MEPS 674:241-255 (2021)  -  DOI: https://doi.org/10.3354/meps13832

ABSTRACT: Sea cage aquaculture can alter the spatial distribution of wild fish populations; however, little is known about the dietary habits of wild fish frequenting sea cages. We used wild juvenile cod Gadus morhua reared in the laboratory and fed either an aquafeed pellet or marine-based diet to determine trophic discrimination factors (TDFs) of stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values in white muscle tissue and baseline liver proportions of vegetable-oil based (VOB) fatty acids. We then used δ¹³C and δ¹⁵N and proportions of VOB fatty acids to investigate the dietary habits of wild cage-associated juvenile and adult Atlantic cod and adult Atlantic redfish Sebastes fasciatus. Cod and redfish were collected in the immediate area of sea cages and reference areas of no aquaculture production. Juvenile cod captured around sea cages had both elevated δ¹³C and δ¹⁵N values and proportions of VOB fatty acids, and isotopic fractionation comparable to laboratory cod fed an aquafeed diet. However, differences in δ¹³C and δ¹⁵N and proportions of VOB fatty acids between cage-associated and reference site adult cod and redfish were predominately absent. Results suggest that sea cages may provide an energetic subsidy to juvenile cod but perhaps not at the level to sustain adult cod or redfish. Therefore, the lack of differences suggests that both adult groups may be using cage sites opportunistically and only for short duration, as isotopic values and fatty acid proportions were not consistent with waste feed consumption despite individuals being collected in close proximity to sea cages.

KEY WORDS: Sea cage aquaculture · Energetic subsidy · Waste feed · Stable isotopes · Trophic discrimination factor · TDF · Fatty acids · Gadus morhua · Sebastes fasciatus