Mackerel predation on herring larvae during summer feeding in the Norwegian Sea

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 degrees N and 69 degrees 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.