American Fisheries Society Symposium 44:529–537, 2004 © 2004 by the American Fisheries Society Evaluating and Understanding Fish Health Risks and Their Consequences in Propagated and Free-Ranging Fish Populations CHRISTINE M. MOFFITT1 USGS, Idaho Cooperative Fish and Wildlife Research Unit Department of Fish and Wildlife Resources, University of Idaho Moscow, Idaho 83844-1136, USA ALF H. HAUKENES2 Department of Fish and Wildlife Resources, University of Idaho Moscow, Idaho 83844-1136, USA CHRISTOPHER J. WILLIAMS Department of Statistics, University of Idaho, Moscow, Idaho 83844-1104, USA Abstract.—Fishery managers and resource conservationists are increasingly interested in understand- ing the fish health and disease risks of free-ranging fishes and whether propagated fishes or features and practices used at fish culture facilities pose a health risk to free-ranging populations. Disease agents are present in most both captive and all free-ranging fish populations, but the consequences and extent of infections in free-ranging populations are often difficult to measure, control, and understand. Sampling methods, protocols, and assay techniques developed to assess the health of captive populations are not as applicable for assessments of free-ranging fishes. The use of chemicals and therapeutics to control diseases and parasites in propagated fishes likely reduces the risk of introducing specific pathogens into the environment, but control measures may have localized effects on the environment surrounding fish culture facilities. To understand health risks of propa- gated and free ranging fishes, we must consider fish populations, culture facilities, fish releases, and their interactions within the greater geospatial features of the aquatic environment. Introduction: Quantifying Diseases in more readily monitored over time because individuals Free Ranging and Propagated Fish in the population are accessible. However, measuring and understanding the disease status of free-ranging In this paper, we review some of the challenges of populations, and measuring the impact of diseases in a measuring and interpreting the health status of propa- population, are difficult as sick fish are often removed gated and free-ranging fish populations. We propose a by predators, the populations are dispersed across space, framework to improve interpretations of the risks of and portions of the population may migrate to other disease and disease control in propagated and free- areas bringing new diseases and parasites with them. ranging fish populations. In propagated fishes, stresses associated with cap- We know much more about diseases and disease tivity and the close proximity of individuals in the processes of fish held in captivity than in free-ranging rearing environment can increase the vulnerability of fishes. The disease status of captive populations can be fish to infection and result in pathogen amplification and increased opportunities for disease transmission. 1 E-mail: [email protected] On the other hand, in controlled rearing systems, wa- 2 Present address: University of Alaska-Fairbanks, School of ter treatments and physical modifications can improve Fisheries and Ocean Sciences, Fishery Industrial Technology water quality and limit the exposure of propagated Center, Kodiak, Alaska 99615, USA fish to other fish, invertebrates, or pathogens. Cov- 529 530 MOFFITT ET AL. ered rearing containers can exclude predators and other affect pathogen development, survival rates, transmis- biological or physical vectors that may transmit patho- sion, and host susceptibility (Harvell et al. 2002). gens or serve as intermediate hosts. Biological, physi- It is likely that the spread of whirling disease, cal, and chemical control measures are often used in caused by Myxobolus cerebralis, was through planting fish culture to modify water quality and to enhance of pathogen positive fish by management agencies or resistance to infectious diseases and reduce pathogen by unapproved releases of positive fish (Nehring and loads and their effect on propagated fish (Winton Walker 1996; Bartholomew and Reno 2002). With 2001). Treatment of hatchery effluents containing Myxobolus cerebralis, there are other habitat compo- chemicals, nutrients, and pathogens reduces the im- nents that are independent of the fish host that can be pact of culture systems on the receiving environment affected by anthropogenic activities such as increased (Aitcheson et al. 2001; Boyd 2003; MacMillan et al. siltation, impounded waters, and other changes that 2003; Tacon and Forster 2003). change the host environment and pathogen relation- Management decisions regarding the siting of fish ships for native species and can multiply the risks of culture facilities are often