Lake whitefish (Coregonus clupeaformis; LWF) is an economically and ecologically important indigenous Great Lakes (GL) fish species. Recently, declines in lake whitefish abundance, growth, and early life stage recruitment have generated substantial concern. Although numerous studies have attempted to elucidate the factors behind these declines, the cause(s) remain undetermined. To investigate the potential role(s) of infectious diseases in these declines, I collected wild adult and juvenile GL-LWF over two years from multiple sites within Lakes Superior, Michigan, and Huron, and performed extensive fish health analyses. For the first time, I recovered Flavobacterium psychrophilum from systemically infected adult GL-LWF and likewise isolated Carnobacterium maltaromaticum from gonadal tissues of infected adult GL-LWF, highlighting potential health risks to their offspring. In this context, I reared juvenile GL-LWF under laboratory conditions for in vivo challenge experiments, but before experiments commenced, juvenile GL-LWF showed clinical signs of disease with mortality quickly following. Upon diagnostic investigation, I discovered that affected fish were systemically infected with C. maltaromaticum. To further investigate effects that infectious microbes have on the health and survival of juvenile GL-LWF, in vivo challenge experiments with Aeromonas salmonicida subsp. salmonicida and Viral Hemorrhagic Septicemia Virus (VHSV) were performed, revealing high susceptibility to both fish pathogens. Collectively, my results highlight multiple fish pathogens as potential contributors to reduced GL-LWF health and survival.
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