Relationships of the Host, Pathogen, and Environment: Implications for Diseases of Cultured and Wild Fish Populations

—Many effects of diseases on cultured fish are known; they are less clear in wild fish populations. Cultured fish represent captive populations that can be subjected to intense scrutiny with an increasing range of diverse and powerful tools. Disease represents a spectrum from acute mortality to rather benign or inconsequential syndromes, all sharing the common feature of a deviation from the normal structure or function of the host. Understanding these deviations among cultured and wild fish populations and balancing their implications against ecological, economic, and political concerns are challenges for both fish health scientists and fisheries managers. The severity of a given disease is dependent on the interaction of numerous variables of the host, the parasite, and the environment. To understand diseases and their impacts on fish populations, we must know which variables are important, how we measure them, and finally how we assess the results of our measurements. We have perhaps been most successful with variables associated with the pathogen. We often can more easily isolate and scrutinize the pathogen than either the host or the environment. The host variables of importance (for which we lack considerable knowledge) include actions of the immune system in general and specifically the influence of genetics and nutrition on host resistance–susceptibility to disease. Lastly, the contribution of the environment, a nebulous term encompassing everything other than the host and pathogen, is only partly appreciated. While we can measure certain physical and chemical parameters of the environment, we have a poor understanding of the biological–ecological variables that influence host–pathogen interactions. Ultimately, diseases of wild fish must be considered in the context of these complex interactions including numerous physical, chemical, biological, and ecological parameters, which may yet be discovered as integral parts of the aquatic habitat. Diseases are an integral part of the existence of all animals including both cultured and wild fish populations. Elton (1931:435) illustrates a widely held misperception of the public and scientific community regarding diseases in wild animal populations. He stated, ‘‘Up to the present time it has been customary to believe that wild animals possess a high standard of health, which is rigidly maintained by the action of natural selection, and which serves as the general, though unattainable, ideal of bodily health for a highly diseased human civilization. This belief is partly true and partly false.’’ Although it is evident that human activities have directly altered the health of fish health populations by direct perturbation of habitats and ecosystems, diseases are natural phenomena in wild fish populations (Sindermann 1990:57; Whittington et al. 1997). Indeed, we must examine the complex interactions of numerous variables if we wish to understand diseases in both cultured and wild fish populations. It is therefore incumbent upon those charged with protecting our resources to understand these variables of the host, pathogen, and * E-mail: [email protected] the environment in making sound decisions on fisheries management. Diseases of Wild and Cultured Fish The knowledge of diseases of captive fish is much greater than that of wild fish for both logistical and historical reasons. Aquaculture provides captive populations that we can scrutinize throughout their existence in a somewhat controlled environment. These cultured fish have been the subject of numerous investigations, many into the roles of pathogens and the environment in disease. These studies have employed an increasing range of diverse and powerful tools that exploit modern advances in molecular biology and human health. The origins of the fish health sciences in North America can be traced to the development of federal and state hatchery systems. The first specialists in fish health worked directly with these captive fish populations, principally salmonids often raised in mitigation hatcheries in the eastern and western USA. This historic connection between the fish health sciences and captive fish propagation has continued to the present; state and federal agencies are the principal employers of fish health scientists. In contrast with captive fish, understanding diseases