made with a goal of mini- infections (Baldwin et al. 2000; Hiner and Moffitt mizing the risks of disease in the propaged fish species 2002; Kerans and Zale 2002). through water source and quality. Fish diseases are Infectious diseases within a population can be part of natural population dynamics (Coutant 1998), modeled with epidemiological models of a popula- and studies have documented a variety of parasites, tion, such as those presented by Anderson and May viral, fungal, and bacterial pathogens in free-ranging (1979) and May and Anderson (1979), to include fishes (e.g., Yamamoto 1975; Grischkowsky and dynamic processes for the number of susceptible, im- Amend 1976; Ellis et al. 1978; Deardorff and Kent mune, and removed individuals (Reno 1998). Pat- 1989; Sakai et al. 1992; Mellergaard and Spanggaard terns of host density and distribution of asymptomatic 1997; Baldwin et al. 1998). carriers are critical limitations in these models. If there Stresses in any environment will increase the like- are intermediate hosts or vectors modeling disease rela- lihood of disease to be expressed (Snieszko 1974; tionships is more complex (Hiner and Moffitt 2002; Wedemeyer 1996; Mesa et al. 2000). No population Kerans and Zale 2002). The disease process in free- of free-ranging fishes can be considered free from the ranging populations has been modeled for the fungal stress of human and habitat alterations associated with parasite Ichthyophonus hoferi in the Atlantic herring global climate change, effluents from human and in- (also known as North sea herring) Clupea harengus dustrial sewage and non point source discharges, har- (Patterson 1996; Mellergaard and Spanggaard 1997), vest, alterations of flow regimes, and habitat (Rapport using inference from samples of fish populations and et al. 1982; Krishnakumar et al. 1999; Arkoosh et al. separating natural and parasite induced mortality from 2001; Harvell et al. 2002). fishing mortality (Haddon 2001). Interest in diseases in fish and wildlife popula- Fish pathogens can spread in both free-ranging tions has increased. Emerging diseases defined by and captive populations, but few studies have made Kiesecker et al. (2004) are those that have increased in observations over a wide geography and over any length incidence, virulence, or geographic range; have shifted of time (Moffitt et al. 1998; LaPatra 2003). A limited hosts; or have recently evolved new strains. Diseases number of studies provided assessments of a selected emerge when a new pathogen is introduced into a parasite, virus, or bacteria in free-ranging and captive naïve host population or when an external factor some- populations on a larger geographic scale (Ching and how increases the vulnerability of current hosts. Munday 1984; Baldwin et al. 1998; Kent et al. 1998; Some exotic pathogens can produce serious health Bruneau et al. 1999a; Kurath et al. 2003; Murray et effects in both propagated and free ranging stocks al. 2003). (Naylor et al. 2001). Movement of water or fish can Propagated fishes have been documented respon- spread exotic pathogens (Lilley et al. 1997, 1998; sible for the spread of some fish diseases especially Blazer et al. 1999; Goodwin 2002). Harvell et al. when the surrounding populations were naïve. Johnsen (1999) reviewed the disease outbreaks in the marine and Jensen (1994) studied the spread of the disease environment and illustrated new diseases emerging furunculosis in salmonids in Norwegian fish farms through host or range shifts of known pathogens. They and rivers and attributed this introduction of infected associated these shifts with climate and human activi- hatchery rainbow trout Oncorhynchus mykiss from ties that accelerate global transport of species. Climate Denmark. From 1985 to 1992, the number of cases change poses many potential synergisms that could of furunculosis at marine and freshwater farms in- EVALUATING AND UNDERSTANDING FISH HEALTH RISKS AND THEIR CONSEQUENCES 531 creased from 16 to 550, and the authors attributed techniques and sampling protocols can show different the rapid spread to escaped fish from infected farms trends over time. Murray et al. (2003) monitored Scot- and transportation of infected fish between farms. tish farms from 1996 to 2001 for infectious pancre- In most circumstances, the pathogens that occur atic necrosis virus (IPNV) in Atlantic salmon. They in cultured fishes are also present in the free-ranging used cell culture and enzyme-linked immunosorbent fishes of the region. Noakes et al. (2000) reviewed the assays on pooled samples and reported that prevalence disease cases in salmon propagation facilities over a 22- increased over time, but they did not adjust for effect year period and found that length of time in captive of pooled samples,