On the food of northern krill Meganyctiphanes norvegica in relation to itsvertical distribution

The feeding behaviour of northern krill (Meganyctiphanes norvegica) was stu died in populations from the Clyde Sea and the Kattegat during summer and w inter. The food spectrum in the stomachs was analysed using traditional tax onomic methods and biochemical techniques. The vertical migration behaviour of krill was monitored through a 30 h series of net samples, whilst the tr ophic environment was characterised through accompanying quantitative analy ses on the depth distribution and biomass of copepods and phytoplankton. Kr ill was found to be more carnivorous in the Kattegat than in the Clyde Sea, which correlated with the higher ratio of copepod to phytoplankton biomass found in the Kattegat compared with the Clyde Sea. High levels of fatty al cohols and other lipid markers in the stomach contents of Kattegat krill we re also indicative of a carnivorous diet. Other food sources included detri tus, terrestrial material and other euphausiids, underlining the opportunis tic nature of northern krill in its choice of prey items. Analyses of stoma ch and intestinal fullness over a diel cycle showed significant variations with time in the Clyde Sea but not in the Kattegat. However, a diel cycle i n the rate of ingestion was evident at both locations when comparing the co pepod mandibles in the stomachs to the distribution of copepods in the envi ronment. The fact that deep-living Calanus was not a major prey item sugges ted that there was little feeding activity during the daytime, when the kri ll occupied the deeper layers. Instead. the majority of mandibles were from species that were dominant in the upper layers, e.g, the genera Temora and Pseudocalanus. The fact that kriIl caught in the daytime contained the sam e relative composition of mandibles in the stomach as those caught at night is probably explained by a cessation in daytime feeding activity and reten tion of the mandibles from the night before. It is proposed that krill in t he Clyde Sea area and the Kattegat show a diel rhythm in feeding activity t hat is believed to be an adaptive response to minimising predation risk.