation hypothesis," a model of relative predation windows of gizzard shad and June sucker to white bass was formulated. Selectivity experiments were also carried out, showing that white bass fed preferentially on gizzard shad (Chesson's alpha 0.677) over June sucker (Chesson's alpha 0.322). We sue the results of all of these experiments to hypothesize a variety of possible scenarios in Utah Lake should additional fish introductions be carried out.

RESUMEN

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