Journal of Fish Biology (2008) 72, 787–802
Thiaminase activity of Baltic salmon prey species: a comparision of net- and predator-caught samples
S. WISTBACKA* AND G. BYLUND
Laboratory of Aquatic Pathobiology, Department of Biology, Åbo Akademi,
20520 Åbo, Finland (Received 2 February 2007, Accepted 9 October 2007)

Thiaminase activity was determined for Gulf of Bothnia (GB) and Gulf of Finland (GF)
Baltic herring Clupea harengus membras, sprat Sprattus sprattus and three-spined stickleback
Gasterosteus aculeatus sampled from either trawl or gillnet catches or from Baltic salmon Salmo
salar stomachs. The thiaminase activity in Baltic herring was about 10-fold higher than that in
sprat, and there was almost no thiaminase activity in three-spined stickleback. Thiaminase
activity of undigested Baltic herring found in Baltic salmon stomachs was significantly higher
than that of trawl-caught Baltic herring from the same sea area, suggesting that there may be
a higher risk of predation for Baltic herring with high thiaminase activity, possibly linked to
their health. Thiaminase activity of the gastrointestinal contents of Baltic salmon, feeding
almost entirely on Baltic herring in the GB, was significantly higher than for Baltic salmon
feeding on both Baltic herring and sprat in the GF. Therefore, Baltic herring may be the major
source of thiaminase for Baltic salmon. A tank experiment demonstrated that thiaminase
activity in Baltic herring may vary, even within very short time periods. The results were
consistent with the hypothesis that the thiaminase content in Baltic salmon forage fish may be
an important link in the aetiology of the thiamine deficiency syndrome, M74, in Baltic salmon.