Stable isotopes as tracers can reveal resource allocation in juvenile golden gray mullets (Liza aurata, Risso, 1810)

Studies on the nutritional physiology of predator fish in the marine environment have contributed to our understanding of how they adapt to the environment and how they have evolved. Despite the fact that herbivorous and omnivorous fish species are numerous and play a significant role in the ecosystem, there is little information on how they process nutrients and how these are allocated to different tissues. This information could be particularly important for the juvenile stages, when small-sized fish are under high predation pressure and have a limited capability to intake and digest large quantities of food. The mullet Liza aurata ingests surface sediment and obtains its nutritional requirements from the organisms associated with the sediment, including microalgae and bacteria or small invertebrates. This paper examines how the carbon and nitrogen derived from benthic micro-organisms are allocated to the liver and muscle tissues of newborn (young-of-the-year, YOY) and one-year-old (OYO) individuals. After the animals were left feeding on 13C-enriched microalgae and 15N-enriched bacteria for 1 h, we traced the 13C and 15N in the liver and muscle tissues as well as in the blood and the gut. The YOY allocated 99% of the 13C and 88% of the 15N to the muscles, while the liver had a negligible amount of tracers (0.4% and 11% for 13C and 15N). Conversely, in the OYO experiment, the tracers were uniformly distributed throughout the muscle and liver (57% of 13C and 45% of 15N were found in the muscle, whereas 43% of 13C and 55% of 15N were in the liver). Negligible amounts were traced in the blood (<0.1%), while a part of the tracers was not assimilated and remained in the gut of both YOY and OYO fish. These results indicated a size-related shift in resource allocation during first year of growth of L. aurata, probably related to changes in the survival strategies among juveniles. Our results also indicated that stable isotope enrichment can be a helpful tool for studying resource allocation in fish.