THE EFFECTS OF PARASITES ON FISH POPULATIONS-THEORETICAL ASPECTS A.P. DOBSON AND R.M. MAY Department of Biology, Princeton University, Princeton, NJ 08544, USA INTRODUCTION The prime task of the fisheries scientist is to emphasize how the innate characteristics of the fish population and its environment determine and constrain its productivity as a resource. Of particular interest is the task of disentangling the roles of natural and man-made factors in generating the observed changes in exploited fish populations. Although parasites may have a considerable effect on a fishery, models for fisheries and epidemiology have not previously been combined to consider how the presence of a parasite or pathogen might affect the management of a fishery. Parasites may be important in reducing fish numbers by mortality, by reducing fecundity or by reducing the weight of individual fish. In this paper we will attempt to blend mathematical models for host-parasite relationships with those used by fisheries biologists to determine how the pathogens can effect the dynamics of exploited fish populations. zyxwvutsrqponm M icroparasites and macroparasites Models for two general classes of pathogen will be explored. Initially we will consider the dynamics of microparasitic pathogens of fish. These diseases are characterized by their ability to multiply withiq infected hosts and their ability to produce some form of immune response in the host to their presence. Models for this type of disease tend to divide the host population into susceptible, infected and immune classes of hosts (Anderson & May, 1979). We are thus concerned with viral, bacterial, fungal and protozoan parasites in this section of the paper. The majority of these pathogens are directly transmitted, and their pathogenicity to the host is dependent upon the natural resistance of the host to infection as well as its nutritional status and environmental factors such as temperature. The tendency of many commercial fish species to live in dense aggregations or “shoals” makes them highly susceptible to these kinds of pathogens. We shall then consider models for the macroparasites, the parasitic helminths and copepods. Mathe- matical models for these species need to consider the statistical distribution of parasites throughout the host population. Pathology is usually dependent upon the number of infective stages present within any individual host and transmission is by either free living egg stages or more commonly via an intermediate host. Table 1 catalogues the effects of a variety of different macro- and microparasites upon a number of fish species of economic importance. This coarse comparison suggests the microparasites are generally more important in causing increased mortality rates and reductions in fecundity, while macroparasites tend to cause reductions in the condition of fish which reduces their market value. Kabata (1970) attempted to estimate the effects of the copepod Lernaeocera branchialis, on the haddock, M elanogrammus aeglefinus. Not only does each copepod cause an average weight loss of 29 grams per fish, but the cost of handling & trimming the fish to detect and remove parasites may increase the cost of packaging fish up to 80%. 363