You know what's funny about that is that I wrote them too and they gave me a different answer. They told me they were Arctic Pond Smelt. Hypomesus olidus. And by looking at the fish in the package I had, and comparing I.D. keys in the internet, they were definitely not menidia. And Ocean Nutrition also told me they were Hypomesus olidus. Either way, niether species contains thiaminase. That particular species of smelt doesn't contain thiaminase according to info I could find.