The effects of parasites on fish populations—theoretical aspects

Abstract

This brief review has indicated how essential aspects of relevant epidemiological considerations may be included into models of fisheries. The main conclusions to emerge from the crude models outlined above are that the ability of a pathogen to establish itself is dependent upon the relative magnitudes of the threshold host density for parasite establishment, H t, and the level of the host population density at its current level of exploitation H(E). If H t is less than H(E), the parasite will always be able to establish itself. Disease prevalence would also seem to be roughly independent of the level of exploitation of the fishery, providing exploited population density is significantly higher than the threshold density for disease establishment. The complications introduced by the presence of disease will in general further increase the levels of uncertainty that fisheries managers have to contend with (Beddington, 1984). In cases where pathogens are having a serious impact on the fishery it would seem sensible to develop methods to quantify the impact of the parasite on the host (Lester, 1984). The simple models discussed here can, moreover, readily be extended to include other factors which can be important in determining management strategies for fisheries where parasites and disease are an important consideration. Three particularly important such considerations are: inclusion of age-structure and more realistic density-dependent recruitment functions in the host population (May, 1980); consideration of the immune response of the host to the parasite (Anderson & May, 1979); and inclusion of environmental stockasticity (May, Beddington, Horwood & Shepherd 1978; Ludwig & Walters, 1981).