Diet of invasive pikeperch Sander lucioperca: developing non-destructive tissue sampling for stable isotope analysis with comparisons to stomach contents analysis

Impact assessments of invasive piscivorous fishes usually rely on dietary analyses to quantify their predation pressure on prey communities. Stomach contents analysis (SCA), typically a destructive sampling method, is frequently used for this. However, many invasive piscivores are exploited by catch-and-release sport angling, with destructive sampling often not feasible. Stable isotope analysis (SIA) provides an alternative dietary analysis tool to SCA, with use of fin tissue, scales and/or epidermal mucus potentially enabling its non-destructive application. Here, the diet of a population of pikeperch Sander lucioperca, an invasive sport fish to Great Britain, was investigated by applying SIA to a range of tissues. Testing SI data of dorsal muscle (destructive sampling) versus fin, scale and mucus (non-destructive sampling) revealed highly significant relationships, indicating that the tissues collected non-destructively can be reliably applied to pikeperch diet assessments. Application of these SI data to Bayesian mixing models predicted that as S. lucioperca length increased, their diet shifted from macro-invertebrates to fish. Although similar ontogenetic patterns were evident in SCA, this was inhibited by 54% of fish having empty stomachs. Nevertheless, SCA revealed that as S. lucioperca length increased, their prey size significantly increased. However, the prey:predator length ratios ranged between 0.08 and 0.38, indicating most prey were relatively small. These results suggest that when non-destructive sampling is required for dietary analyses of sport fishes, SIA can be applied using fin, scales and/ or mucus. However, where destructive sampling has been completed, SCA provides complementary dietary insights, especially in relation to prey size.

Using faecal samples in lizard dietary studies

While the use of faecal pellets is widely accepted as a primary methodological source of data for dietary studies, a recent paper advocated for the use of gut contents. This was due to the fact that faecal samples would give biased results of the diet of arthropod predators, due to a lower representation of soft-bodied prey in faecal pellets. To test this assumption, we compared the spring diet of several populations of two insular lizards from the Balearic Islands (Spain), Podarcis lilfordi and Podarcis pityusensis, using both faecal pellets and gut contents. Our results do not support the supposed bias of dietary analyses based on faecal pellets. Indeed, soft-bodied prey and particularly insect larvae are often equally represented in faecal pellets and gut contents. Alternatively, soft bodied prey are represented in different proportions in gut contents and faecal pellets, but in some cases with higher proportions being observed in the gut contents, and in other cases with higher proportions in faecal samples. We conclude that faecal pellets can be a reliable source of information for dietary studies.