Population fluctuations and recruitment in marine populations

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Abstract
This paper is not concerned with the effect of oil pollution as such, but sets out to consider the range of natural variation and the extent to which this might mask the effects of other factors such as possible pollutant effects. To detect the effect of a pollutant (or a change in fishing effort) it is necessary to allow for natural variations, both random and periodic. Some examples are given to illustrate the extent of natural variations, in a variety of marine populations. For Arcto-Norwegian cod, information is available on catches from about the middle of the last century to the present day. Large catches are about 3-6 times the size of small catches. The difference between large and small catches is about equal to the mean catch. Further, the time taken to change from a small to a large catch level is very variable. The Greenland cod provides an example of a stock that increased very considerably due to a northerly increase of the limits of distribution of the species. This increase was associated with a warm period in the North Atlantic and with increased catches. Many fish stocks and in particular many species of pelagic fishes, exhibit much larger fluctuations in stock size, e.g. the Hokkaido herring, the Japanese sardine, the Bohuslan herring, the Atlanto-Scandian herring and the Califonian anchovy and sardine. Fluctuations also occur in invertebrate species and evidence is given of changes that have occurred in North Sea phytoplankton and zooplankton. The Peruvian anchovy provides an example of a stock that decreased very considerably, due partly to fishing and partly to changes in the hydrographic regime that caused the fish to become more available for exploitation. Fluctuations in fish stocks are primarily due to fluctuations in recruitment. The factors affecting recruitment are not yet fully understood but are known to be determined during the first year of life and probably during the larval or early juvenile stages. Whatever the mechanism, however, it is the variations in recruitment that determine a very large part of the variations in adult fish stocks and hence it is variations in recruitment and the causes of these that are important. In conclusion, the examples show that natural communities can exhibit large natural fluctuations, of varying periodicity, in the long term. Apart from incidents where there is gross pollution, an effect of pollution can therefore only be convincingly demonstrated for those species for which background information is available for a long enough period to allow for long-term periodicity as well as for short-term and irregular variability.