Are you what you eat? Stable isotopes and stomach content analyzes revealed contrasting food importance and assimilation patterns of an omnivorous fish at a subtropical coastal lagoon

Background/Question/Methods

Traditionally, the statement “You are what you eat” makes sense to trophic ecology studies based on stomach food content. However, since stable isotope techniques allowed the possibility to evaluate which food is really incorporated in the consumer’s tissues, “You are what you assimilate” makes even more sense. The unequal assimilation of distinct types of food ingested by a consumer could be due several factors, like the nutritional quality of food and species-specific efficiency for food-specific digestion. Jenynsia multidentata is an omnivorous fish that occurs along a salinity gradient in a subtropical coastal lagoon, encompassing a freshwater wetland, estuarine zone and a site near the connection of the lagoon with the sea. We hypothesized that (1) the species changes its diet along sites and (2) food are not assimilated in the same proportion they appear in stomachs. In order to test these hypotheses, we collected J. multidentata, and its main food items. Diet were evaluated by the alimentary index of importance (IAi) that consider the frequency of occurrence (%FO) and the volume (%V) of each food (IAi=%FO*%V). We also applied several isotope mixing models (SIAR) to estimate the assimilation of food ingested, considering source specific trophic enrichment factors (TEFs).

Results/Conclusions

The inspection of 121 stomachs revealed that the diet was comprised by both algae and animals, but there was a marked and progressive shift from herbivory at freshwater wetland (algae = 80.95%) to carnivory (polychaets = 96.39%) at lagoon’s mouth. Despite this diet shift, all SIAR mixing models revealed that animal-derived organic matter was the major food source assimilated in the muscle tissues of the consumer. After accounting for changes in isotope baselines along the gradient, one-way ANOVA showed no differences in consumer’s trophic position among sampling sites (p>0.05). These findings strongly suggested that, despite of its high frequency in the wetland, algae was ingested accidentally when the consumer was foraging for animal preys hidden in dense mats of filamentous algae. Conversely, the high digestibility of soft body animals underestimated its importance in the stomach content in the wetland. The model with specific TEF for each food source achieved the best resolution, with algae and animal contribution ranging from 0% to 22% and 78% to 100% (95% credible interval), respectively. Our results show the importance of stable isotopes in elucidating trophic pathways and food assimilation patterns, which could be misinterpreted if using only stomach content analysis.