Nutrient-driven poleward expansion of the Northeast Atlantic mackerel (Scomber scombrus) stock: A new hypothesis

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.

Environmentally Driven Ecological Fluctuations on the Faroe Shelf Revealed by Fish Juvenile Surveys

“…Silicate concentrations across the north Atlantic have similar trends and peaks ( Figure 4D) and are linked to the subpolar gyre dynamics as represented by the gyre index (Hátún et al, 2017b). The silicate concentration off the Faroe shelf may therefore influence the intensity of the CS primary production, in addition to the oceanic phytoplankton dynamics (Allen et al, 2005;Pacariz et al, 2016).…”

Geographical expansion of Northeast Atlantic mackerel (Scomber scombrus) in the Nordic Seas from 2007 to 2016 was primarily driven by stock size and constrained by low temperatures

“…Astthorsson et al (2012) Temperature cannot explain the lack of mackerel presence in the Iceland Basin (see Fig. 1 (Pacariz et al, 2016). This approximately three-week difference in survey dates could explain why mackerel abundance and mesozooplankton density was high in the Iceland Basin in 2015 compared to the other years.…”

“…Even small changes in temperature (< ~2°C) can influence stock distribution on various temporal and spatial scales (Drinkwater, 2003(Drinkwater, , 2006Sundby and Nakken, 2008). Temperature changes often coincide with large-scale changes in circulation patterns, nutrient upwelling and plankton production, which are likely to influence prey availability of pelagic plankton feeders like mackerel (Nye et al 2014;Hátún et al 2016;Pacariz et al, 2016). Astthorsson et al (2012) Temperature cannot explain the lack of mackerel presence in the Iceland Basin (see Fig.…”

“…This approximately three-week difference in survey dates could explain why mackerel abundance and mesozooplankton density was high in the Iceland Basin in 2015 compared to the other years. Pacariz et al (2016) furthermore suggested that the ongoing nutrient decline throughout the subpolar North Atlantic (Hátún et al, 2017a;Rey, 2012), together with the west-east (high-low) horizontal nutrient gradient, might have added to the density-dependent depletion of food resources in the east, forcing mackerel to migrate farther north and west in their search for food.…”

Influence of Phytoplankton Advection on the Productivity Along the Atlantic Water Inflow to the Arctic Ocean

“…Two main regions along the AWI show that the timing of phytoplankton growth is altered in such a way as to result in a longer productive season: the spring (May) bloom starts earlier in the NAC in comparison to the Norwegian and Greenland Seas where peak productivity is delayed until June (Supplementary Figure S1). In the absence of sea ice, early productivity implies a better use of light within the current, though it is likely that the NAC may replenish its nutrient load sooner due to its shallower nature and proximity to coastal processes (Saetre, 2007;Pacariz et al, 2016;but see Johnson et al, 2013). In addition, advection, by bringing carbon to the north, enables productivity (GPP) in the WSC as late as September, extending the growth season.…”