Recent Norwegian studies have shown that the indirect effects of fish farming on wild populations, particularly regarding behavioral and environmental changes, are more concerning than the direct transmission of viruses or bacteria.
An annual study conducted by the Norwegian Institute of Marine Research found minimal viral and bacterial transmission between farmed fish and post-smolt wild salmon and sea trout, while a different study by the same institution found farmed fish may be impacting the behavior of wild cod.
“To ensure sustainability along the coast, the environmental impact from farming must be minimized, and we need solid knowledge about how aquaculture can affect the wild fish stocks along the coast,” Marine Researcher Thomas Bøhn, a co-author of the latter study, said.
For the first study, researchers analyzed wild salmon and sea trout for four viruses commonly found in farmed fish, as well as the bacterium Renibacterium salmoniraum, as a part of an annual monitoring program mandated by the Norwegian Food Safety Authority.
About 138 wild fish were tested, with only a few positive results for viruses but none for the Renibacterium salmoninarum – despite outbreaks in nearby farms of the bacterial kidney disease (BKD) that this bacterium causes. This supported previous results that indicated a low incidence of infection transmitting from migrating wild fish to farmed fish.
The other study completed by the institute and published in the journal Reviews in Aquaculture found that salmon farming in open pens along the Norwegian coast raises the possibility of environmental harm done to wild Atlantic cod.
The authors identified several major potential risks that salmon farming poses to wild populations, including those that affect behavior, physiology, and survival in wild cod.
Cod, according to the study, are sometimes attracted to farm nets due to the availability of feed, scarcity of prey, and bright lights.
“Large amounts of waste feed, probably 60,000 to 100,000 metric tons (MT) per year, can cause wild fish to congregate around a farm, something that can change the interactions and health of the ecosystem,” Bøhn said.
This waste feed can unnaturally increase growth due to a difference in fatty acids from feed compared to natural nutrients.
Sea lice treatments and materials used on farms may also impact populations and the surrounding ecosystem by exposing fish to new chemicals and changes in available prey.
Both studies touch on how climate change may exacerbate these risks.
Currently, about 50 percent of global aquatic food consumption stems from aquaculture, and although it reduces stress on wild populations by providing an alternative protein source, large-scale aquaculture may also represent an additional stressor to wild populations in the areas where overlap of the two occur, according to the studies.
The authors of both studies emphasized a need for continuous improved scientific knowledge, monitoring, and risk assessments to adapt aquaculture practices as needed, which should help mitigate the negative impacts of fish farming on wild populations while still meeting global demand for seafood protein sources.
