Seasonal dynamics of Atlantic herring (Clupea harengus L.) populations spawning in the vicinity of marginal habitats.

Florian Eggers,Aril Slotte,Richard D M Nash,Even Moland,Cecilie Kvamme,Lísa Anne Libungan,Arne Johannessen,Esben M Olsen

doi:10.1371/journal.pone.0111985

Abstract

Gillnet sampling and analyses of otolith shape, vertebral count and growth indicated the presence of three putative Atlantic herring (Clupea harengus L.) populations mixing together over the spawning season February–June inside and outside an inland brackish water lake (Landvikvannet) in southern Norway. Peak spawning of oceanic Norwegian spring spawners and coastal Skagerrak spring spawners occurred in March–April with small proportions of spawners entering the lake. In comparison, spawning of Landvik herring peaked in May–June with high proportions found inside the lake, which could be explained by local adaptations to the environmental conditions and seasonal changes of this marginal habitat. The 1.85 km2 lake was characterized by oxygen depletion occurring between 2.5 and 5 m depth between March and June. This was followed by changes in salinity from 1–7‰ in the 0–1 m surface layer to levels of 20–25‰ deeper than 10 m. In comparison, outside the 3 km long narrow channel connecting the lake with the neighboring fjord, no anoxic conditions were found. Here salinity in the surface layer increased over the season from 10 to 25‰, whereas deeper than 5 m it was stable at around 35‰. Temperature at 0–5 m depth increased significantly over the season in both habitats, from 7 to 14°C outside and 5 to 17°C inside the lake. Despite differences in peak spawning and utilization of the lake habitat between the three putative populations, there was an apparent temporal and spatial overlap in spawning stages suggesting potential interbreeding in accordance with the metapopulation concept.

Highlights

Fish species may be split into populations based on their degree of reproductive isolation from each other in space and/or time, which could be reflected in genetic or phenotypic differences driven by diverging environmental conditions [1,2,3]
Salinity ILV at 0–1 m increased over the season from 1% in March to 7% in June, but was stable around 20–25% deeper than 10 m
There were no anoxic conditions in Strandfjorden, the salinity at 0–1 m increased from 10% in March to 25% in June and was stable at 35% deeper than 5 m

Summary

Introduction

Fish species may be split into populations based on their degree of reproductive isolation from each other in space and/or time, which could be reflected in genetic or phenotypic differences driven by diverging environmental conditions [1,2,3]. Under such circumstances exploitation on one population should have little effect on the population dynamics of a neighboring population, and it is common to assess and manage such populations separately [4,5]. When referring to fisheries management, the term ‘‘stock’’ is used, otherwise the term ‘‘population’’ is preferred

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