Great Lakes Lake Trout Thiamine Status and Influencing Factors in Lake Ontario

Abstract

Chapter 1: Though stocking lake trout (Salvelinus namaycush) in the Great Lakes has found success in establishing some self-sustaining populations, a return of consistent natural recruitment of lake trout in Lakes Michigan, Erie and Ontario remains unachieved. A major impediment has been thiamine deficiency complex (TDC). Insufficient maternal deposition of thiamine (vitamin B1) into eggs during development is a major consequence of TDC and leads to mass mortality of fry prior to their first feeding. Monitoring of egg thiamine concentrations is a critical indicator for TDC impacts on lake trout populations. Since 2001, the USGS Great Lakes Science Center in cooperation with partner agencies has monitored egg thiamine concentrations in lake trout eggs throughout the Great Lakes region. Our analysis of the resulting dataset reinforces prior findings connecting the presence of alewife (Alosa pseudoharengus) in the diet with reduced egg thiamine concentrations. Relationships between total length of egg producing females and total egg thiamine concentrations of lake trout eggs in Lake Ontario suggest that a shift in diet from a round goby (Neogobius melanostomus) dominated diet, to an alewife dominated diet, results in lower total egg thiamine concentrations among larger lake trout. Long-term trends in lake trout egg thiamine concentrations are closely linked to changes in prey base composition. Following the crash of the alewife population in Lake Huron, lake trout egg thiamine concentrations increased considerably. Similarly, reductions in site specific alewife abundance preceded lake trout egg thiamine increases in Lake Michigan and may have helped to facilitate a recent trend of increasing natural recruitment. The introduction of alewife to Lake Champlain in 2004 lead to large declines in lake trout egg thiamine concentrations in the lake, but also appears to have facilitated natural recruitment of the same lake trout population. Egg thiamine concentration from Cayuga Lake lake trout increased following the 2013 introduction of round goby, presumably offering lake trout an alternative to alewife as prey, however, lake trout egg thiamine concentrations in Cayuga Lake have returned to pre-introduction concentrations. Chapter 2: The specific mechanism behind the induction of TDC by a diet rich in alewife is yet to be fully explained. The thiamine degrading enzyme thiaminase I is present in the visceral organs of alewife and represents the leading hypothesized cause of TDC. However, oxidative stress in the form of lipid peroxidation and the consumption of thiamine through its role as an antioxidant is thought to be the mechanism behind thiamine deficiency in Atlantic Salmon (Salmo salar) in Northern Europe. In this chapter we explored relationships among lake trout egg thiamine concentrations, belly flap fatty acid compositions and concentrations of the primary lipid peroxidation disrupting antioxidant, α-tocopherol (vitamin E), in lake trout livers and eggs. α-tocopherol concentrations in lake trout eggs appeared to be driven by diet and have a negative relationship with length in both Lake Ontario and Lake Superior. Egg and liver α-tocopherol concentrations did not appear to have a direct impact on egg thiamine concentrations. Integrating recent findings on the roles of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) in cellular antioxidant defense activation into a calculation of peroxidation potential, led to an improved negative relationship between belly flap unsaturation index and lake trout egg thiamine concentrations, potentially implicating lipid peroxidation as a source of thiamine consumption.