Casini, Michele

Gräwe, Ulf

Haase, Stefanie

Hilvarsson, Annelie

Hüssy, Karin

Krumme, Uwe

McQueen, Kate

Mion, Monica

The eastern Baltic cod stock is in distress, but the underlying mechanisms are still poorly understood. To improve the understanding of their behaviour and ecology, 1260 cod were internally tagged with data storage tags (DSTs) during 2016 to 2019, within the international project TABACOD. The cod were caught, tagged and released in the Baltic Sea off the coast of Denmark, Germany and Sweden. 15 of the cod recaptured so far spent more than 90 days at liberty. The DSTs recorded pressure and water temperature experienced by the cod. Given the characteristic bathymetry between adjacent Baltic basins and the recapture distribution of conventionally externally tagged cod (T-bar anchor-tags), we could distinguish different patterns depending on release location. We found a surprising variety in horizontal movements; however, between June and November, all tagged cod apparently spent time in the Bornholm Basin, a major spawning area of eastern Baltic cod. While cod released near Bornholm (Denmark) displayed localized behaviour and frequently went to the deep Bornholm Basin (stationary behaviour), most of the cod tagged in Hanö Bay (Sweden) and near Rügen (Germany) returned to their shallower release areas after spawning (migratory behaviour). Additionally to these horizontal movements, cod displayed distinct vertical movements, again varying between individuals. However, a clear twilight-triggered pattern emerged year-round: with the onset of sunset, cod swam to shallower water or rose in the water column (up to 30m within 30 minutes) and returned to deeper water at sunrise. These vertical movements resulted in temperature changes of up to 9.6°C during single excursions crossing the thermocline. The unexpected spatio-temporal dynamics challenge our present understanding of eastern Baltic cod ecology. Moreover, the extended use of shallow water areas and the open water column suggest that the present survey approach of the Baltic International Trawl Survey has an unknown bia

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From dusk till dawn: Diversity and similarities in the movement patterns of eastern Baltic cod from DSTs

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From dusk till dawn: Diversity and similarities in the movement patterns of eastern Baltic cod from DSTs

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