These proposals for possible measures are based on the results of many years’ work. The scientists have described in detail how salmon behave and respond to the cage environment, and how fish farmers can best use that knowledge to increase production.
- With more than 20 years of data and observations, we now have a better overall understanding of how cage environments vary and how salmon react to them. If fish farmers make use of this knowledge, it will allow them to break new ground in terms of cage environments, feeding regimes, the use of light, production efficiency, delousing and salmon welfare, says researcher and project manager Frode Oppedal of The Institute of Marine Research.
Salmon choose their environment
It is normal for the environment inside a cage to vary considerably over time and depending on the water depth. The depth at which salmon swim and the fish density depend on a number of factors: temperature, oxygen levels, salinity, currents, light, feeding regimes, appetite, chemicals, perceived fear, etc. These behavioural choices mean that the full volume of the cage is not used, and that fish density is greater at some depths than at others.
- It is typical to observe fish densities that are 1.5-5 times higher than what you would have if the fish were evenly distributed, and in extreme cases up to 20 times higher, explains Oppedal.
The scientists’ most important recommendations are:
1. Expand monitoring from the current low levels to include continuous measurement of the environment at several depths (temperature, oxygen and salinity) and in several cages (oxygen), to allow farmers to actively respond to unfavourable conditions. In the short term, feeding regimes can be adjusted, while in the longer term it may be relevant to reassess biomass, cage size, cage location and the general location of farms.
2. Adjust the depth at which feed is released in relation to where the salmon want to swim. The current practice is generally to release the feed on the surface, which forces the fish to swim up to the often unfavourable surface environment in order to eat. This measure would probably both improve fish welfare and increase production.
3. Guide the salmon away from the surface using underwater light and/or feeding during periods when there are high levels of sea lice near the sea surface, in order to reduce infection intensity. Reducing the number of lice on farmed fish will reduce the production of sea lice larvae, which have the potential to infect both wild and farmed fish. This will in turn reduce the stress caused by having to delouse the farmed fish, increasing fish welfare.
4. Abnormal behaviour is often an early warning of disease. An operational welfare index should therefore be developed, based on salmon behaviour in cages, which is easy for workers at fish farms to use.
Based on this work, two new projects have been granted funding by The Research Council of Norway this year. They will focus on areas such as how individual fish experience the cage environment and the consequences of this, as well as tolerance levels of salmon of different sizes to variations in oxygen concentration and water currents.