Abstract
The Norwegian spring-spawning (NSS) herring (Clupea harengus), blue whiting (Micromesistius poutassou) and Northeast Atlantic (NEA) mackerel (Scomber scombrus) are extremely abundant pelagic planktivores that feed in the Norwegian Sea (NS) during spring and summer. This study investigated the feeding ecology and diet composition of these commercially important fish stocks on the basis of biological data, including an extensive set of stomach samples in combination with hydrographical data, zooplankton samples and acoustic abundance data from 12 stock monitoring surveys carried out in 2005–2010. Mackerel were absent during the spring, but had generally high feeding overlap with herring in the summer, with a diet mainly based on calanoid copepods, especially Calanus finmarchicus, as well as a similar diet width. Stomach fullness in herring diminished from spring to summer and feeding incidence was lower than that of mackerel in summer. However, stomach fullness did not differ between the two species, indicating that herring maintain an equally efficient pattern of feeding as mackerel in summer, but on a diet that is less dominated by copepods and is more reliant on larger prey. Blue whiting tended to have a low dietary overlap with mackerel and herring, with larger prey such as euphausiids and amphipods dominating, and stomach fullness and feeding incidence increasing with length. For all the species, feeding incidence increased with decreasing temperature, and for mackerel so did stomach fullness, indicating that feeding activity is highest in areas associated with colder water masses. Significant annual effects on diet composition and feeding-related variables suggested that the three species are able to adapt to different food and environmental conditions. These annual effects are likely to have an important impact on the predation pressure on different plankton groups and the carrying capacity of individual systems, and emphasise the importance of regular monitoring of pelagic fish diets.
Highlights
The Norwegian Sea (NS) is the feeding ground of some of the largest fish stocks in the world, including two of the ten with highest global yields [1], namely Norwegian spring spawning (NSS) herring (Clupea harengus) and blue whiting (Micromesistius poutassou)
Institute of Marine Research (IMR), which is the responsible institution for monitoring Norwegian Spring Spawning herring, mackerel and blue whiting in Norway, and responsible for giving advice to managers in Norway, is given specific research quotas and special permission to sample these species at any location within the Norwegian economical zone by the Directorate of Fisheries, Bergen, Norway
Our results, based on diet data of mackerel, herring and blue whiting acquired during spring and summer 2005–2010, showed that mackerel and herring diets largely overlapped, with calanoid copepods being their main prey item, while the blue whiting diet consisted of larger prey items, amphipods
Summary
The Norwegian Sea (NS) is the feeding ground of some of the largest fish stocks in the world, including two of the ten with highest global yields [1], namely Norwegian spring spawning (NSS) herring (Clupea harengus) and blue whiting (Micromesistius poutassou). Prokopchuk and Sentyabov [15] found Calanus finmarchicus to be the principal prey of mackerel in summer (June and July) 2001 and 2002, while C. finmarchicus was an important prey of herring only in July 2001 and June 2002, while in July 2002 appendicularians, amphipods and euphausiids dominated their diet. Such opportunistic predation on larger prey, complementing the usual calanoid-copepod-based diet, had already been observed by Dalpadado et al [3]. The later study by Langøy et al [16] confirmed this general picture of prey composition, and these authors found that mackerel in particular were opportunistic, adjusting their feeding activity and diet to prey availability (see [13])
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