Feb. 14, 2011 The salmon swim bladder requires an air supply in order to regulate the fish's buoyancy. Until now, however, no one realised just how seldom salmon need to surface for air. This need is being quantified by researchers at CREATE, one of 14 Norwegian Centres for Research-based Innovation established four years ago with funding from the Research Council of Norway.
"We kept salmon in completely submerged cages for 17 days," reports Arne Fredheim, Research Manager at CREATE. "The individuals swam somewhat faster than usual but they did not exhibit stress. We also tested the pain threshold for how long salmon can continuously stay below the surface. It took four months for them to become excessively stressed."
The centre's researchers believe that submerged cages can be used for extended periods -- as long as the cages are lifted to the surface at regular intervals to give the fish access to air.
"But more research is needed to adequately determine their threshold limits," asserts Dr Fredheim.
Using submerged cages is advantageous for avoiding storm damage and resisting invasion by algae and sea lice. There is also growing evidence that submersion may increase the effectiveness of sea lice medicines.
CREATE carries out research and develops technology for the sea-cage phase of "fattening up" production fish for the market.
"Our research is part technology and part biology, and interactions are crucial. For instance, we are examining the effects of aquatic temperature, salinity and oxygen content on fish behaviour and welfare. This is fundamental knowledge on which we will base our technology development," explains Dr Fredheim.
In the project "Cage Environment," researchers at CREATE studied hypoxia (oxygen depletion in the aquatic environment) to ascertain critical threshold limits for fish welfare. At the same time, they gained a better understanding of how seawater flows through cage netting. Even though aquaculture sites are placed in sea areas with strong natural currents, there can be too little oxygen in the water at times -- which impedes fish digestion.
"We have developed models for calculating how strong the current must be in relation to cage size and fish density," says Senior Scientist Pål Lader, who works at CREATE. "We have also studied how fish group behaviour affects the current. They often swim in circles en masse, creating a rising spiral current."
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