Aquatic pollution: effects on the health of fish and shellfish

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Abstract
SUMMARYAs there is little evidence of pollution affecting the health of fish and shellfish on a global scale, this paper attempts to put into perspective the pollution/fish disease relationship by reviewing examples of studies and reports in the historic and current literature. Although there is no dispute that pollution can affect the health of aquatic organisms under laboratory conditions and may be responsible for the decline of populations of such animals in some inland waters and some estuaries, most of the evidence for pollution causing or increasing disease in fish in open waters is circumstantial. Historical data proves that almost all fish and shellfish diseases known today have been described since the end of the last century. However, it is also known that water pollution, especially in inland waters, has for the past 400-500 years been the result of urbanization and industrialization. This has resulted in some major rivers becoming devoid of or deficient in fish stocks. The concern that pollution may influence the health status of fish and shellfish stocks has increased over the past 20 years. Initial attention was paid to epidermal diseases, including fin-rot in demersal fish, and protozoan diseases in molluscs in the heavily polluted bays and estuaries in North America. As the interest in this subject spread, it became political, and often controversial, especially amongst the North Sea countries. The disagreements have largely been settled amongst scientists because international bodies, such as the International Council for Exploration of the Sea (ICES), established workshops to investigate sampling methods and disease-reporting techniques. Recommendations from those workshops have contributed to some form of standardization for field work and the subject, although largely subjective, has some objective approaches which are described. As there are variable, interacting biological and physical influences in the aquatic environment, it is difficult to establish the background prevalences of diseases in populations offish and shellfish. Examples of the influences of climatic changes are presented, and these show that short-term catastrophes can be directly related. However, a more long-term problem is water acidification resulting largely from anthropogenic activities. In parts of Scandinavia this has, and is, leading to decimation offish stocks in inland waters. In general, diseases in fish and shellfish are very localized, but there is concern amongst scientists that certain cancers, especially liver tumours, occurring in demersal fish inhabiting polluted estuarine and coastal waters, are related to the release of chemicals, e.g. hydrocarbons, pesticides and heavy metals. This subject is discussed in detail, with examples of the author's own studies in North Sea fish. It is concluded that cancers in fish are of extremely low prevalence, and only present in a very few species, and then only in the oldest animals. Though changes in disease pattern may well be an indication of adverse environmental effects, further research is necessary for conclusive evidence.