Funk, Steffen and Möllmann, Christian (2023) Monitoring winter spawning activity of Western Baltic cod (Gadus morhua) (2021-25), Cruise No. AL606, January 22nd – January 30th 2024, Kiel (Germany) – Kiel (Germany), Winter cod 2021-25. . Alkor-Berichte, AL606 . Institute of Marine Ecosystem and Fisheries Science, University of Hamburg, 35 pp. DOI 10.3289/CR_AL606.

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Abstract

This cruise was the fourth out of five proposed cruises in the framework of the winter cod 2021-
25 cruises of the IMF, which aim to investigate the early winter spawning activity of Western
Baltic cod. For this purpose, ichthyoplankton samples and cod samples were taken on a pre-defined
station grid. Cod samples were used to investigate maturity stages and condition of adult cod.
Plankton samples were analysed with a focus on the occurrence of cod eggs and larvae. The data
will be used to identify whether there is a shift towards earlier spawning activities of Western
Baltic cod and how spawning activity differs spatially between parts of the Belt Sea.
During the cruise AL606 spawning cod were found in the Kiel Bight, the Mecklenburg Bight,
and the Fehmarn Belt. First analyses of BONGO-net samples revealed no occurrence of cod larvae
in the sampling area during this year’s January cruise 2024. This absence of cod larvae might be a
result of the relatively harsh and cold winter of 2023/24 and an associated delay in the maturation
and spawning of western Baltic cod compared to the warmer previous years 2021-2023. The
observation of fewer larvae of other fish species (e.g., plaice) compared to previous years supports
this hypothesis.
However, especially the observation of spawning and post-spawning cod in the Mecklenburg
Bight during AL606 provided further evidence for the hypothesised temporal shift in spawning
phenology towards an earlier onset than described in the past.

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