Stable isotope analysis suggests nutrient connectivity between salmon and kelp within a commercial scale open coast integrated multi-trophic aquaculture system

This study explores nutrient connectivity between Atlantic salmon (Salmo salar) and sugar kelp (Saccharina latissima) within a commercial-scale integrated multi-trophic aquaculture (IMTA) system in Bantry Bay, Ireland. In this system the kelp farm is located next to a salmon farm that restarted production in mid-2023 following four years of no production. This allowed a comparison between baseline conditions at the same site during a kelp-only production season in 2023, prior to salmon farming, and a salmon-kelp production season in 2024. Fieldwork campaigns were conducted during each year to collect samples of kelp, water, and proxy nutrient sources (fish feed, faeces, particulates, seaweeds). Stable isotope analysis and a Bayesian mixing model were used to assess uptake of nutrient sources by the kelp. The δ¹⁵N values differed between the study period of each year, with consistently lower δ¹⁵N values observed during 2024. Mixing model estimates showed a pronounced shift after salmon farming commenced. Comparative growth data between years revealed increases in kelp blade length, width, wet weight, and tissue nitrogen content during 2024. The findings suggest nutrient transfer from salmon farming to the kelp, with stable isotope analysis serving as a complementary method to growth and water sampling for determining nutrient connectivity.