Salmonid Conservation in an Invaded Lake: Changing Outcomes of Predator Removal with Introduction of Nonnative Prey

Abstract

AbstractCulling of predators is a traditional tool in inland fisheries management. There is a long history of removing Northern Pike Esox lucius from certain Irish lakes in an attempt to enhance Brown Trout Salmo trutta fisheries. In recent decades, some of these systems have experienced ongoing warming, eutrophication, and the establishment of large populations of a nonnative cyprinid, the Roach Rutilus rutilus. Availability of this abundant new fish prey resource may have modified predator–prey interactions between Northern Pike and Brown Trout and consequently the potential efficacy of Northern Pike removal as a trout fisheries management tool. Statistical analysis of long‐term fish survey data (1978–2015) and Northern Pike removal data (1980–2014) from Lough Sheelin, Ireland, indicated that the Northern Pike diet (stomach contents) changed significantly after the Roach invasion. There was a strong reduction in the proportion of Northern Pike stomachs containing trout, and the incidence of Roach in Northern Pike stomachs increased. Northern Pike removal was found to have a generally positive effect on abundance of Brown Trout in the following year, but this positive effect became neutral or negative at intermediate and peak levels of Roach abundance (>33rd percentile of annual survey CPUE). Brown Trout abundance also declined in years of high chlorophyll‐a concentration. Removal of top predators may have unanticipated effects on target fish stocks in systems with multiple anthropogenic pressures.