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Fisheries biology: War dividend

via The Economist: Science and technology on 8/19/10

Fisheries biology

War dividend

The second world war led to a boom in North Sea fish numbers

Aug 19th 2010

SOME experiments are hard to conduct. Fisheries biologists are, for example, reasonably confident that creating protected areas in the sea, in which fishing is forbidden, encourages the recovery of those species that stay put in the area. This has worked in several places in the tropics, notably the Great Barrier Reef in Australia, where fish populations in protected zones have doubled in five years. They are less confident, however, that it applies to places where the fish of interest are migratory, as is often the case in temperate-zone fisheries like those of the North Atlantic and its adjacent seas.

Closing such places to fishing in order to find out is politically difficult. But 71 years ago politics did dictate one such closure, and a group of biologists, led by Doug Beare at the European Commission's Office of Maritime Affairs, has now taken advantage of it. The closure in question was the little matter of the second world war, and Dr Beare and his team have been looking at its effects on the population of cod, haddock and whiting in the North Sea.

The information they worked with was collected by Britain's Ministry of Agriculture and Fisheries from a region known as Buchan, off the north-eastern coast of Scotland, between 1928 and 1958. During the war British trawlers continued to plough the waves of the eastern Atlantic, between Britain and Iceland, but more or less stopped fishing in the North Sea, which was nearer Germany and thus much riskier. The reports reveal that fishing activity in the Buchan region dropped from 300,000 hours a year in 1938 to practically zero after the war started in 1939. The fish species in question, however, migrate between these two areas, so the question was how much the reduction of fishing effort in part of their range would affect their numbers.

As they reveal in Naturwissenschaften, Dr Beare and his colleagues found that populations of all age groups of all three species declined steadily between 1928 and 1939. There was a particularly steep fall—over 80%—in the number of two-year-old haddock. During the war, though, most of the populations rebounded, with older fish showing the biggest increase. The number of ten-year-old haddock, for example, went up nearly twelvefold during the six years the conflict lasted, though year-old haddock actually declined by 50%. The team propose the theory that such yearlings are truly, in this case, the exception that proves the rule. Such small fry are prey, and as the number of older, predatory fish increased, yearling haddock suffered disproportionately.

Once hostilities ceased in 1945, the populations of all three species resumed their pre-war decline though, intriguingly, there was a surge in numbers around 1955. The reasons for this post-war baby-boom are obscure—and show that there is still much to learn about fisheries biology. But one lesson is clear. Laying off, even for just six years, has as big an effect on migratory fish as it does on sedentary ones.

Science and Technology