Evidence of density-dependent cannibalism in the diet of wild Atlantic bluefin tuna larvae (Thunnus thynnus) of the Balearic Sea (NW-Mediterranean)

The heavy exploitation rates of Atlantic bluefin tuna (ABFT) during the nineties propitiated research into the larval ecology of ABFT and its associated species. The transition from a planktivorous to a piscivorous diet is considered a major bottleneck in the survival of ABFT larvae observed in aquaculture experiments. Although larval piscivory was reported in the Gulf of Mexico (GOM), the most important spawning grounds of this species in the W Atlantic, trophic studies have not been able to reveal piscivory in Mediterranean ABFT larvae. This study analyzes the trophic behavior of Mediterranean ABFT larvae by stomach content analysis. The results show that more than 90% of the larvae had at least one prey in their stomachs. The diet shifted from copepods and cladocerans to gastropod larvae in pre-flexion stages and to ABFT larvae in post-flexion stages. This is the first time that cannibalism is reported for wild ABFT larvae from the Mediterranean Sea. Intracohort cannibalistic feeding was observed when the requisite density-dependent processes aligned, namely the spatio/temporal overlap of a wide range of ABFT larval cohorts of different size class. Moreover, stomach contents of ABFT larvae revealed the ingestion of microplastic fibres. Whether these plastic contaminants were passively or actively ingested, they may affect the condition of larvae. The presence of microplastic strands in fish larvae undoubtedly raises concern because its impact on the survival of ABFT larvae still remains uncertain and is open to scientific experimentation.