Predatory fish diets shift towards an invasive mullet in a traditional Hawaiian aquaculture system

Abstract Traditional Hawaiian fishponds, called loko iʻa , are a low‐impact and culturally important aquaculture system that historically produced significant fish yields. To better understand the structure of the contemporary food web of the restored Heʻeia fishpond, a mark‐recapture experiment was conducted to estimate the population abundance of the three dominant predatory fishes, and gut content, genetic barcoding, and stable isotope analyses were used to assess their diets. Great barracuda Sphyraena barracuda had the largest estimated population at 190 individuals, followed by 89 giant trevally Caranx ignobilis and 19 bluefin trevally Caranx melampygus . The three species were caught throughout the pond, with C. ignobilis concentrated in the deeper northwest area near a mangrove island. Diets of the three species consisted mainly of nearshore, estuarine fishes, and crustaceans. Prey taxonomic resolution was greatly improved with the use of genetic barcoding since most fish were too degraded to be visually identified. Trophic level calculations and isotopic niche breadth comparisons indicated that C. ignobilis and S. barracuda occupied similar ecological niches in the fishpond, and stable isotope mixing models revealed that their long‐term diets were mostly comprised glass shrimp ( Palaemon sp.), mangrove swimming crab ( Thalamita crenata ), and an invasive Australian mullet ( Osteomugil engeli ). The native mullet ( Mugil cephalus ), a primary food fish species from the fishpond, was not identified in the diets of the predatory fishes in this study nor in a previous diet study decades ago prior to the introduction of Australian mullet. The inclusion of the invasive Australian mullet in the contemporary diets of the predatory fishes demonstrates a shift towards the newly available prey which may benefit the native mullet populations. These findings improve the understanding of food web dynamics in contemporary Hawaiian fishponds and provide an example of native predatory fishes consuming an invasive species as part of their diets.