As noted in prior posts, the creation of protected zones in which fishing is prohibited has been shown to be an effective method for assisting the recovery of fish that do not migrate over great areas. For example, protected zones in the region of the Great Barrier Reef in Australia have seen fish populations double over five years. But, can such a strategy assist species that migrate over great areas?
Some researchers had a clever idea. They decided to assess the impact of World War II on migratory fish stocks (cod, haddock, and whiting) in the North Sea. The researchers used data generated by the British Ministry of Agriculture and Fisheries for a region known as Buchan, which is off the northeastern coast of Scotland, from 1928 to 1958. Because of its proximity to Germany, fishing in this region more or less stopped during WWII. The change was significant. Fishing activity was reported to be at around 300,000 hours in 1938, but nearly zero in 1939. The species in question in this area migrate toward Iceland, where fishing did continue.
The researchers found that the populations of all age groups for all three species declined between 1928 and 1939. During WWII, most of the populations rebounded, with older fish showing the largest increase. The youngest haddock did decline; the researchers believe this reflects the large population of older fish that preyed on the smaller, younger fish. Once WWII was over in 1945, the population of all 3 species resumed their decline. There was a surge in numbers around 1955 for reasons unknown, indicating that we do not fully understand the entirety of population dynamics. However, the key result was that laying off, even for just 6 years, had as big an impact on migratory fish as on sedentary ones.
The report can be found at http://www.ncbi.nlm.nih.gov/pubmed/20625698.