“Escapes” of Eggs from Farmed Cod Spawning in Net Pens: Recruitment to Wild Stocks

Abstract

Escapes of farmed fish from aquaculture installations, as well as large scale stock enhancement and sea ranching activities, are regarded as risk factors for negative genetic impacts on native gene pools. These aspects have been discussed for several decades, and a number of recent experiments on Atlantic salmon have clearly demonstrated dramatically reduced overall fitness of farmed offspring compared with offspring of wild origin. The Atlantic cod (Gadus morhua L.) is now the most promising new marine species for large scale farming in Norway, and more than 500 commercial farming licenses have been issued, corresponding to a maximum annual production capacity of about 300,000 tonnes. In comparison with the environmental problems caused by escapes from the salmon farming industry, cod farming presents new challenges. As a marine species, cod are able to spawn in the net pens during the ongrowing period and thus release genetic material into the environment. Coastal cod stocks in Norwegian waters are depleted and therefore may be particularly vulnerable to genetic interaction with farmed cod. Thus, there is an urgent need for more detailed knowledge of potential hazards. A pilot experiment performed in Heimarkspollen in Austevoll (a practically landlocked fjord) demonstrated for the first time that farmed cod are capable of producing viable cod larvae that mix with larvae from wild cod. The farmed cod used in the pilot experiment were homozygotic for a rare allele (GPI-1*30), and mature fish were placed in a net pen to allow spawning and egg drift into the surrounding water to take place. Surveys of eggs found the highest densities in the vicinity of the net pen. Later larval surveys found that 20% to 25% of the larvae sampled in the area possessed the unique genetic marker and could therefore be traced to the spawning cod in the net cage. Numerous genetically marked larvae were also found outside Heimarkspollen, indicating a high degree of dispersal of eggs and larvae by tidal advection.

Keywords:

• Atlantic cod
• Gadus morhua
• genetic marking
• larval recruitment
• farmed/wild interactions

ACKNOWLEDGEMENT

We are indebted to the staff at Parisvatnet Field Station and Austevoll Research Station for reestablishment of the genetically marked cod strain.