Science - July 7, 2010

Plaice is thriving again

The North Sea is full of plaice. Stocks are almost back to nineteen eighties levels: well over 400 thousand tons. According to Jan Jaap Poos, fisheries biologist at Wageningen Imares, reduced fishing pressure has made a big contribution to the recovery of the population. But fishers question the research methods.

'North Sea plaice is doing fine again. The total stocks have grown considerably over the past five years. After record stocks mid-nineteen eighties, a combination of factors led to a big dip ten years later. There were fewer young fish coming in and there was a lot of fishing going on. But since 2002, deaths through fishing have gone down a lot. Fishing quotas were cut and the fleet shrank. The price of plaice was low too, so that this sort of fishing didn't pay. The fishers concentrated on the more expensive sole.
'We are in a transition phase now in fisheries management. For a long time, the catch quotas were based not just on biologists' advice but on a political game. The final quotas were often higher than the advice. But since 2007 there has been an EU management plan for plaice as well as for sole, and it should lead to increased stocks of these species. The basis of the plan is a big reduction in fishing pressure.
'About 60 percent of the plaice population used to be fished. Thanks to the management plan, that is now down to 30 percent: approximately 120 thousand. Since about 40 to 50 percent of the total catch is too small to sell and is thrown overboard, dead, the current quota of saleable plaice is more than 70 thousand. The EU espouses the principle of the 'maximum sustainable catch'. If you applied those principles now, it would mean a slightly lower plaice quota of no more than 64 thousand tons.'
'The fishers' criticisms of our method of estimating stocks are understandable. Those estimates are made with models that calculate on the basis of samples of the spawn population. That always means a degree of uncertainty, so it is not surprising that estimates have to be adjusted later in the light of new information. Fishers see these adjustments as 'mistakes', whereas biologists see this as a logical consequence of the margin of error in estimates. In the case of plaice it turned out we had underestimated stocks, while for sole our estimate was too high.'