Seasonal Dynamics of Atlantic Herring (Clupea harengus L.) Populations Spawning in the Vicinity of Marginal Habitats

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.

Individual habitat transitions of Atlantic herring Clupea harengus in a human-modified coastal system

“…It is also an interesting area with regard to potential population differences in behaviour and internal drivers, such as maturation status of individual fish. Here, 3 putative herring populations were observed to co-occur at maturing, spawning and spent stages from March to June, where 1 population, 'Landvik herring', showed high fidelity to the artificial estuary (Eggers et al 2014). In addition to 'Landvik herring', coastal Skager rak spring spawning herring (CSS) exist in neighbouring fjords, without conducting large an nual migrations like those of Norwegian spring spawning herring (NSS), which are also found here.…”

“…Typically, Landvikvannet now has a highly stratified water column with a transition depth at 4 m. In May the upper layer has low salinity (< 20 PSU) and higher temperature (> 8°C), and is oxygen rich (>1 ml l −1 ). In contrast, the lower layer has a constant temperature (7 to 8°C) and high salinity (> 20 PSU), and no oxygen but toxic hydrogensulphide (for details, see Eggers et al 2014). Strandfjorden is about 2 km long and sheltered from the outer coast.…”

“…While the environmental conditions in the Kiel Canal are similar to the Baltic Sea and herring found inside the canal did not form an individual population, the herring in Østerbøvatn are classified as a single population, distinct from the neighbouring population in the full marine habitat. Also, the varying residence time of tagged herring in Landvikvannet of 2 to 36 d suggests that the lake may be suitable for long-term residence for herring populations adapted to the environmental conditions of this marginal habitat (Eggers et al 2014), whereas populations not adapted may choose to leave quickly. Besides the artificial estuaries Landvikvannet and Østerbøvatn, herring have colonized habitats with similar environmental conditions and can be phenotypically distinct from other populations (Neb 1970, Hognestad 1994.…”

“…In other cases, humaninduced habitat changes may also present animals with new opportunities for expanding their range of movements, for instance when new canals connect aquatic ecosystems that have previously been isolated from each other (Silva et al 2013, Eggers et al 2014. Such human-induced alterations of connectivity may influence population dynamics as well as interspecific competition and predator−prey relationships.…”

“…In addition to 'Landvik herring', coastal Skager rak spring spawning herring (CSS) exist in neighbouring fjords, without conducting large an nual migrations like those of Norwegian spring spawning herring (NSS), which are also found here. NSS occur in this area mostly in March before starting their annual migration (Eggers et al 2014). Using individual acoustic tagging and monitoring, herring habitat transitions in this complex artificial estuary-canalfjord-ocean system were quantified and their pur-pose evaluated.…”