Great Lakes Fish Diseases Past, Present, Future

Over the last three decades, numerous mortality events have occurred within the Laurentian Great Lakes Basin (LGLB), some attributed to infectious agents, such as the Viral Hemorrhagic Septicemia Virus, Renibacterium salmoninarum, and , Koi Herpesvirus.  Novel bacterial infections have also been detected such as Pantoea agglomerans, Carnobacterium maltaromaticum, Chryseobacterium piscum, and over 200 new Flavobacteria spp. that were never reported from North America before. As a result, it became apparent that some novel fish pathogens had invaded the LGLB, while others resurged.  These epizootics and the associated sociopolitical repercussions brought LGLB fish health programs to the forefront of priorities. Fisheries management programs moved forward and many agencies now include fish health metrics as indicators of ecosystem health in their monitoring and assessments. For example, the Viral Hemorrhagic Septicemia (VHS) resulted in the issuing of a Federal order that placed restrictions on the movement of fish in the Great Lakes states. Greater emphasis than ever has been placed on biosecurity strategies and pathogen testing for both public agencies and private hatcheries.  Despite the emergence of these novel pathogens, relatively little has been done to expedite the development of sensitive assays for their detection and development of effective means to control or minimize their impacts.  For example, although the epizootic epitheliotropic disease virus (EEDV) of lake trout caused catastrophic mortality in state and federal hatcheries in the 1980’s, efforts to understand the epizootiology of this herpesvirus were hindered by lack of a specific diagnostic tool.   Since the development of an EEDV PCR assay in 2008, new and archived tissues have been tested, expanding the host range to include coregonids. The lack of knowledge on reservoirs and geographic ranges of emerging pathogens makes the development of control strategies difficult.  Greater emphasis is needed on statistically appropriate pathogen surveys of wild fish populations.  When these data are available, successful control strategies have been implemented in the LGLB including one for Renibacterium salmoninarum which is currently at very low levels in targeted sportfish species.  Multidisciplinary research efforts are needed to determine: 1) the relationship between energy pathways and disease resistance; 2) the effects of anthropogenic stressors on health of LGLB fish; and 3)  the contribution of disease to natural mortality rates. Last, it is critical that the current fish disease control strategies and fishery management practices be periodically re-evaluated.