Tracking multiple pathways of waste from a northern bluefin tuna farm in a marine-coastal area

Aquaculture of bluefin tuna in Mediterranean coastal waters has generated growing concern about the negative environmental effects. In the present isotopic study we examined the dispersal and fate of organic matter derived from a Mediterranean tuna farm in the surrounding environment. An overall enrichment in the heavy nitrogen isotope was found in the feed and in farmed tunas, indicating the input of isotopically traceable organic matter in the system. Waste was clearly traceable in the water column up to 1000 m from the cages, while only slight accumulation occurred in the sediment just below the cages. Waste was isotopically shown also to contribute to the diet of demersal and benthopelagic wild fish collected around the cages. As a result, waste undertook multiple pathways. In the water column its was diluted and dispersed due to hydrodynamism, which prevented great accumulation of aquaculture-derived organic matter in sediments. In addition, biological constraints such as benthopelagic and demersal fish further prevented organic matter accumulation through the benthic trophic route.

► δ15N was found to be an effective tool for tracing tuna farm-derived organic matter.
► Uningested feed was intercepted by local wild fish and transferred to local food webs.
► Tuna farm-derived organic matter was detected in the water column.
► Low accumulation occurred in the sediments due to the dispersive nature of the site.
► Physical and biological constraints are involved in minimizing biodeposition.