Feeding specialists on fatty acid-rich prey have higher gonad weights: Pay-off in Baltic perch?

Individual specialization is a common phenomenon throughout the animal kingdom. Many studies have identified intraspecific competition as one of the main drivers for individual feeding specialization. These studies have mainly considered the quantity of resources, commonly overlooking qualitative aspects of the diet. For example, highly unsaturated fatty acids of the omega-3 class (omega-3 HUFAs) are related to optimal health and growth in consumers. However, little is known on direct fitness consequences for consumers of natural populations that specialize on high-quality resources, such as those rich in omega-3 HUFAs. Despite being such an important qualitative aspect of the diet, it is still unknown whether natural populations show among-individual variation in their choice on prey items that are either rich or poor in HUFAs, and how it affects individual performances. In this study, we investigated whether there is individual feeding specialization and whether it is related to fitness benefits, in a population of perch (Perca fluviatilis) in the Baltic Sea. The contribution of pelagic planktivorous fish to the diet varied from 17% to 61% among perch individuals, as depicted by stable isotope mixing models. This variation in diet was also qualitative, as the omega-3 HUFA content differed among prey types. Specialization on the high-quality resource pelagic planktivorous fish was associated with the proportions of omega-3 HUFA in the individuals' muscles and individuals among those with the highest proportions of omega-3 HUFAs had the greatest relative gonad weight (gonadosomatic index, GSI), a proxy for reproductive investment. Thus, our results highlight the function of food quality for individual specialization and its potential to have direct fitness benefits, playing a major role in shaping ecological interactions.