Morphological and metabarcoding dietary analysis of the cunner wrasse (<scp><i>Tautogolabrus adspersus</i></scp>) revealed significant regional variation, with large overlap between its common prey species and biofouling animals living on salmonid se

The stomach-less cunner wrasse (Tautogolabrus adspersus) has been experimentally used as a biological control agent for salmon lice that infest Atlantic salmon (Salmo salar) and to remove biofouling inside sea cages. The cunner demonstrates a strong population structure, suggesting that its diet, and therefore its usefulness for biological control, could differ among its populations along 1086 km of eastern Canada, in response to the biogeography of its prey species. Gastrointestinal tract samples were collected across 14 locations throughout five distinct regions from Southern Nova Scotia to Eastern Newfoundland between 2018 and 2022. Primary constituents of diet, identified using morphology and by percentage weight, were mussels, bryozoans, ascidians, gastropods, unidentified digested material and barnacles. Dietary DNA (dDNA) metabarcoding identified mussels in 46% of guts, amphipods in 45%, bryozoans in 31%, ascidians in 28% and sea anemones in 18%. Sea lice were rare yet present in samples from three separate regions. Permutational multivariate analysis of variance (PERMANOVA) based on DNA metabarcoding suggested that sampling region, location and year all significantly influence diet composition. Regional divergence in diet was greatest between Southwestern Nova Scotia and Northeastern Newfoundland. Invasive cionid ascidians were present almost exclusively in Nova Scotian samples, whereas brittle stars were present almost exclusively in Northeastern Newfoundland samples. dDNA metabarcoding enabled the detection of soft-bodied prey and often identified prey to the species level. Cunner were demersal feeders on neritic sessile or slow-moving benthic animals that comprise the biofouling community. In addition to preying on sea lice and invasive ascidians, we predict that cunner wrasses will reduce the density of biofouling communities on structures used in marine aquaculture.