Abstract
Abstract The Northeast Atlantic mackerel (Scomber scombrus) stock has increased and expanded its summer feeding migration west- and northwards since 2006, entailing large geopolitical challenges for the countries harvesting this species. A common perspective is that climatic warming opens up new regions for biota in the north. It has also been suggested that the presently large pelagic fish stocks deplete prey resources in the eastern North Atlantic during their summer feeding phase, forcing the stocks west towards the Irminger Sea in their search for food. Here, we suggest that the declining nutrient (silicate) concentrations observed along the northern European continental slope reduce primary and thus secondary production, exacerbating food scarceness in the east and adding to the incentive to migrate westward. The new westward feeding route requires that the fish cross the Iceland Basin, which during the summer season quickly becomes nutrient-depleted and thus might act as a barrier to migration after the spring bloom. Using mackerel and zooplankton abundance data from the International Ecosystem Summer Surveys in the Nordic Seas, we suggest that the oligotrophic waters in the central Iceland Basin force the fish to migrate through a narrow ‘corridor’ along the south Iceland shelf, where nutrients are replenished and both primary and secondary production are higher.
Highlights
The Northeast Atlantic mackerel (Scomber scombrus) is a highly migratory species that, after spawning along the European shelf, gradually moves northwards (Figure 1a) into the summer feeding areas in the Norwegian Sea (Uriarte et al, 2001; Iversen, 2002; ICES, 2014a)
The position of the 6–7 °C isotherm, the lower bound of the temperature niche of mackerel (Utne et al, 2012), has been relatively stationary throughout the period 2010–2014 (ICES, 2014b). it appears that average summer temperatures ( July–August) in the Norwegian Sea have not changed enough to explain the post-2006 expansion of the feeding area
The paper is divided into two main parts: first, we propose and test a new hypothesis that mackerel migration through the Iceland Basin is restricted by food availability caused by nutrient limitation to primary production; second, we generalize the findings from this test and discuss the post-2006 large-scale westward expansion in a nutrient limitation context
Summary
The Northeast Atlantic mackerel (Scomber scombrus) is a highly migratory species that, after spawning along the European shelf, gradually moves northwards (Figure 1a) into the summer feeding areas in the Norwegian Sea (Uriarte et al, 2001; Iversen, 2002; ICES, 2014a). A portion of the stock migrates southwards and into the North Sea. After 2006, the mackerel stock has been steadily increasing and expanding northwards into the northern parts of the Norwegian Sea, and westwards into Icelandic waters. Recent studies have shown that the expansion areas in the Irminger Sea, Icelandic waters and the northern Norwegian Sea have, for decades, had summer temperatures tolerable (> 5 °C), but well in the temperature range that mackerel occupy (Astthorsson et al, 2012; Utne et al, 2012; MacKenzie et al, 2014)
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