Connecting the dots: evidence of interactions between wild Atlantic cod Gadus morhua, the benthic environment and salmon farm discharges

Rapid expansion in coastal salmon farming has coincided with declines in coastal Atlantic cod Gadus morhua populations in Norway, raising suspicions over a causal relationship. One established effects pathway concerns the discharge of exogenous waste feed; however, food-web-related pathways remain poorly understood. Here we present a study conducted in 3 neighbouring fjords that are also important cod spawning areas, one of which has several active salmon farms. The study combines spatially explicit information on benthic ecology, cod stomach contents (via eDNA), dietary groups (via fatty acid profiling) and cod source-stock and demographics. Results confirmed that in addition to an anticipated fish diet, wild cod fed on a diverse array of benthic invertebrates, including shrimps, echinoderms, crabs, benthic worms and other infauna. Cod diets differed strongly with proximity to fish farms. Most (69%) cod caught adjacent to a farm had fatty acid trophic markers in their stomachs and/or livers consistent with a salmon pellet diet, and their stomachs contained benthic fauna that proliferate with organic enrichment. This trend was very localised, indicating a degree of residency and feeding specialisation or adaptability. When feeding off the bottom, cod are also likely to ingest contaminated sediments, which can (1) be toxic to marine life, (2) have a strongly altered microbiome and (3) contain known fish pathogens. We conclude that cod are influenced by farm-induced changes to food-web pathways, both in terms of the type and availability of suitable prey species and the composition of their tissues.