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.

Pelagic marine fish often display highly dynamic migration patterns. However, such movement behaviour is usually studied at the population or school level, while less is known about individual movement characteristics and habitat transitions. During March 2012 to June 2013, we used acoustic tags and moored receivers to monitor the behaviour of Atlantic herring Clupea harengus L. (N = 47) throughout a range of habitats on the Skagerrak coast in southern Norway. Five of the tagged herring entered a former lake transformed into an artificial estuary by a humanmade canal linking the former lake to the open ocean. Herring resided in this system for up to 36 d. All tagged herring left the fjord where they were tagged by early August 2012. This habitat transition was detected by the receivers as 3 main pulses of tagged individuals, which were assumed to be formed by putative populations mixing in the area. Most transitions occurred during nighttime regardless of tidal cycle, and it is suggested that spawning is the primary driver for entering the fjord and artificial estuary. Later detections at a separate receiver system 17 km to the northeast suggest that some herring may overwinter in coastal areas. In the spring of 2013, 3 of the tagged herring returned to their original fjord tagging location. Our study reveals new aspects of herring migration dynamics linked to anthropogenic modifications of connectivity, and suggests that capacity for individual behaviours in schooling fish may be underestimated. KEY WORDS: Schooling fish · Acoustic telemetry · Movement behaviour · Migration · SkagerrakResale or republication not permitted without written consent of the publisher

The Italian spined loach (Cobitis bilineata) is an elongated, small‐sized (<12 cm) spined loach native to northern Italy, Slovenia and Croatia. As for loaches in general, little is known about the individual movements of this loach in nature. Passive integrated transponders (PIT‐tags) are small (typically 7–32 mm), relatively cheap and allow tracking of individual fish movements and behaviour. A fundamental assumption in animal telemetry is that the performance of a tagged animal does not deviate substantially from its natural performance. Although PIT‐tagged fish often display high survival and tag retention, the effect varies between species and contexts, and few studies have looked at behavioural effects of PIT‐tagging. Here we demonstrate a PIT‐tagging methodology for spined loaches, and compare survival, activity and provoked escape response (maximum swimming speed) between tagged and control fish. We also track tag retention in the tagged fish. Italian spined loaches tagged with 12 mm PIT‐tags displayed high tag retention and no extra mortality, and no effects of tagging on activity or maximum swimming speed were observed. The tag‐to‐fish weight and length ratios in our study ranged from 2% to 5% and from 10% to 16%, respectively, and we conclude that PIT‐tagging, within these ratios, appears suitable for Italian spined loach.