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The Joint Research Centre (JRC) is the European Commission's science and knowledge service which employs scientists to carry out research in order to provide independent scientific advice and support to EU policy.
A study published by a team of European scientists shows how the cessation of commercial fishing in the North Sea caused by World War II led to a profound change in the age structure or ‘complete demographic transition’ in the populations of resident fish.
Understanding the mechanisms via which commercial fishing pressures, management regimes and environmental changes interact with the ecology of wild fish populations is a key question for fisheries management science. Such questions are, however, difficult to address using recent time-series datasets as fishing pressure has been sustained and ubiquitous over recent decades. An unintended ‘experiment’ in fisheries science did however occur in the North Sea during World War II when commercial fishing almost ceased because fishing vessel movements were restricted due to wartime dangers, and the fact that many fishermen were called up, and their vessels requisitioned for war service.
This study, published on Naturwissenschaften  and started at the JRC's Institute for the Protection and Security of the Citizen (IPSC) as an Exploratory Research Project, examines the unintended effects of the 6-year closure of the North Sea during World War II on migratory fish species, such as cod, haddock and whiting.
Creating protected areas where fishing is not allowed has proven useful to help the recovery of sedentary fish populations, but the utility of Marine Protected Areas (MPAs) for protecting migratory fish stocks in the temperate areas of the North is still debated. This study demonstrates that migratory fish populations benefit from the protection of marine areas against commercial fishing as well as sedentary species.
Scientists combined commercial landings and effort data with scientific trawl survey data, spanning the period 1928 to 1958 demonstrating how the war-mediated closure resulted in an instant increase in catch per unit effort in cod, haddock and whiting. Using only survey data they then examined how age-structures changed as populations transited the war period. They show that the mean levels of the oldest fish responded first, and that this response was followed by younger counterparts in an age-dependent wave or cascade.
This is the result of the cumulative effects of enhanced survival on the older fish during the war. After WWII, when fishing recommenced, the wave was again observed in reverse order with the older fish succumbing first to the increasing (fishing) mortality. At first sight this can seem rather obvious but no allusions to the observation could be found anywhere in the scientific literature.
Furthermore, the war mediated closure provided an opportunity to split potential environmental drivers (temperature, salinity) from those due to commercial fishing; a genuine and recurring issue in modern fisheries science and management.
 An unintended experiment in fisheries science: a marine area protected by war results in Mexican waves in fish numbers-at-age
Doug Beare, Franz Hölker, Georg H. Engelhard, Eddie McKenzie and David G. Reid
Naturwissenschaften - 2010, Volume 97, Number 9, Pages 797-808