Spatial Variability of the Feeding Conditions for the Norwegian Spring Spawning Herring in May

Abstract

The abundant and ecologically important copepods Calanus finmarchicus and Calanus hyperboreus within the Norwegian and Iceland Seas are key prey species of Norwegian spring spawning herring. The volume flux of East Icelandic Water, which carries the mentioned zooplankton species into the southern Norwegian Sea, is highly variable. The years 1996–2002 have previously been characterized as high influx years, 2003–2016 as low influx years and since 2017 a reversal to higher influx has been apparent. Hydrographic and zooplankton data, as well as fish data on herring (size and diet content), from the International Ecosystem Surveys in the Nordic Seas in May have been used in this study. Focus is on the south-western Norwegian Sea, where herring has regularly been observed in May since 2005 and where changes in plankton availability and in the hydrographic environment have also been observed. Diet biomass from 2017–2020 (higher influx period) showed higher stomach fullness compared to 2007–2011 (lower influx period). Furthermore, the highest stomach fullness is observed in the westernmost feeding region—referred to as the Feeding Spot. The scrutinized diet content showed a notably higher biomass of ingested C. hyperboreus in 2020 compared to 2007–2011, indicating a preference for the larger and more nutritious copepod. Zooplankton analysis from May 2020 revealed that the core of the western feeding region, i.e., the area with highest zooplankton biomass, was located within the subarctic waters immediately north/west of the junction between the Iceland-Faroe Front and the Jan Mayen Front. The core area was mainly represented by overwintering stages and the derived adult stages of C. finmarchicus and C. hyperboreus. Interplay between food availability and accessibility, in terms of temperature, to the Feeding Spot, is discussed.

Arctic and Atlantic Waters in the Norwegian Basin, Between Year Variability and Potential Ecosystem Implications

“…First, the fueling of zooplankton via the EIC (Wiborg, 1954;Kristiansen et al, 2022). Kristiansen et al (2019) found a coinciding decrease of MEIW and C. hyperboreus abundance at Section N in the early 2000s but an increase of C. hyperboreus since 2017 within the southwestern Norwegian Sea (Kristiansen et al, 2022). This paper shows that the increase in C. hyperboreus abundance with increasing Arctic Waters can also be traced to the eastern part of the Norwegian Basin (Figure 9).…”

“…Related to the changes in the Arctic Water two partially related mechanisms are at play. First, the fueling of zooplankton via the EIC (Wiborg, 1954;Kristiansen et al, 2022). Kristiansen et al (2019) found a coinciding decrease of MEIW and C. hyperboreus abundance at Section N in the early 2000s but an increase of C. hyperboreus since 2017 within the southwestern Norwegian Sea (Kristiansen et al, 2022).…”

The Dynamics of 0-Group Herring Clupea harengus and Sprat Sprattus sprattus Populations Along the Norwegian Skagerrak Coast

“…We only used data from the North Sea (51.5°N -62.0°N and 4.0°W -8.0°E) and Skagerrak-Kattegat (east of 8.0°E). Further, we only included abundance data of Calanus finmarchicus, C. helgolandicus, and large and small copepods which are important prey items for herring and sprat (Falkenhaug and Dalpadado, 2014;Kristiansen et al, 2022). Small copepods (<2 mm) were identified based on the "traverse" counting method, i.e.…”

Changes towards stable good somatic condition and increased gonad investment of Norwegian spring-spawning herring (Clupea harengus) after 2005 are linked to extended feeding period

“…The variability in the Norwegian Sea Gyre is linked to the input of colder East Icelandic water into the Norwegian Sea. This, in turn, governs the relative contribution of both C. hyperboreus and large overwingering stages of C. finmarchicus, which are both important as prey to herring early in the feeding season (Kristiansen et al, 2019;Kristiansen et al, 2022). The influx of subarctic waters from the Iceland Sea is also a key nutrient source for the Norwegian Sea (Skagseth et al, 2022).…”

“…The Jan Mayen front has been identified as an important transitional zone in both zooplankton biomass, abundance and diversity, changing significantly across the front (Melle et al, 2020). Both prey abundance and stomach content of herring was higher on the western, colder side of the front (Melle et al, 2020;Kristiansen et al, 2022), where Calanus hyperboreus was important part of the diet. On the Atlantic side, the herring mainly fed on overwintering stages of Calanus finmarchicus, i.e.…”