Great Lakes fish are vulnerable to multiple environmental stressors, from habitat degradation to contamination of nearshore spawning habitats. An additional problem facing certain commercially and recreationally important fish in the Great Lakes is Thiamine Deficiency Complex (TDC). Sport fish such as coho salmon, chinook salmon, steelhead, and lake trout are at risk for developing TDC, which leads to embryonic mortality. Consumption of non-native forage fish like alewives and rainbow smelt leads to a nutritional deficiency (lack of thiamine) in these sport fish and rendering them unable to produce viable offspring, in turn preventing the species from producing future generations, potentially leading to extinction. The USGS-GLSC is working to assess the status of thiamine in Great Lakes salmonines and uncover ways to increase thiamine levels to prevent future declines of these important fish populations. The GLSC has undertaken long-term monitoring of thiamine levels in lake trout eggs in lakes Huron and Michigan since 2001 and will continue this program into the future.