Blue whiting Micromesistius poutassou diel feeding behaviour in the Irminger Sea

With warming ocean temperatures, the abundance of blue whiting Micromesistius poutassou is increasing in the waters around Greenland. However, in this region, knowledge about the species’ trophic role in the ecosystem is scarce. Consequently, we investigated the diet composition and diel feeding behaviour of blue whiting in the north-eastern part of the Irminger Sea in Greenland waters by analysing their stomach contents and the vertical position/movements of their prey from zooplankton samples and hydroacoustic measurements. We collected the data during a designated experimental survey in July 2016 with repeated sampling at the same location. Results from the stomachs of 624 blue whiting individuals ranging from 22-39 cm long (total length) showed that the highest food intake took place from noon until late evening, with minimum feeding occurring in the morning. The most essential prey groups consisted of euphausiids, copepods, amphipods and fish, in that respective order. Regarding copepod prey, blue whiting had a strong affinity for Calanus hyperboreus and Paraeuchaeta spp. and showed potential for local depletion of these large copepods. On the other hand, the more abundant but smaller C. finmarchicus was almost absent in the fish stomachs, in contrast to findings in other regions. This new understanding provides an early indication of some of the emerging trophodynamics in the Irminger Sea and similar subarctic zooplankton communities with increasing numbers of blue whiting. Our results confirm the importance of accounting for diel and size-specific differences in blue whiting feeding when studying various aspects of its food intake.