Evaluating The Efficacy Of Predator Management Concurrent With Changes In Prey Communities

Abstract

Double-crested cormorants (Phalacrocorax auritus; hereafter cormorants) are piscivorous waterbirds considered top predators in aquatic systems. In order to accurately evaluate effects of cormorants on prey fish, several pieces of information which overlap in space and time are needed: (1) cormorant abundance and consumption in the study system; (2) cormorant diet composition; and (3) accurate information on prey abundance in the system. This research expands the knowledge of bird-fish interactions and predator-prey, dynamics and evaluates the effects of predator removal and changing prey communities. I examine patterns in cormorant consumption and how consumption was influenced by changing prey communities in two large lakes, Lake Champlain and Oneida Lake, New York. I also examine percid population dynamics in response to the removal of cormorants from Oneida Lake. Cormorants were found to significantly affect walleye (Sander vitreus) and yellow perch (Perca flavescens) subadult survival in Oneida Lake and were subsequently managed on the lake. This research evaluates the efficacy of cormorant management on Oneida Lake by utilizing the long-term data set available for cormorants and the fish community. I document spatial and temporal variation in cormorant diets in Lake Champlain and Oneida Lake associated with changes in prey fish communities, specifically alewife (Alosa psuedoharengus) establishment in Lake Champlain and gizzard shad (Dorosoma cepedianum) resurgence in Oneida Lake. Also, cormorants were found to select for and switch to emerald shiner (Notropis atherinoides) and gizzard shad when present in Oneida Lake. Gizzard shad were found to buffer yellow perch from cormorant consumption. Cormorant management resulted in decreased percid subadult; however, it should be noted that the prey fish community and cormorant diets shifted at this time as well. Only the walleye adult population reached the expected abundance. Yellow perch failed to reach predicted adult abundance, likely due to low age-1 abundances, possibly a result of walleye, largemouth bass, and smallmouth bass predation on age-0 and age-1 yellow perch. Cormorant management implemented to protect sportfishes should include fish abundance and cormorant diet monitoring to influence management actions and adapt them if impacts to sportfish change.