Seasonal variation of crude digestive protease activity in Antarctic krill Euphausia superba

Abstract

Crude krill digestive protease (CKP) was extracted on board a fishing vessel from fresh whole krill caught around the Antarctic Peninsula, and its protease activity was measured by using the azocasein method. CKP activity was high from December to February (in summer) and was low from March to August (in winter). CKP activity and mean feed intake index had a high correlation coefficient by Pearson’s correlation coefficient test (P<0.05). These results suggest that the seasonal variation of CKP activity coincides with the seasonal prevalence of some phytoplanktons as feed for krill. In summer, phytoplankton is plentiful so that krill feed on them and the krill protease activity remains high, but in winter phytoplankton abundance is low and krill do not have to keep their proteolytic activity high.