- Nytt fra HAVBRUK nr 1/2009 (PDF-310.7 KB)
What happens when farmed cod meet wild cod
Why do cod escape, and what impact do they have on the wild cod stocks? The latest issue of the HAVBRUK programme newsletter takes a close look at escaped production cod.
"Cod are real escape artists. They are good at finding openings - and if necessary, they will even bite a hole in their cage to get out," says Anders Fernø of the University of Bergen. "This behaviour could mean that cod escaping from their cages may become a bigger problem than escaped salmon."
The University of Bergen's Justin Meager, Jon Egil Skjærsaasen, Anders Fernø and Frank Midtøy study production cod in a tank. (Photo: Skjalg Ekeland)
Together with researchers Justin Meager and Jon Egil Skjæraasen, Professor Fernø has been studying what happens when escaped production cod meet their wild counterparts in the fjord. Would the farmed cod seek out the spawning grounds? Would they disrupt spawning? Would the farmed and wild cod interbreed?
Researchers are seeking answers to questions like these to determine the potential harm of escapes.
Cod and escapes is the main topic in the most recent HAVBRUK programme newsletter ("Nytt fra HAVBRUK" no. 1/2009, available in Norwegian only).
Selective females
Professor Fernø and his colleagues have conducted both laboratory and field experiments. In one laboratory trial, the researchers placed wild and farmed cod of both genders in large tanks, then filmed their behaviour. Wild and farmed cod were shown to keep a certain distance from one another. Wild females preferred wild males during spawning, although this is no guarantee against cross-fertilisation.
"During spawning, a female normally selects a specific male to spawn with," explains Justin Meager. "As soon as she releases her eggs, however, other males race toward her, which means that farmed male cod nearby could also fertilise the eggs."
Dr Meager found furthermore that wild males are not as selective in choosing a "partner" at spawning. It makes no difference to them whether they release their milt over the eggs of a farmed cod or a wild one.
Field trials
Laboratory experiments are one thing, but behaviour out in the fjord could be something quite different. In one field trial, 48 cod (females and males, wild and farmed) were tagged with sensors and released into a fjord. The trial showed that farmed cod and wild cod do behave differently, and in a way that from the outset reduces the chance of their interbreeding after escape.
"Among other findings, we saw that production cod and wild cod swam to different depths, even if they were in the same place in the fjord," notes Jon Egil Skjæraasen. A likely explanation is that farmed cod are accustomed to their feed sinking from the surface, so they swim higher in the water column than wild cod.
Not the same, but similar enough
"The project has documented that production fish males and wild fish males behave differently," reports Professor Fernø, "and that this to some extent hinders hybridisation."
"In a competitive one-on-one situation, the wild fish would have the advantage and rebuff the approach of a farmed cod. But this does not help much when the escaped fish gain numerical superiority," adding that in one recent incident alone, 150,000 production cod escaped en masse. "Escapes like that cause a serious threat to a wild stock that is already overtaxed."
"In addition to the threat of interbreeding, a large number of escaped cod would compete with the wild fish for area and resources, which could lead to the spread of disease and in the long term to changes in behaviour and 'culture' in the wild cod stock."
Technical solutions
Professor Fernø believes that although the trials they have conducted show somewhat of a natural barrier between wild cod and farmed cod, it is still necessary to work toward better technical solutions to prevent escapes.
"Land-based facilities would be the most secure, of course. More realistic approaches would include more secure cages with double nets, and thorough studies in order to avoid placing the facilities near known spawning grounds."
- Published:
- 20.05.2009
- Last updated:
- 27.05.2009
Attachments
Contact
- Rolf Giskeødegård +47 22 03 70 97 rg@forskningsradet.no
