Eaten by a cormorant: Unexpected return of a tagged Baltic cod

Temperature-depth data storage tags (DSTs) continuously store individual measurements of water depth and water temperature of the habitat used by free-ranging fish. To analyse the data, the DSTs need to be returned. Usually they are returned by fishers whereas returns of fish tagged with DSTs that died due to natural causes are rare.

During an international tagging project, one DST was returned by an ornithologist which found the data logger in a pellet in a cormorant colony. Although the cod itself was not available since it was already digested, the DST provided 90 days of data prior to the predation event. The cod mostly remained in waters shallower than 40 m and conducted daily in- and offshore movements covering the whole water column, leading to daily temperature differences of up to 8.8°C.

In May, during one trip where the cod used very shallow waters (1.5m water depth) in the morning (09:00 h), it was taken by a cormorant. Digestion, indicated by temperatures of 39.3°C to 41.2°C, took 31 hours. This returned DST emphasizes that large-scale tagging projects should not only raise awareness in the fisheries sector but also among ornithologists when avian predators target the tagged fish species.