Devils Lake, Red River, and Lake Winnipeg Parasite/Pathogen Monitoring - FINAL REPORT FOR LAKE WINNIPEG FISH HEALTH SURVEY – LIGHT MICROSCOPY
Lumsden, John S.
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Executive Summary The objectives of the Canadian portion of the project were to; 1. Determine the presence and estimate the prevalence of fish parasites and pathogens in resident fish from Lake Winnipeg; 2. Provide a comprehensive, and scientifically credible survey of fish parasites and pathogens in fish from Lake Winnipeg that may be used in performing risk analysis associated with transfer of fish parasites and pathogens from the outlet on Devils Lake to aquatic ecosystems in the Red River basin including Lake Winnipeg; 3. Use the comprehensive survey of fish collected during this proposed survey to meet the overall framework for biological monitoring in the Red River basin that is included in the “Work Plan” of the International Red River Board. The purpose of this particular portion of the project was to perform a fish health survey of 10 species of fish from Lake Winnipeg using light microscopy. Sixty fish from each species were examined in summer of 2006 and an additional 30 fish of each species from the spring of 2007. Summaries for each species, including a table of lesions and/or organisms noted, are found at the beginning of each segment of the report. The summary includes the tissues examined, common lesions present that were not systematically evaluated and any issues peculiar to a species. Next, the notable lesions that were systematically evaluated are described as well as the number of fish affected. Organisms or lesions that affected fish at both time points examined are listed first (see methods); followed by those that were noted only in summer and finally those noted only from fish examined during the spring. For each of these a morphologic diagnosis (summary phrase) is also included. The majority of these lesions/organisms are illustrated with a photograph. The remainder of the document is a list of the morphological diagnoses that were noted for each individual fish. Fish examined during the summer (2006) are listed first and those examined during the spring (2007) are listed last. For easier orientation when scrolling through the document, fish examined during the summer are underlined and those examined during the spring are not. Occasionally, a short description is given for an additional lesion affecting a fish that wasn’t common or more noteworthy. The vast majority of agents found were metazoan or protozoan (used loosely including myxosporeans and microsporeans) that caused local tissue reactions. In the majority of instances, parasites that are well-adapted to their host cause relatively minimal mortality but may impair growth, reproduction, etc. depending on location and numbers. Parasites that have moved to a new host/location typically cause the greatest impact. A relevant example to this study is the discovery of the Asian tapeworm in fish from Lake Winnipeg by Dr. Terry Dick, University of Manitoba. All of the types of fish examined by light microscopy here had intestinal cestodes likely representing numerous different metazoan species. The majority of the intestinal cestodes produced no, or limited, histological lesions and this is not unusual. The effects of the presence of the Asian tapeworm will vary markedly between species but is not best assessed by tissue lesions and/or overt mortality. Unless fortuitous sections reveal discriminating features, cestodes are typically not possible to identify using light microscopy to a family, let alone species, level. This is also true for trematodes and less so for nematodes, crustaceans, myxosporeans, protozoans, etc. 3 Lesions associated with bacterial and viral agents were rare. Evidence for bacterial infections were limited to epitheliocystis (chlamydia-like intracellular bacteria) in white bass, yellow perch, fathead minnow and channel catfish. These organisms produce microcolonies, typically in the gill epithelium, that are visible by light microscopic sections. Evidence for virus was limited to the detection of lymphocystis in walleye, caused by a piscine iridovirus that produces characteristic histological lesions and dermal sarcomas in two walleye. Similar lesions are caused by the walleye dermal sarcoma virus; an oncogenic retrovirus that has a wide distribution in North America. The lymphocyte-intense lesion noted in the dermis of one Northern pike may be an early lymphosarcoma, which in this species can be caused by a separate oncogenic retrovirus. There were numerous lesions present in these fish, as would be expected for most wild fish. However, many of these lesions were not associated with a visible organism. The majority of the lesions not directly associated with an agent are most likely remnants of metazoan migrations, etc. or lesions left after the inflammatory/immune response has removed the agent. Lesions that are most likely to have a significant impact on fish health are (as judged by the lesions produced and number of fish affected): branchial myxosporeans of emerald shiners, branchial epitheliocystis of white bass, branchial trematodes in white bass, and the intracardial trematodes of walleye and sauger. A single agent of those found that would likely have the greatest impact on fish health (but this could still be relatively minimal) is listed for each species below. Those agents judged to have the most significant impact on fish health for each species are; Brook stickleback – intralenticular Diplostomulum sp., although very few fish were affected. Channel catfish – no clear agent. Emerald Shiner – branchial myxosporeans or intestinal cestodes. Fathead minnow – unclear or possibly hepatic nematodes. Goldeye – intestinal cestodes. Northern pike – unclear. Sauger – intracardial Sanguinicola sp., Walleye – intracardial Sanguinicola sp. White bass – branchial monogenetic trematodes and/or epitheliocystis and/or intestinal cestodes. Yellow perch – intestinal cestodes. Finally, even with the limited utility of light microscopy for exact classification of parasites, there appears to be numerous organisms identified in this study that are either not previously recorded for a species of fish or not recorded from a particular species within the Hudson Bay watershed that includes Lake Winnipeg. An exhaustive literature review and in many cases coupled with additional efforts, such as electron microscopy, would be required to clarify the exact status of the organisms in question.