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

Abstract

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.

Managing Freshwater Fish in a Changing Climate: Resist, Accept, or Direct

“…In Ireland, there is a long history of removing Northern Pike Esox lucius because they prey on Brown Trout S. trutta , the preferred sportfish (Shephard et al 2019). But warming and eutrophication have allowed a nonnative species, Roach Rutilus rutilus , to proliferate to the detriment of Brown Trout.…”

Evaluating management options for two fisheries that conflict through predator–prey interactions of target species

“…In many of the designated lakes roach are, from an ecological perspective, as important a component of the fish community as trout and pike and may influence the interactions and population dynamics of pike and trout (Shephard et al, 2018). Therefore, a more realistic description of the ecosystem within the current modelling framework Our results also show that angling can be an important pressure on trout populations in large Irish lakes.…”

“…In all cases, the model indicates that a greater biomass of alternative prey (in the same size range as trout) diminishes the predation mortality on trout, which modifies the potential utility of pike removal as a trout conservation tool. This effect has been observed empirically in one of the designated Irish trout lakes (Lough Sheelin), where non-native roach have become established (Shephard et al, 2018) and now constitute an important prey species for pike (Pedreschi et al, 2015). Model representation of ecosystem function is such that any reductions in predation pressure on trout lead to enhanced projected steady-state brown trout biomass.…”

“…Many of the designated lakes have shown strong environmental changes in recent decades (Champ, 1998), which may modify the impacts of fish predation (Haught and von Hippel, 2011). Environmental change has facilitated the establishment of non-native roach (Rutilus rutilus) as an abundant alternative prey species for pike (Pedreschi et al, 2015); availability of roach has been shown to change the relationship between pike removal rate and subsequent trout abundance (Shephard et al, 2018). These factors demand formal assessment of the performance of pike removal and other interventions.…”

Fisheries Volume 47 Number 6 June 2022

“…In Ireland, there is a long history of removing Northern Pike Esox lucius because they prey on Brown Trout S. trutta , the preferred sportfi sh (Shephard et al 2019 ). But warming and eutrophication have allowed a nonnative species, Roach Rutilus rutilus , to proliferate to the detriment of Brown Trout.…”