Declining Pre-bloom Calanus finmarchicus Egg Production Adjacent to Two Major Overwintering Regions in the Northeastern Atlantic

Abstract

Calanus finmarchicus is a key secondary producer in the North Atlantic. Shortly prior to the spring bloom the animals ascend from diapause at depth to surface waters, where the females spawn partly, based on winter lipid reserves. C. finmarchicus eggs are an important prey of first feeding fish larvae inhabiting North Atlantic shelves during early spring and are thus essential for larval survival. Comprehensive late April surveys have been carried out on and around the Faroe shelf, which is located between the Northeast Atlantic and the Nordic Seas, for more than two decades. One aim is to investigate the critical match-mismatch between the spring bloom development, zooplankton reproduction and occurrence of first feeding fish larvae. In this study, we examine spatial and temporal changes in pre-bloom reproductive activity of C. finmarchicus on and around the shelf using a unique dataset of more than 8,000 examined females sampled during the period 1997–2020. Enhanced productivity was observed on the north-western side of the shelf, where the main flow of oceanic water to the inner permanently well mixed shelf takes place. We attribute this increased productivity to enhanced food (phytoplankton) availability in the seasonally stratified outer shelf, slightly upstream of the main egg production area. Both individual egg production rates and the fraction of spawning females declined throughout the Faroe shelf during the examined period. This decline could not be explained by the employed environmental parameters. The declining pre-bloom egg production may have consequences for first feeding fish larvae.

Influence of larval prey availability on year class strength in Faroe Plateau cod (Gadus morhua)

“…Previous studies have suggested that the production and abundance of C. finmarchicus eggs is relatively high in the Faroe area (Stenevik et al 2007), indicating that newly spawned Faroe Plateau cod larvae in most cases have sufficient food resources during the first feeding period. However, phenological changes resulting in earlier emergence of C. finmarchicus have been observed in and around the Faroe area after the gut content study was conducted (Kristiansen et al 2016;Jacobsen et al 2018), and recent research has revealed that there has been a marked decline in C. finmarchicus pre-bloom egg production on the Faroe shelf after 2007 (Jacobsen et al 2022). Thus the feeding conditions of first-feeding cod larvae might have changed in recent years.…”