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Abstract The presence of wild fish in and around aquaculture habitats is often assumed a response to food resources within these habitats, either from input feed, the presence of cultured species, and/or the assemblage of biofouling that naturally colonises aquaculture structures. The nutritional quality of the food resources consumed by wild fish in aquaculture habitats is also important in determining their nutritional condition and subsequent productivity. Few studies have investigated the nutritional quality of prey in aquaculture habitats, and these have mostly focused on fed aquaculture by tracking manufactured fish pellets into the diets of wild fish. However, in non‐fed aquaculture, the assemblage of cultured and biofouling species may also provide a nutritional benefit to fish feeding in these habitats. The Australasian snapper, Chrysophrys auratus, are commonly present as adults within coastal mussel farms in New Zealand and tend to become a resident species. This study investigated the nutritional quality of the gut contents of snapper in soft‐sediment habitats within and outside of New Zealand green‐lipped mussel farms. Total lipid, protein, carbohydrate and total calorific content were measured from the gut contents of snapper sampled from mussel farm and natural (i.e. control) habitats. Snapper in mussel farms had double the dietary intake of lipid (16% vs. 8%) from consuming lipid‐rich bivalves and barnacles which are in abundance in mussel farms. Higher lipid intake can contribute to improved nutritional condition, reproduction and growth in snapper. However, the higher dietary lipid intake of snapper in mussel farms did not increase their overall body condition (i.e. Fulton condition index). This may be due to the coarse nature of this measure, or the use of the additional lipid in more rapid somatic growth or reproductive outputs, possibilities that warrant examination through further research. Overall, this study shows for the first time the potential ecosystem benefits of shellfish aquaculture in provisioning nutritionally valuable prey for coastal fish populations.

Abstract Wild fish utilise aquaculture habitats for shelter and/or food resources. It is often assumed that fish respond to feed input, the abundance of the farmed species or the associated assemblage of biofouling which naturally colonises the structural habitats. However, few studies have directly analysed the composition of the diet of fish within aquaculture habitats, and of these most have focused on fed finfish aquaculture. Snapper are commonly present as adults within coastal mussel farms and tend to become a resident species of these farms. Therefore, they are a suitable case study species for exploring differences in diet between natural and aquaculture habitats. This study investigated the gut contents of snapper in soft‐sediment habitats within and outside of New Zealand green‐lipped mussel farms. Visual gut analysis and DNA metabarcoding methods were used to provide complementary analyses on the composition of gut contents between the mussel farm and natural (i.e., control) sites. Snapper within mussel farms were consistently found to have consumed different prey groups compared to the control snapper. Prey groups identified from mussel farm snapper gut contents could be directly linked to species commonly present in the farms, that is cultured green‐lipped mussels, blue mussels and barnacle biofouling. There was good alignment between the visual gut and genetic analyses for the key species identified. Overall, the results show that the highly abundant prey groups consumed by snapper in mussel farm habitats are likely to be beneficial to the snapper population, reducing foraging effort and potentially supplying more nutritious prey. These findings provide evidence towards the supporting services of mussel farm habitats through the provision of food resources.