Introduction
The expansion of aquaculture production into more exposed and harsher ocean environments increase the risk during vessels berthing and mooring operations alongside the cages. A possible solution to deal with this increased risk is to utilize Dynamic positioning (DP) systems, which allows vessels to maintain a specific position and orientation without having to moor the vessels alongside the cages. DP may lead to increased use of thrusters in vicinity of the cage systems , which may be negative for fish welfare according to existing knowledge on effects of anthropogenic noise on fish.
The knowledge about effects of anthropogenic sounds on farmed fishes is scarce . However , it is shown that high anthropogenic noise levels may alter behavioural response, increase stress and cause decreased growth rate for farmed fish. A tlantic salmon (Salmo salar) have been found to only det ect low frequency sounds below 380 Hz, c oinciding with the dominant frequencies from commercial ships at frequencies below 300 Hz . For other farmed fish species, it has been found that noise from vessels can lead to reduced foraging behaviour , whereas other studies has shown that farmed fish can adapt to anthropogenic sounds without any long-term effects on growth rate. The results from this project may be used for risk assessment of future vessel operations close to sea cages.
Method
Full- scale experiments are performed at a commercial open net pen sea farm for Atlantic salmon grow- out production to identify possible behavioural response during controlled vessel thruster activity towards a cage. Sensors and cameras are used to observe possible abnormal behaviour compared to reference measurements in undisturbed conditions. Expected measurable responses are groups of fish moving suddenly away from the noise or propulsion jet source and/or groups of fish suddenly unschooling. The physical properties of the thruster and propulsion jet, such as noise level, noise frequency and water speed will be measured at various positions within the cage. This provides useful information for evaluating t he fish desire and ability to escape from the noise and propulsion jet within the cage.
Results and discussion
The results from the full-scale experiments are being processed. Preliminary results based on interviews with vessel captains and fish farmers to map current practice on use of vessel thrusters close to sea cages indicates that there have not been observed any occurrences of abnormal fish behaviour during or after vessel thruster operations. This may indicate that farmed salmon have the possibility to move adequately away from the disturbance sources and/or that the fish adapts to the noise levels present at existing sea farms . It should be noted that the results from these experiments could only be used to map the fish’s immediate behavioural response and that any potential negative effects on fish welfare needs to be further investigated.