Overwintering Calanus finmarchicus in Ullsfjord

Abstract

Calanus finmarchicus is a key species in the North Atlantic ecosystems that serves as an important link between primary producers and higher trophic levels, including commercially important fish. C. finmarchicus spends the unproductive time of the year overwintering in deep water layers. While it typically overwinters in deep oceanic basins, overwintering populations with high abundances have also been observed in North Norwegian fjords, possibly providing an earlier food source for local fish larvae. However, the spatial distribution and overwintering success of these C. finmarchicus populations, especially in shallower fjords, remain largely unknown. The aim of this study was to detect and describe the changes in abundance, vertical distribution and origin, of the overwintering C. finmarchicus population in Ullsfjord over the unproductive season. Monthly to bimonthly sampling was conducted from October 2024 to April 2025 in one station in Ullsfjord. Data on the zooplankton and environment was gathered using WP2 nets, the Laser Optical Plankton Counter (LOPC), CTD and water samples. This study established that a large overwintering C. finmarchicus population (average winter abundances between 173 ind m-3 (LOPC) and 417 ind m-3 (net samples)) was present throughout winter in the deeper layers (>100 m) of Ullsfjord, in Norwegian Coastal Water, and dominated the local zooplankton community at depth. The C. finmarchicus overwintering population suffered high mortality over winter (>99%), likely primarily due to high predation pressure in the fjord. The overwintering population was dominated by CV stage copepods that started moulting into adults in January and ascended to surface in April, possibly reproducing locally and providing a food source for local fish larvae. Based on δ13C and δ15N stable isotope values, the C. finmarchicus overwintering population in Ullsfjord originated from the same area and did not feed during overwintering.