Resurgence of Salmonid Herpesvirus-3 Infection (Epizootic Epitheliotropic Disease) in Hatchery-Propagated Lake Trout in Michigan.

Over the past century, populations of Lake Trout Salvelinus namaycush have declined throughout the Great Lakes basin due to overfishing, habitat destruction, introduction of invasive species, and associated recruitment issues from high thiaminase, as well as emerging infectious diseases. To combat these declines, state and federal fishery management agencies undertook substantial stock enhancement efforts, including more stringent regulation of sport and commercial catch limits and increasing hatchery propagation of Lake Trout stocked into Great Lakes basin waterways. One state fish hatchery involved in these rehabilitation efforts experienced mass mortality events in 2012 and 2017. In 2012, following a period of abnormally heavy rain, hatchery staff observed abnormal behavior followed by increased mortalities in two strains of Lake Trout fingerlings, reaching upwards of 20% mortality and totaling a loss of approximately 100,000 fish. In 2017, following another heavy-rain season, 6-8% of 2-year-old Lake Trout experienced morbidity and mortality similar to that observed in 2012. During the 2012 event, Brook Trout Salvelinus fontinalis and splake (Lake Trout × Brook Trout hybrid) reared in flow-through systems receiving water from diseased Lake Trout remained clinically unaffected. Molecular analyses revealed all lots of affected Lake Trout were infected with the salmonid herpesvirus-3 (epizootic epitheliotropic disease virus [EEDV]), a disease that caused complete depopulation of this hatchery in the late 1980s and until 2012 was never again detected in this hatchery or in Michigan. Further sampling detected EEDV in apparently healthy 5-year-old Lake Trout and in wild Mottled Sculpin Cottus bairdii collected in the hatchery source water. The ability of the virus to replicate in tissues of infected fish was verified by exposing naïve Lake Trout to the filtered tissue homogenates of infected fish resulting in similar disease signs. Despite the virus going undetected for many years, these two EEDV episodes clearly demonstrate the continued presence of this deadly herpesvirus in the Great Lakes basin.

Epidemiological investigation of Renibacterium salmoninarum in three Oncorhynchus spp. in Michigan from 2001 to 2010.

Bacterial kidney disease (BKD) has caused mortalities and chronic infections in wild and farm-raised salmonids throughout the world. In the Laurentian Great Lakes of North America, BKD was associated with several large-scale mortality events of Oncorhynchus spp. throughout the 1980s and 1990s. In response to these mortality events, the state of Michigan implemented several enhanced biosecurity measures to limit the occurrence of BKD in state-operated hatcheries and gamete-collection weirs. The objectives of this study were to assess if infection levels (prevalence and intensity) of Renibacterium salmoninarum, the causative agent of BKD, have changed in broodstock and pre-stocking fingerlings of three feral Oncorhynchus spp. (Chinook salmon (O. tshawytscha), coho salmon (O. kisutch), and steelhead (O. mykiss)) over a decade, following the implementation of the enhanced biosecurity measures. Between 2001 and 2010, a total of 3,530 broodstock salmonids collected from lakes Huron and Michigan tributaries during spawning runs and 4,294 propagated pre-stocking salmonid fingerlings collected from three state of Michigan fish hatcheries were tested for the presence of R. salmoninarum antigens using the enzyme-linked immunosorbent assay. Substantial declines in the overall prevalence of the bacterium were detected in each of the examined broodstocks. Most propagated pre-stocking fingerlings also exhibited substantial declines in R. salmoninarum prevalence. Prevalence was typically higher in Chinook salmon from Lake Michigan than from Lake Huron; prevalence was also generally higher in the Hinchenbrooke strain of coho salmon than in the Michigan-adapted strain. For most strains and stocks examined, intensity of R. salmoninarum infection was found to have declined. Although there were declines in the potential for shedding the bacteria for both male and female Chinook and coho salmon, overall shedding rates were generally low (<15%) except for Hinchenbrooke coho salmon strain, which had shedding prevalences in excess of 50% at the beginning of the study. This study provides evidence that enhanced biosecurity measures at culture facilities and collection sites are capable of severely curtailing disease infection in wild populations even at the scale of Lake Michigan fisheries.