Inland fisheries can be diverse, local and highly seasonal. This complexity creates challenges for monitoring, and consequently, many inland fish stocks have few data and cannot be assessed using methods typically applied to industrial marine fisheries. In such situations, there may be a role for methods recently developed for assessment of data‐poor fish stocks. Herein, three established data‐poor assessment tools from marine systems are demonstrated to highlight their value to inland fisheries management. A case study application uses archived length, catch and catch‐per‐unit‐effort data to characterise the ecological status of an important recreational brown trout stock in an Irish lake. This case study is of specific use to management of freshwater sport fisheries, but the broader purpose of the paper was to provide a crossover between marine and inland fisheries science, and to highlight accessible data‐poor assessment approaches that may be applicable in diverse inland systems.
Culling of predators is a traditional tool in inland fisheriesmanagement. 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.
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