PREDICTING DIET COMPOSITION OF SYNGNATHID FISHES USING STABLE ISOTOPES: APPLICATION OF LAB-BASED EXPERIMENTS TO FIELD RESEARCH
Consumers incorporate the isotopic composition of the food, with a consistent trophic enrichment factor (TEF) at each trophic level, so that the diet of a given species can be inferred from δ13C or δ15N signatures in potential prey and consumer. The analysis of stable isotopes (SIA) is a useful non-invasive tool to infer the diet composition in wild fishes. SIA is particularly interesting in studies concerning endangered or threatened species.
Two groups of adult seahorses Hippocampus guttulatus (Syngnathidae) were maintained in captivity and fed on two diets (Artemia + Mysidae or only Mysidae) for the whole breeding season. Stable isotopes in dorsal fin clip samples were determined at both the onset and the end of the breeding season. The resulting experimental TEFs as well as others commonly used in similar studies were applied to infer the diet composition of the Syngnathidae Syngnathus acus from the Atlantic Islands National Park (NW Spain). Monte Carlo simulations (mixing polygons with consumers and five groups of potential sources) and hierarchical Bayesian mixing model analyses (MixSIAR, with or without priors) were applied to estimate the diet. Separate models were run for each TEF and for five prey cluster grouping.
Mixing model polygons showed that our experimentally derived TEFs were more appropriate than others previously reported. SIMM predictions from TEFs (Table 1, columns a and d) suggest that S. acus is a generalist consumer, foraging on several isotopically similar prey sources. The species consumed greater amounts of Amphipoda and Copepoda prey, followed by Isopoda and to a lesser extent by Caridea and Mysidacea (<10%) (Table 1). However, there was broad overlap in the predicted mean diet composition and 95% credibility intervals overlapping considerably across most prey sources (Bhattacharyya's Coefficients). This study provides novel TEFs for adult Syngnathid fishes.
This study was financed by projects CGL2015-68110-R (Spanish Ministry of Economy, Industry and Competitiveness) and 1541S/2015 (Spanish Agriculture, Food and Environment Ministry - National Parks Autonomous Organism).
