Bottom-up shifts in benthic food webs by salmon farm effluents in deep Norwegian fjords: a multi-tracer approach

Particulate organic waste from salmonid aquaculture represents a potential food source for benthic communities in the vicinity of fish farms. This study investigated the incorporation of farm-derived organic matter into the benthic food web at 2 deep-water fish farms in western Norway, using fatty acid (FA) and stable isotope (δ 13 C and δ 15 N) tracers. Sediments and infaunal polychaetes were collected using a grab during production and fallowing periods, while decapods and demersal fishes were collected using trawls and baited pots. Sediments and fauna collected during production generally showed stronger aquaculture-derived biochemical signatures than those collected during fallowing, although responses differed between farms and species. Tracer FAs associated with salmon feed, especially 18:2(n-6), 18:3(n-3), 20:1(n-9), and 22:1(n-11), were elevated in sediments and opportunistic polychaetes, suggesting a rapid assimilation of farm-derived organic matter at the base of the food web. Decapods and demersal fishes also showed elevated tracer FA levels, revealing both direct and indirect trophic transfer of aquaculture waste to higher trophic levels. δ 13 C was less conclusive for tracing waste inputs, but δ 15 N revealed significant shifts in trophic structure between production and fallowing periods. During production, reduced δ 15 N separation among trophic groups suggested a compressed food web supported by a common farm-derived food source. During fallowing, most FA tracers declined and δ 15 N values increased, indicating reduced reliance on aquaculture-derived material and a shift toward more natural trophic interactions. Overall, these findings show that aquaculture activities alter deep-sea benthic food webs by increasing the uptake and trophic transfer of farm-derived organic matter.