FARMED COD SPAWNING IN NET PENS: OFFSPRING RECRUITMENT UNDER NATURAL CONDITIONS AND INTERBREEDING WITH WILD LOCAL STOCK

Escapes of farmed fish from the aquaculture industry as well as large-scale stock enhancement/ranching activities are regarded as a risk factor for negative genetic impacts on native gene pools. The Atlantic cod (Gadus morhua L.) is now the most interesti ng new marine species for large-scale farming in Norway, and more than 500 commercial farming licenses have been issued (production capacity - 300 000 tonnes). Compared with the salmon, cod farming presents new challenges. Cod are able to spawn in the net pens during the on-growing period and thus release genetic material into the environment. Wild coastal cod stocks are depleted and possibly vulnerable to genetic interaction with farmed cod. In a pilot experiment in 2006, it was demonstrated for the firs t time that farmed cod are able to produce viable cod larvae that mix with larvae from wild cod. The farmed cod used in the pilot experiment was homozygote for a rare allele (GPI-1*30), and 20 - 25% of the larvae sampled in the area possessed this unique genetic marker and could be traced back to the spawning pen. In 2007 a full-scale spawning experiment with the same genetically tagged strain was carried out. This year the contribution of larvae from the farmed cod was estimated to about 35 %. Now a sig nificant fraction of the two last year classes of cod in this area consist of offspring of farmed cod, and there are urgent needs to investigate the recruitment success of these fish and the extent of interbreeding with the wild cod population. Through in tensive fishing of juveniles and mature fish, reproductive success of the farmed offspring and interbreeding with the wild cod can be estimated through the presence of the genetic tag in the new yearclasses produced.