Florencia Botto

Combining a geographic information system, known dietary, foraging and habitat preferences, and stable isotope analysis to infer the diet of Magellanic Penguins in their austral distribution

Abstract The Magellanic Penguin (Spheniscus magellanicus) is the most abundant and widely distributed top marine predator of Atlantic Ocean waters off the Patagonian coast. During the breeding season, this species consumes thousands of tonnes of its prey and has a significant effect on populations of fish and squid near the Penguins breeding colonies. We combined data on stomach contents, known habitat preferences of their prey, stable isotope analysis and a geographic information system to describe the diet of Magellanic Penguins in their austral distribution. On this basis, we present a comprehensive analysis of the trophic ecology of the species, the diet of which is poorly known (only a few analyses of stomach contents). Stable isotopes revealed a mean trophic level of 5.3 (range 4.5-5.8) and a diet in accordance with spatio-temporal overlap analysis. This study shows that Magellanic Penguins forage on few prey species not targeted by commercial fisheries in this region, mostly Patagonian Sprat and Longfin Squid. Moreover, we demonstrated that a combination of complementary tools can be used to describe the trophic relationships of wide-ranging marine predators, such as seabirds.

Important sources of variation to be considered when using fin clips as a surrogate for muscle in trophic studies using stable isotopes

White muscle is the prevalent tissue for C and N stable isotope analysis in fish, requiring the death of the fish or biopsy procedures that could lead to infections or severe damage. Given that caudal fin-clipping does not seriously affect growth or condition, the present study assessed the suitability of caudal fin tissue as replacement for muscle tissue in trophic studies. Clips of caudal fin were a useful non-lethal surrogate of muscle samples in four studied reef-fish (Diplodus argenteus, Pagrus pagrus, Acanthistius patachonicus and Pinguipes brasilianus). Fin clips were easy to collect in quantities adequate for mass spectrometry analyses and had C:N ratios similar to those of white muscle with low lipid content. However, results showed that fin-muscle correction models should be specific and sampling design should be conducted to reduce spatial and temporal variation. Moreover, species-specific correction factors may not be valid for other populations of the same species if the presumed range of δX values differ from the population used to estimate the correction models. Results also showed that the fin-muscle relationship could vary with size. Thus, unless a non-ecological meaningful fin-muscle correlation with body size was previously identified, correction models should be estimated sampling a representative size range and fin samples should be used with caution to study size-related trophodynamics.