Do you believe it? It's an honest question - I didn't save any of the citations, but when I looked at studies of thiaminase-producing bacteria in alewives the results seemed inconsistent (didn't always find bacteria where they found thiaminase). If the studies you read had really solid results, that would be interesting.
I think so, at least for the really high values, plus there is a very high level of variation in the amount of thiaminase present.

This is digressing a bit, but incidentally, thiaminase-producing bacteria have also been found in the digestive tracts of some people with chronic thiamine deficiency!
Which raises the question, "have Alewives(and other fish) been found to have a thiamine deficiency with this bacteria"

I will basically have to look at the papers to see if the level of thiaminase is really unrelated to thiamine activity, or if there is something underlying. From the way they do the assay, they might have a confound in their data, in that they grind up the fish, breaking down cell membranes and permitting more thiaminase to come into contact with the thiamine present. If this is the case, one would not necessarily expect the two concentrations to be related... Or so I think.

Also the temporal variability might also be consistent with a bacterial infection. Maybe...