Historical mackerel expansion in the Northeast Atlantic during the last decade: what is happening and why?

• ¹ Institute of Marine Research, Norway

Migrations of fish are often associated with exploitation of spatially and temporally varying food resources. Long-distance migrations and extensive distributions are key features of pelagic planktivorous fish species. Less than 0.01% of all fish species make long-distance migrations, but these species are also highly abundant and of substantial economic value worldwide. The Northeast Atlantic (NEA) mackerel (Scomber scombrus L.) is a widely distributed fish species that play a key ecological role in several coastal and offshore marine ecosystems, and is also one of the most commercially valuable fish species in the Atlantic Ocean. International catches of NEA mackerel reached staggering 1.4 million tonnes in 2014, worth approximately 2 billion euros! Mackerel is a pelagic schooling fish, and mackerel now makes extensive northward feeding migrations in the Nordic seas. The Norwegian Sea provides a large and productive feeding ground up in the high north, including Arctic regions, for pelagic fish in spring and summer. The mackerel population has rapidly increased in abundance and expanded its geographic distribution during the last decade. The mackerel stock expanded its geographic range during the feeding season from 1.3 mill. km2 in 2007 to at least 2.9 mill. km2 in 2014, mainly towards western and northern regions of the Nordic seas. Mackerel has recently been recorded as far north as Svalbard and west to Cape Farwell in Greenland. The recent rapid increase in stock size of mackerel has increased the intra- and inter-specific competition for limited food resources. Increasing temperature, heat content, and changing prey conditions and reduced zooplankton concentrations during the last 15 years in the Norwegian Sea, may have forced mackerel to expand their feeding migration and distribution, in addition to expand their feeding season. Thus, a major driving force behind the historical expansion of mackerel in the Norwegian Sea and surrounding waters seems to be limited available food resources, where intra-specific competition is leading to pronounced density-dependent growth in the mackerel stock. Since mackerel is also competing for common prey resources (copepods) with e.g. Norwegian spring-spawning herring (Clupea harengus L.), inter-specific competition also seem to be operating within this large oceanic ecoystem. High abundance of mackerel combined with limited food resources, now force mackerel to enter new and productive regions towards north and west. However, until recently is was not known how mackerel exploit the spatially varying feeding resources, and their vertical distribution and swimming behaviour were also largely unknown. About 250 mackerel schools were tracked with modern multibeam sonars in combination with ADCP measurements, pelagic trawling, zooplankton sampling and oceanographic recordings. Swimming direction of the tracked schools mainly coincided with the prevailing northerly Atlantic current direction in the Norwegian Sea. Swimming with the current saves energy, and the current also provides a directional cue towards the most productive areas in the northern Norwegian Sea. Average mean swimming speed was about 3.8 body lengths sec–1. However, fish did not swim in a straight course, but often changed direction, suggesting active feeding in the near field, either by filter feeding or particulate feeding. Mackerel swim close to the surface in summer, with schools staying above the thermocline (<30 m) in waters of at least 6°C. In surface waters, mackerel encounter improved foraging rate and swimming performance. Going with the flow until temperature is too low, based on an expectation of increasing foraging rate towards the north and west while utilizing available prey underway, could be a simple and robust feeding strategy for mackerel.