Healthy food may alleviate stress among farmed fish

In the same way as people, fish can find loud noises and unexpected movements stressful—and this can adversely affect their physical health. In fact, density at intensive farming facilities may lead to stress and decelerated growth. New studies by a team of researchers from DTU now indicate, however, that a positive and durable effect can be achieved on the behaviour of pike perch (Sander lucioperca) in situations of stress by adding a long-chain non-saturated omega-3 fatty acid (DHA) to their normal animal plankton feed while they are still in their larval stage.

“Stress is one of the principal obstacles to streamlining the production of pike perch. But the fact that larvae and fry on a DHA diet react and swim more quickly than pike perch reared on feed without DHA seems to indicate that administering the right nutrition as early as the larval stage make the pike perch more robust,” explains Ivar Lund, Project Manager and Senior Researcher at DTU Aqua, who conducted the survey in partnership with his colleagues Erik Høglund and Peter Skov.

DHA-feed thus makes the growing pike perch better able to adapt to conditions at fish farms. There is broad consensus that DHA has a positive impact on the early neural development of learning capacity in both people and animals, and a previous study by the same team of researchers demonstrated that pike perch larvae given feed with a low DHA content for a limited period of time simply developed smaller brains.

It is uncommon to conduct studies of the effect of nutrition on cerebral development and behaviour on fish, but the results from the DTU Aqua survey are backed by studies in mice and rats, which have revealed correspondingly positive effects of DHA on learning and sensory reactions.

“We don’t know precisely which parts of the pike perch’s brain benefit from DHA, so we will need to carry out additional studies involving a greater number of fish and clearer focus on the neural effects in the brain if we are to test the correlation between nutrition and behaviour,” explains Ivar Lund. This is done in a new EU project (www.diversifyfish.eu) involving 6 new species in aquaculture.

The pike perch was chosen for the study because it has proved to be extremely sensitive to stress compared with other species of fish reared in aquaculture facilities.

“For example, you need to fit dimmer switches over their pools because if you suddenly switch on the light, you risk losing thousands of fish. Moreover, when we put fish larvae in a beaker in the lab, we noted on more than one occasion how they splashed around frantically before settling on the bottom of the beaker—dead,” adds Ivar Lund.

The pike perch has long been bred extensively in Eastern Europe, but the species has only recently been introduced to intensive farming programmes that involve recirculation technology. For this reason, fish farmers are hungry for knowledge about the best possible rearing and nutritional conditions for the pike perch.