Species-specific drivers explain fish feeding and individual niche variation.

The trophic ecology of animals often results from complex interactions between environmental and biological drivers. Yet, studies that explore trophic ecology across multiple fish species while considering a wide range of environmental variables remain limited. In this study, we analysed diet and niche variation in fish communities across five protected areas in the northern Iberian Peninsula, examining their responses along environmental and biological gradients. Our multiple regression analyses showed that both environmental and biological drivers significantly influence fish feeding behaviour and individual specialisation, but these effects are species-specific. We propose that the primary mechanisms shaping fish feeding strategies, whether benthic foraging or surface-drift feeding, are determined by intrinsic factors such as species-specific feeding behaviours and ontogenetic stage, along with benthic macroinvertebrate density and fish density. Specifically, benthic macroinvertebrate density positively affected individual niche specialisation in brown trout (Salmo trutta) and Pyrenean stone loach (Barbatula quignardi), whereas it negatively impacted the Pyrenean minnow (Phoxinus bigerri). An increase in total fish density, measured as the number of fish per square meter, negatively influenced individual niche specialisation of brown trout. However, brown trout density had a positive effect on individual niche specialisation of Pyrenean stone loach and Pyrenean minnow. The findings have direct implications for conservation and restoration efforts, highlighting the importance of increasing habitat heterogeneity to meet species-specific habitat needs and promoting benthic macroinvertebrate production to support fish feeding requirements.