Population niche breadth and individual trophic specialisation of fish along a climate-productivity gradient

A mechanistic understanding of how environmental change affects trophic ecology of fish at the individual and population level remains elusive. To address this, we conducted a space-for-time approach incorporating environmental gradients (temperature, precipitation and nutrients), lake morphometry (visibility, depth and area), fish communities (richness, competition and predation), prey availability (richness and density) and feeding (population niche breadth and individual trophic specialisation) for 15 native fish taxa belonging to different thermal guilds from 35 subarctic lakes along a marked climate-productivity gradient corresponding to future climate change predictions. We revealed significant and contrasting responses from two generalist species that are abundant and widely distributed in the region. The cold-water adapted European whitefish (Coregonus lavaretus) reduced individual specialisation in warmer and more productive lakes. Conversely, the cool-water adapted Eurasian perch (Perca fluviatilis) showed increased levels of individual specialism along climate-productivity gradient. Although whitefish and perch differed in the way they consumed prey along the climate-productivity gradient, they both switched from consumption of zooplankton in cooler, less productive lakes, to macrozoobenthos in warmer, more productive lakes. Species with specialist benthic or pelagic feeding did not show significant changes in trophic ecology along the gradient. We conclude that generalist consumers, such as warmer adapted perch, have clear advantages over colder and clear-water specialised species or morphs through their capacity to undergo reciprocal benthic–pelagic switches in feeding associated with environmental change. The capacity to show trophic flexibility in warmer and more productive lakes is likely a key trait for species dominance in future communities of high latitudes under climate change.

Predation of European catfish on anadromous fish species in an anthropised area

Increases in both food availability and intraspecific competition downstream of dams have the potential to trigger the emergence of trophic specialisation among fish predators, with considerable implications for prey species populations. The aim of this study was to assess whether the presence of a dam located on the River Garonne (France) affected the dietary preference of the European catfish Silurus glanis towards anadromous prey. Stable isotope analysis showed that the contribution of marine-derived carbon in the diet of the European catfish was substantial (on average 53% of the diet) and was similar between individuals caught downstream of the dam and those caught from the free running part of the river. In contrast with previous studies, a significant relationship between the size of European catfish individuals and their consumption of marine-derived nutrients was found in this study. Anadromous fish populations are in decline; therefore, this significant predation is concerning.