At present, little direct evidence exists for negative interactions of escaped and wild Atlantic cod juveniles or adults, despite predictions that negative consequences will result (Bekkevold et al. 2006). Cod farming is a relatively new industry, thus if negative consequences exist they may not have had sufficient time to manifest and/or be detected. Telemetry studies of simulated cod escapes have indicated that escapees, regardless of whether they originated from stocks of coastal or oceanic origin, mix with wild populations in fjord environments and can move to spawning grounds in the spawning season (Uglem et al. 2008, 2010). Behavioral studies have further indicated that escaped farmed cod are likely to hybridise with wild cod (Meager et al. 2009). However, farmed cod may have limited reproductive success in sperm competition with wild cod, which lowers the risk of genetic introgression from escapees (Skjæraasen et al. 2009).
Other possible ecological effects of escaped farmed cod include increased predation pressure on out-migrating wild salmon smolt (Brooking et al. 2006) and transmission of pathogens and parasites to wild populations (Øines et al. 2006), although direct evidence for these effects is at present lacking. Recaptures of Atlantic cod escapees equipped with acoustic transmitters in local commercial and recreational fisheries in Norway are known to be high (approximately 40%; Uglem et al. 2008), indicating that local fisheries receive temporary increases after escape events and may be partially effective in reducing escaped cod numbers.
Possible impacts of ‘escape through spawning’ of Atlantic cod
In the culture of Atlantic cod, some fish mature during the first year of culture, while a majority of farmed cod are believed to mature during the second year. This means that almost the entire culture stock in any particular farm has the potential to spawn in sea-cages before they are slaughtered. Spawning of Atlantic cod within a small experimental sea-cage containing 1000 farmed cod and dispersal of their spawned eggs in a fjord system has been demonstrated (Jørstad et al. 2008). In the proximity of this experimental sea-cage, 20-25% of the cod larvae in plankton samples were determined by genetic analyses to have originated from the 1000 farmed cod (Jørstad et al. 2008). Furthermore, preliminary results indicate that 4-6 % of juvenile cod (35-40 cm total length) caught in the area around the farm in following years were offspring of the farmed cod (van der Meeren & Jørstad 2009). This illustrates that if spawning occurs within commercial cod farms where numbers of farmed individuals are far greater, the contribution of ‘escaped’ larvae to cod recruitment within fjord systems may be substantial.
Escape of large quantities of eggs from caged cod could lead to ecological and genetic effects in wild populations (Bekkevold et al. 2006, Jørstad et al. 2008) as; 1) coastal cod populations in some areas of Norway are presently weak, most likely due to overfishing (ICES 2008); 2) coastal cod have a high fidelity to specific spawning grounds (e.g. Wright et al. 2006); and 3) sea-cage cod farms are often located within short distances of known wild cod spawning grounds (Uglem et al. 2008). Recent research also suggests that cod eggs may be entrained in the vicinity of the spawning grounds long after spawning (Knutsen et al. 2007). Therefore, there is considerable potential for larvae from escaped cod eggs to experience favourable conditions for survival and recruitment to coastal cod stocks if spawning in sea-cages occurs during the natural spawning season of wild cod.