Spatial Distribution of Different Age Groups of Herring in Norwegian Sea, May 1996–2020

Abstract

The commercially important Norwegian spring spawning herring is characterized by its extensive annual migrations and, on a decadal timescale, large shifts in migration patterns. These changes are not well understood, but have previously been linked to temperature, food availability, and size and age composition of the stock. Acoustic and trawl data from the International Ecosystem Surveys in the Nordic Seas, carried out annually in May since 1996, were used to analyze the spatial distribution of herring in the period 1996–2020. The dataset was disaggregated into age classes, and information about where the different age classes feed in May was derived. The analysis of herring feeding patterns in May confirms that the youngest age classes are generally found close to the Norwegian shelf, whereas the older age classes display larger variations in where they are distributed. During the period 1996–1998, the oldest age classes were found in the central and western Norwegian Sea. During the period 1999–2004, the whole stock migrated north after spawning, leaving the regions in the southern Norwegian Sea void of herring. Since 2005 the oldest herring has again congregated in the south-western Norwegian Sea, in the frontal zone between the cooler East Icelandic water and the warmer Atlantic water. There was a significant positive relationship both between stock size and distribution area and between stock size and density. Moreover, it is likely that the strong year classes 1991/1992 and 1998/1999, which were relatively old when the respective changes in migration patterns occurred, were important contributors to the changes observed in 1999 and 2005, respectively.

Spatial Variability of the Feeding Conditions for the Norwegian Spring Spawning Herring in May

“…Understanding herring behavior is complicated as multiple factors may influence stock structure, such as temperature, food availability, recruitment, stock size and inherent learned behavior between generations (Misund et al, 1997;Fernö et al, 1998;Skjoldal et al, 2004). However, Eliasen et al (2021) find no link between internal stock dynamics such as the entrance of strong year classes and changes in the distribution pattern in May, and thus concluded that the observed major distribution shifts of old herring are primarily induced by the external factors such as temperature and/or food abundance.…”

“…During the years 1996-1998, the herring mainly migrated along the eastern side of the Jan Mayen Front (JMF) (Misund et al, 1998;Holst et al, 2002). In 1999, the herring stock became abruptly confined north of 66 • N (Holst et al, 1999(Holst et al, , 2004, with nearly no herring east of Iceland (Eliasen et al, 2021). After 2003, and more persistently since 2005 to present, the herring stock did again adopt a southwestern migration toward the southwestern Norwegian Sea.…”

“…After 2003, and more persistently since 2005 to present, the herring stock did again adopt a southwestern migration toward the southwestern Norwegian Sea. Adult herring are presently densely aggregating against the slope of the Jan Mayen Ridge (Eliasen et al, 2021) (∼65.5 • N, 5 • W in Figure 1), which the authors refer to as the "feeding hotspot"-hereafter simply the Feeding Spot (FS). Homrum et al (in review) has shown that the best growth conditions of herring, during this early feeding period, are observed in the southwestern Norwegian Sea, which might reflect the abundant food availability in this region.…”

“…This reversed after 2015 to this present period. This period is characterized by an intensifying subpolar gyre (Hátún and Chafik, 2018) and the largest drop in seawater salinities ever observed in the northeast Atlantic (Holliday et al, 2020), and together with an (Eliasen et al, 2021). The Icelandic monitoring sections (KR and LA) are shown in blue, while the Faroese section (N) is divided into an Atlantic (red), frontal (black), and subarctic (blue) regions.…”

“…The present work is done in parallel with the following four papers: Eliasen et al (2021); Hátún et al (2021), Homrum et al (in review), and Skagseth et al (in review) in association with a Research Topic in Frontiers in Marine Science. We here provide information on some key aspects of temperature, zooplankton composition and abundance and herring stomach content in the vicinity of the FS.…”