Characteristics of the Norwegian Coastal Current during Years with High Recruitment of Norwegian Spring Spawning Herring (Clupea harengus L.).

Øystein Skagseth,Richard D M Nash,Aril Slotte,Erling Kåre Stenevik,Brian R Mackenzie

doi:10.1371/journal.pone.0144117

Øystein Skagseth, Richard D M Nash + Show 3 more

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https://doi.org/10.1371/journal.pone.0144117

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Abstract

Norwegian Spring Spawning herring (NSSH) Clupea harengus L. spawn on coastal banks along the west coast of Norway. The larvae are generally transported northward in the Norwegian Coastal Current (NCC) with many individuals utilizing nursery grounds in the Barents Sea. The recruitment to this stock is highly variable with a few years having exceptionally good recruitment. The principal causes of recruitment variability of this herring population have been elusive. Here we undertake an event analysis using data between 1948 and 2010 to gain insight into the physical conditions in the NCC that coincide with years of high recruitment. In contrast to a typical year when northerly upwelling winds are prominent during spring, the years with high recruitment coincide with predominantly southwesterly winds and weak upwelling in spring and summer, which lead to an enhanced northward coastal current during the larval drift period. Also in most peak recruitment years, low-salinity anomalies are observed to propagate northward during the spring and summer. It is suggested that consistent southwesterly (downwelling) winds and propagating low-salinity anomalies, both leading to an enhanced northward transport of larvae, are important factors for elevated recruitment. At the same time, these conditions stabilize the coastal waters, possibly leading to enhanced production and improved feeding potential along the drift route to Barents Sea. Further studies on the drivers of early life history mortality can now be undertaken with a better understanding of the physical conditions that prevail during years when elevated recruitment occurs in this herring stock.

Highlights

The recruitment of Norwegian Spring Spawning herring (NSSH) Clupea harengus L. is highly dynamic both in regard to the number of recruits and distribution of recruits across nursery areas [1,2,3,4]
The fjords along the Norwegian coast are important nursery grounds for NSSH, annually providing recruits to the stock but generally it is the juveniles that reside in the Barents Sea nursery that make up the majority of very strong year-classes [2, 5]
Further modeling work has demonstrated that the southern spawning and subsequent northward dispersion generally resulted in a high spatio-temporal overlap of herring larvae with spring phytoplankton blooms developing along the larval drift route to the Barents Sea [33]

Summary

Introduction

The recruitment of Norwegian Spring Spawning herring (NSSH) Clupea harengus L. is highly dynamic both in regard to the number of recruits and distribution of recruits across nursery areas [1,2,3,4]. The fjords along the Norwegian coast are important nursery grounds for NSSH, annually providing recruits to the stock but generally it is the juveniles that reside in the Barents Sea nursery that make up the majority of very strong year-classes [2, 5]. NSSH stock has been influenced by fishing pressure and shown to vary in phase with the temperature of the Atlantic Water throughout the 20th century [6]. The long lasting lack of recovery during the 1970s was thought to be due to low spawning stock size, and coincided with a period of relatively low water temperatures [6]. Studies of the North Sea herring indicate that the intertwining of environmental forcing, ecosystem state and other factors result in a complex relationship which affects survival in young herring [8]

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