Variable diet plasticity in Eurasian perch (Perca fluviatilis): Current versus seasonal food uptake

Diet plasticity is often studied in Eurasian perch (Perca fluviatilis), a species commonly described as having generalist populations composed of specialised individuals. Perch diet was examined using gut content analysis (GCA) and stable isotope analysis (SIA), and individual specialisation was calculated in two study lakes within 2 years. Mostly only one diet category was present in the perch stomach, with more variation in the diet in the Most lake compared to the Milada lake between 2013 and 2014. The calculated degree of individual specialisation indicated higher specialisation in the Most lake. Interestingly, despite the different or almost uniform diet composition between the years, the total niche width (based on SIA) of the population remained similar in both lakes. This suggests that the overall variation in the sources utilised by the entire population remained consistent between the years. GCA mostly indicated zooplankton as the prevailing food source, whereas SIA indicated significant utilisation of YOY fish earlier that year, an information that was completely missed by the GCA of fish caught in September. The differences between GCA and SIA results could be attributed to the different time intervals reflected by the methods, but possibly to the conversion of the diet into the body tissues that is reflected by SIA and may depend on the diet's nutritional values rather than the proportion of different prey consumed.