Science - May 29, 2012

Farmed sole reproduces after harsh winter

For the first time, farmed sole has managed to reproduce, bringing commercial sole farming one step closer.

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Exposure to a cold winter seems to have done the trick, although the underlying mechanism still remains unclear. These results were presented last week by researchers from Imares , who are now working to get them published.
Accidentally
The team discovered accidentally last year that it was either daylight or the cold which had set off a signal for reproduction. In another project, the researchers kept soles in open air ponds throughout one winter. To everyone's surprise, the ponds were teeming with small fishes in spring. A DNA analysis dispelled any doubts: the soles had indeed reproduced. 'It was really a lucky coincidence,' says Robbert Blonk, Imares project leader.
Harsh winter
More than half a year later, it now becomes clear that a cold winter is the crucial factor. The researchers proved this by cultivating soles under different conditions. The fishes were given artificial or day light, combined with a 'mild' or a 'harsh' winter, with water temperatures at 7 and 4 degrees Celsius respectively.
Conjecture
Blonk is now looking for the mechanism behind this. He thinks that wild animals time their reproduction according to natural cycles, such as the seasons. Farmed soles are not exposed to this aspect during their lifespan, and only a stronger signal could trigger a reproductive behaviour. Blonk is cautious, though. 'This is only a conjecture.'
This finding removes one of the obstacles in commercial sole farming. Farmed soles do not normally reproduce in captivity and new fishes have to be brought in from the North Sea time and again. Therefore, it has not been possible to breed a suitable fish in the farm, and returns keep falling short. This research work forms part of the Zeeland Sole project and is financed by various government sectors and a private sector consortium.

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