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

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.