Farmed cod are capable of spawning out of season and producing surviving offspring, raising fears of farmed stocks interfering with wild populations, according to a recent study from Norway's Institute of Marine Research.

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A knowledge review conducted by the institute has found that even when farmed cod spawn out of season at the 'wrong' time of year, their offspring will not necessarily perish.

Given the reduced genetic variation among farmed cod, this has raised fears that escapee fish could mingle with wild cod and inter-breed, reducing variation within wild stocks.

"The greater the variation in the genes, the greater the probability that there are individuals in a population that cope with, for example, environmental changes better than other individuals," noted researcher Terje van der Meeren in a release. "Such changes could be increased temperature or ocean acidification from climate change."

The report was commissioned by the fisheries directorate due to fears of sexually mature fish escaping from farms along the west coast of Norway, especially following an escape from Norcod's farm in Nesna, Nordland county, which was discovered on Nov. 28, following a tear in the net pen.

The study notes that the success of out-of-season cod spawning is dependent on the water temperature, as cod prefers temperatures below 10 degrees Celsius. Yet researchers also found that due to the depths of cod farming facilities, the temperature is low enough that eggs can survive outside of the spawning season for large parts of the year.

Even at higher temperatures, eggs can still be successfully fertilized, the study found, which is especially important where farmed cod is concerned.

"There are so many farmed cod in a cage that if many of these spawn, the number of eggs will be so high that there is certainly a chance that some of these will hatch and develop normally regardless of the temperature being a bit high," van der Meeren said.

In recent years, wild cod has also been found successfully spawning outside the main season from February to April in both the Baltic Sea and around the Canadian province of Newfoundland.

For fish that spawn in late autumn or early summer, there is also food available in the form of jumping crayfish, the researcher found.

"It is, therefore, likely that some of the cod eggs that are spawned at the 'wrong' time have the opportunity to develop into hatching and that the cod larvae find enough food to grow up," the institute concluded.