Stomach content analysis of Euthynnus affinis, Auxis thazard and Auxis rochei of the coastal waters of Sri Lanka by DNA barcoding.

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Analysis of the content of the gut of fish helps in the understanding of their inter- and intra-specific interactions, fish behaviour, condition and energy intake. The stomach contents of the commercially important neritic tuna species of Sri Lanka, kawakawa (Euthynnus affinis), frigate tuna (Auxis thazard) and bullet tuna (Auxis rochei) were analysed to determine their feeding habits and to identify prey species. The weighed stomachs of fish were dissected to reveal the types of prey found within. The prey was categorised into prey categories and each prey species was identified morphologically. Prey items which were partially digested were identified using DNA barcoding. The main prey category was small fish, followed by crustaceans and cephalopods. While the highest occurring prey category for E. affinis and A. rochei was fish, crustaceans dominated the A. thazard diet. DNA barcoding identified 11 prey items that were partially digested, which could not be identified to species-level morphologically. Of the prey items identified by DNA barcoding, four species of fish, three species of cephalopod and four species of crustaceans were identified. These prey item identifications confirmed that E. affinis, A. thazard and A. rochei are all nonspecific feeders. This exhibits the value of molecular tools in the identification of species which have lost their distinguishable features due to digestion. Further, it illustrates the predator-prey relationships between these species, aiding in the management of prey and predator populations, ensuring that both populations remain stable, helping in the maintenance of the balance of the ecosystem.