Farming of salmonids in open sea cages can attract wild fish species (Banno, et al., 2022; Dempster, et al., 2009; Uglem, et al., 2014). The attraction can be caused by excess fish feed or the construction itself (Uglem, et al., 2014). Presence of sea cages may impact the migrating behavior of wild fish species, and large aggregations of wild fish may increase the risk of transferring diseases or parasites between farmed and wild fish (Uglem, et al., 2014). Previous studies have focused on the potential impact of fish aggregations around salmon farms in terms of environmental challenges and the wild fish itself. However, there are currently limited knowledge on how salmon are influenced by the presence of wild fish. For example, smolt of wild Atlantic salmon have been found in the stomach of Atlantic cod (Gadus morhua) and saithe ( Pollachius virens) , suggesting these fish species prey on salmon when they are migrating through the fjord (Hvidsten, Lund, 1988). It is not known whether salmon contained in a closed environment will respond to any wild fish approaching the sea cage. Farmed Atlantic salmon were anecdotally observed altering its behavior on a random video recording in a commercial sea cage when a large cod approached the net. Such behavioral alterations may impact the welfare of salmon if it occurs on a regular basis when wild fish approach the sea cage. In the present case study, we investigate d the immediate behavioral response of farmed Atlantic salmon when Atlantic mackerel, saithe and cod approached the sea cage with salmon .
This study took place at a commercial farming site in Møre and Romsdal when wild fish were expected to be present in the area. A ROV were first used to identify high densities of wild fish (saithe or mackerel ) in the vicinity of a particular sea cage. Secondly, a 360- degree camera were placed in the same area inside the sea cage . This made it possible to assess the swimming behavior of salmon at the same time as wild fish outside the cage approached salmon inside the cage. Videos were collected from one cage containing large salmon ( ≈ 2.5 kg) and three cages containing small salmon (< 1 kg). More than 700 minutes of video were collected and were analyzed for any escape response (defined as a clear change in swimming speed or swimming direction) among salmon when wild fish instantly appeared close to salmon.
No escape response was observed. In total, Mackerel instantly appeared close to large and small salmon, 10 and 48 times respectively. Saithe instantly appeared close to large and small salmon, 59 and 69 times respectively. This indicates that salmon are not regularly frightened by mackerel or saithe, but more research on other potentially larger wild fish is required.
Banno, K., Kaland, H., Crescitelli, A.M., Tuene, S.A., Aas, G.H., Gansel, L.C., 2022. A novel approach for wild fish monitoring at aquaculture sites: wild fish presence analysis using computer vision. Aquac. Environ. Interact. 14, 97-112.https://doi.org/10.3354/aei00432
Dempster, T., Uglem, I., Sanchez-Jerez, P., Fernandez-Jover, D., Bayle-Sempere, J.T., Nilsen, R., Bjørn, P.-A., 2009. Coastal salmon farms attract large and persistent aggregations of wild fish: an ecosystem effect. Mar. Ecol. Prog. Ser. 385, 1-14.https://doi.org/10.3354/meps08050
Hvidsten, N.A., Lund, R.A., 1988. Predation on hatchery-reared and wild smolts of Atlantic salmon, Salmo salar L., in the estuary of River Orkla, Norway. J. Fish Biol. 33, 121-126.https://doi.org/10.1111/j.1095-8649.1988.tb05453.x
Uglem, I., Karlsen, Ø., Sanchez-Jerez, P., Sæther, B.S., 2014. Impacts of wild fishes attracted to open-cage salmonid farms in Norway. Aquac. Environ. Interact. 6, 91-103.https://doi.org/10.3354/aei00112