Vertical dynamics and reproductive behaviour of farmed and wild Atlantic cod Gadus morhua

Abstract

Rapid development of fish mariculture and efforts to restock depleted populations have raised concerns about the effects on wild populations and the risk of genetic introgression from hybridisation. Reproduction of many marine fish species involves substantial vertical movements, but nothing is known of the vertical dynamics of farmed fish in spawning shoals. We combined laboratory observations and field biotelemetry to examine the depth-related reproductive behaviour and vertical dynamics of individual farmed and wild Atlantic cod Gadus morhua (L). We first tested the prediction that the depth distributions of male and female wild cod differ both in the laboratory and in the field. We then tested the prediction that farmed cod have a shallower depth distribution than wild cod and thus lack the sex-based vertical dynamics of wild cod. In both the laboratory and field, male farmed and wild fish tended to be located deeper than females, and wild fish were deeper than farmed fish. In the field, wild males were near the seafloor, considerably deeper than wild females and farmed fish. Residency on the spawning ground was longer for wild fish, but a substantial proportion (61%) of farmed fish remained for more than 3 d. Our study reinforces the idea that vertical dynamics play a key role in reproduction of an important fisheries species, and demonstrates clear differences in vertical distribution between farmed and wild fish on a spawning ground. The results suggest that escaped farmed cod, and in particular, farmed females, are likely to hybridise with wild cod.

Environmental regulation of individual depth on a cod spawning ground

“…Recently fertilised cod eggs were found in all 12 samples. Further information about the wild and farmed fish can be found in Meager et al (2009Meager et al ( , 2010.…”

“…The inter-batch interval of cod females is highly affected by ambient temperature (Kjesbu 1989), and females are likely to seek out a depth that optimises the developmental time of oocytes. The spawning event itself, where a male and female engage in a mating embrace (Brawn 1961, Skjaeraasen & Hutchings 2010, is associated with upwards vertical movement (Meager et al 2009). Finally, although cod appetite during spawning is believed to be reduced (Fordham & Trippel 1999, Skjaeraasen et al 2004, cod do feed at this time (Michalsen et al 2008), and fish may thus undertake foraging excursions towards the surface.…”

“…Studies of coastal spawning populations are particularly problematic, because they are often either associated with bathymetric features (Marteinsdottir et al 2000, Ames 2004, Lindholm et al 2007 or are within heterogeneous inshore habitats such as fjords and inlets (Green & Wroblewski 2000, Knutsen et al 2007. Recent experiments designed to examine interactions between farmed and wild cod on a natural spawning ground (Meager et al 2009 provided a unique opportunity to explore the responses of individual cod to environmental gradients in detail, as well as the possible effects of previous experience by comparing wild cod and cod raised in captivity. Individual movements of fish were here tracked 3-dimensionally in real-time on the spawning ground using biotelemetry along with simultaneous collection of data on temperature, salinity, light, water current and weather conditions.…”

Net-biting and escape behaviour in farmed Atlantic cod Gadus morhua: effects of feed stimulants and net traits

“…The potential for negative ecological consequences is significant (Bekkevold et al 2006). For example, research has shown that escaped farmed cod are present on spawning areas for wild cod during the spawning season and that wild and escaped cod are likely to interbreed (Uglem et al 2008, Meager et al 2009). Within sea cages, farmed cod are also capable of producing viable cod larvae which subsequently mix with larvae from wild cod in the areas around cod farms (Jørstad et al 2008).…”

Spatiotemporal distribution of coastal and oceanic Atlantic cod Gadus morhua sub-groups after escape from a farm

“…It has been shown that escaped farmed cod are present in spawning areas for wild cod during the spawning season and that wild and escaped cod are likely to interbreed , Meager et al 2009). Farmed cod are also capable of producing viable cod larvae within sea cages, which subsequently mix with larvae from wild cod in the areas around cod farms (Jørstad et al 2008).…”