Abstract
Cod (Gadus morhua) aquaculture is a rapidly developing industry in coastal areas of the North Atlantic. Preventing escapes from sea cages has proved extremely difficult, and the close proximity of these cages to coastal cod populations means that interactions of escapees with wild stocks are inevitable. Very little is known, however, of how escapeeswill affect populations that are already over-exploited and at historical lows. Experience with salmon indicates that escapees may interbreed with wild stock causing genetic introgression and fitness depression in wild populations. Farmed escapees may also directly influence wild stocks by entering spawning shoals and disrupting the mating behaviour of wild fish. Substantial evidence indicates that the cod mating system is highly complex, involving both behavioural and acoustic courtship displays, and considerable vertical movement. Females are thought to attend male dominated aggregations when they are ready to spawn. The risks imposed by escapees may therefore depend on gender. As mating success of cod in the laboratory is highly skewed towards aggressive males, understanding behaviour at the individual level is also important. However, problems associated with elucidating the behaviour of individual cod in the field has, to date, limited our understanding of not only interactions between escapee and wild cod, but of cod spawning behaviour in general. In our study, we tested for genderbased differences in the behaviour of escapee and wild coastal cod on a spawning ground in Western Norway. Forty-eight individual cod were released at the start of the spawning season in 2007 and 3-D fish movements in and around the spawning shoal were tracked with a stationary positioning system for 35 days. A subset of 8 fish was tagged with continuous transmitters for high-temporal resolution. Ongoing analysis is examining fish movements and behaviour in relation to water currents, surface light, salinity, temperature and sea-floor topography. Preliminary analysis indicates that while farmed cod may enter wild spawning shoals, there are significant gender-based differences in diel vertical movements of farmed and wild cod, and in other behavioural traits. These results are discussed in relation to possible effects of escapees on wild spawning shoals.