Population Structure of Atlantic Mackerel (Scomber scombrus)

Abstract

Atlantic mackerel (Scomber scombrus) occurs on both sides of the north Atlantic and has traditionally been grouped into 5 spawning components, some of which were thought to be isolated natal homing stocks. Previous studies have provided no evidence for cross Atlantic migration and no or weak support for isolated spawning components within either side of the North Atlantic. We question the de-facto accepted hypothesis of isolation between spawning components on the basis of spawning and age distribution data. The spawning intensities, proxied by larval abundances, are negatively correlated between the North Sea and Celtic Sea, which indicates that the two spawning components may be connected by straying individuals. This finding is based on unique larvae samples collected before the collapse of North Sea component, thus showing that the exchange is not a recent phenomenon due to the collapse. The analyses of old as well as more recent age distributions show that strong year classes spread into other areas where they spawn as adults (“twinning”). Our findings are in accordance with the lack of solid evidence for stock separation from previous analyses of tagging data, genetics, ectoparasite infections, otolith shapes, and blood phenotypes. Because no method has been able to identify the origin of spawning mackerel unequivocally from any of the traditional spawning components, and in the light of our results, we conclude that straying outweighs spatial segregation. We propose a new model where the population structure of mackerel is described as a dynamic cline, rather than as connected contingents. Temporal changes in hydrography and mackerel behavior may affect the steepness of the cline at various locations. The new interpretation of the population structure of Atlantic mackerel has important implications for research, assessment and management.

Nursery areas and recruitment variation of Northeast Atlantic mackerel (Scomber scombrus)

“…This simplistic population model has recently been challenged by results, indicating structures within the spawning components and substantial straying among the spawning components (Jansen and Gislason, 2013). A more complex population model is, therefore, needed to represent actual details and dynamics.…”

“…The elongate band of high density that runs parallel with the shelf edge was, to some extent, segmented by two areas of relatively low density, namely west of the English Channel (49.58N 88W) and around the Fair Isle Channel (608N 2.58W) (Figure 8). The Fair Isle Channel is also known to form a discontinuity in the spawning area and has been regarded as the border between the western and North Sea spawning components (Jansen and Gislason, 2013). However, the area west of the English Channel (49.58N) is in the middle of the spawning area for the western component, and a study that suggested spatial segregation within the western area did not indicate an abrupt shift around 49.58N .…”

Thermal Niche Tracking and Future Distribution of Atlantic Mackerel Spawning in Response to Ocean Warming

“…However, from that year on (based on tagging experiments, which validated the migratory route of mackerel in the Northeast Atlantic waters), it was agreed to unify the population in a single stock. This stock is divided into three spawning components (ICES, 1996): North Sea Spawning Component (NSC), Western Spawning Component (WSC, Biscay to northwest of Scotland), and Southern Spawning Component (SSC, Gibraltar to southern Biscay); the existence of these spawning components still remains controversial Some studies on natal homing studies indicate the existence of structures within the spawning components (Nesbö et al, 2000; with mixing between them (Jansen and Gislason, 2013), although most of the studies failed to identify patterns (Jamieson and Smith, 1987;MacKenzie, 1990;Abaunza et al, 1995).…”

Opportunistically recorded acoustic data support Northeast Atlantic mackerel expansion theory

“…Recent work comparing larval time series between the 1950s and 1970 s suggested that the spawning dynamics are more complicated and that the North Sea spawning stock should not be considered an isolated natal homing stock but instead is linked with the western spawning population (Jansen and Gislason, 2013).…”

“…Reasons for this have been attributed to possible decreases in foraging opportunities due to increased windinduced turbulence and decreasing zooplankton concentrations (Jansen, 2014). Another factor could have been the fishery in the northern North Sea during autumn and winter which targeted mackerel which could have otherwise contributed to spawning in the North Sea (Jansen and Gislason, 2013).…”