Based on two large-scale surveys in the Norwegian Sea in July 2004 and 2006 we investigated the potential overlap in distribution and diets and association with environmental variables for mackerel, herring and blue whiting. Mackerel and blue whiting had low overlap in both distribution and diets, and were associated with warm Atlantic and cold prey-rich Arctic waters, respectively. Also, herring and mackerel were negatively correlated, associated with different environmental variables and even had low diet overlap in Atlantic water where the highest overlap in distribution was observed, but these trends were much clearer in 2006 than in 2004. Prey was patchily distributed and data from single stations indicated that feeding was opportunistic, in particular for mackerel. However, mackerel diet width and feeding incidence were similar between the years, whereas herring was more selective in 2004, probably reaching the end of the feeding season. A delayed peak in Calanus availability was indicated in 2006 and may have prolonged the feeding season, partly explain the difference between the years. Our data did not indicate a prey shift due to encounters between mackerel and herring. Concentrated efforts in limited areas are needed to further elucidate this aspect of interaction.

In the course of the past two decades, Atlantic mackerel, Scomber scombrus, have expanded their summer feeding distribution in the Norwegian Sea substantially, and now potentially overlap with pelagic larvae of Norwegian spring-spawning herring, Clupea harengus, as these drift northwards. Mackerel are known to be opportunistic predators, and the aim of this study was to evaluate mackerel predation in an area of overlap between mackerel and herring larvae, with particular focus on predation on herring larvae. In early June 2013, we followed a predefined transect in the expected core larvae distribution area on the Norwegian coastal shelf between about 66°N and 69°N. The transect was repeated twice, and samples of mackerel for stomach analyses and subsequent herring larvae samples were obtained at pre-defined stations. Mackerel were caught in all but one of the trawl hauls, but were hardly ever observed acoustically, suggesting that they were dispersed close to the surface throughout the study area. Herring larvae were caught in all samples. Calanoid copepods were the dominant prey of the mackerel, but 45% of the mackerel guts contained herring larvae, with a maximum of 225 larvae counted in a single gut. Both the frequency of guts containing herring larvae and the average amount of herring larvae increased in line with increasing abundance of larvae. On the other hand, no spatial correlation between mackerel abundance and herring larvae abundance was found at the station level. The results suggest that mackerel fed opportunistically on herring larvae, and that predation pressure therefore largely depends on the degree of overlap in time and space. Rough areal projections suggest that the mackerel would be capable of consuming the herring larvae present in the investigation area in 6–7 d, and that such predation therefore could have regulatory effects on stocks of Norwegian spring-spawning herring.

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The Norwegian Sea is a migration and feeding ground for fin whales (Balaenoptera physalus) and humpback whales (Megaptera novaeangliae) in summer. During the last decade, significant structural changes in the prey community, including northerly expansion and movement in the distribution of pelagic fish species, have been reported from this ecosystem. However, little information on whale feeding ecology exists in the Norwegian Sea and surrounding waters. A total of 59 fin whales and 48 humpback whales were sighted during 864 h of observation over an observation distance of about 8200 nmi (15,200 km) in the Norwegian Sea from 15 July to 6 August 2006 and 2007. The fin whale group size, as mean (AESD), varied between one and five individuals (2.1 AE 1.2 ind.) and humpback whale group size varied between one and six individuals (2.5 AE 1.7 ind.). Fin-and humpback whales were observed mainly in the northern part of the study area, and were only found correlated with the presence of macro-zooplankton in cold Arctic water. Humpback whales were not correlated with the occurrence of adult Norwegian spring-spawning herring