Early Life Stage Mortality Syndrome in Fishes of the Great Lakes and Baltic Sea

<em>Abstract</em>.—Smelt <em>Osmerus mordax </em>and alewives <em>Alosa pseudoharengus </em>were collected from Lakes Huron and Michigan in spring and fall and from Lake Superior in spring to determine the activity of thiaminase, a thiamine-destroying enzyme, in those species. Greater thiaminase activity was found in the viscera (1,902 pmol • g^-1 • min^-1 for smelt and 1,705 pmol • g^-1 • min^-1 for alewives) than in the eviscerated body (180 pmol • g^-1 • min^-1 for smelt and 235 pmol • g^-1 • min^-1 for alewives). The average whole body thiaminase activity when all of the samples were pooled was 362 pmol • g^-1 • min^-1 for smelt and 357 pmol • g^-1 • min^-1 for alewives. Large differences were found in thiaminase activities between smelt and alewives from different locations in the Great Lakes region and at different sampling times. These differences may be species-, location-, or season-specific.