Vertical distribution: hybridisation barrier between escapee and wild cod?

Most farmed cod are produced in net-pens in sheltered coastal areas used as habitat by coastal cod. As future expansion of the industry is expected, interactions between farmed escapees and local coastal stock are inevitable. Genetic introgression from es capees has caused fitness depression in wild salmon populations and similar effects may occur with cod. Further, even if excluded from mating, escapees may still disrupt spawning events by invading local shoal spawning shoals, which can decrease the repro ductive output of wild populations. Results from stock enhancement research indicate that reared cod remain within a few kilometres of their release site and enter local spawning grounds. However, ongoing laboratory research indicates that farming produce s behavioural deficits in cod that may reduce their spawning success in the wild. In particular, farmed cod do not seem to develop natural vertical migratory behaviour, but maintain positions close to the surface. This may spatially separate escapees from wild spawning shoals. It may also limit the reproductive success of farmed cod, because mating of wild cod typically involves substantial vertical movement. Vertical distribution may therefore provide a barrier to both hybridisation between escapees and wild cod, and to disruption of spawning behaviour in wild shoals. We will test if behaviour provides a hybridisation barrier between farmed and wild cod by simulating an escape of farmed fish close to a natural spawning shoal. For the first time, the ver tical distribution and behaviour of farmed and wild cod will be examined, individually, and in relation to natural spawning shoals, using biotelemetry, video and sound recordings. This information is crucial to understand the possible ecological and genet ic impacts of cod escapes on local wild populations, and at the heart of Sub-Program 3: Sustainability of the HAVBRUK program. The results will also aid in efficient and focused recapture of escapees.