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January 23rd – January 31st 2023, Kiel (Germany) – Kiel (Germany) Winter cod 2021-25

March 1st – March 10th 2023, Kiel (Germany) – Kiel (Germany) BALTEACH – 1

Understanding individual growth in commercially exploited fish populations is key to successful stock assessment and informed ecosystem-based fisheries management. Traditionally, growth rates in marine fish are estimated using otolith age-readings in combination with age-length relationships from field samples, or tag-recapture field experiments. However, for some species, otolith-based approaches have been proven unreliable and tag-recapture experiments suffer from high working effort and costs as well as low recapture rates. An important alternative approach for estimating fish growth is represented by bioenergetic modelling which in addition to pure growth estimation can provide valuable insights into the processes leading to temporal growth changes resulting from environmental and related behavioural changes. We here developed an individual-based bioenergetic model for Western Baltic cod (Gadus morhua), traditionally a commercially important fish species that however collapsed recently and likely suffers from climate change effects. Western Baltic cod is an ideal case study for bioenergetic modelling because of recently gained in-situ process knowledge on spatial distribution and feeding behaviour based on highly resolved data on stomachs and fish distribution. Additionally, physiological processes such as gastric evacuation, consumption, net-conversion efficiency and metabolic rates have been well studied for cod in laboratory experiments. Our model reliably reproduced seasonal growth patterns observed in the field. Importantly, our bioenergetic modelling approach implementing depth-use patterns and food intake allowed us to explain the potentially detrimental effect summer heat periods have on the growth of Western Baltic cod that likely will increasingly occur in the future. Hence, our model simulations highlighted a potential mechanism on how warming due to climate change affects the growth of a key species that may apply for similar environments elsewhere.

Seafood trade is a global business, where catches, processing, and consumption are increasingly separated. An increasingly integrated global market creates telecouplings, i.e. connections between fish stocks that are ecologically separated. These telecouplings may spread the impact of vulnerabilities, such as climate change, between unconnected fisheries. The effect of climate change on fisheries is often analyzed on a fish stock basis, which may overlook the spread of these vulnerabilities. Atlantic cod (Gadus morhua) stocks, an iconic fish species, are no exception. Depending on the geographical location, stocks have been impacted differently by climate change, with North-East Arctic (NEA) cod, the stock in the Barents Sea, reaching record high biomass levels and other stocks being extremely depleted. Here, we investigate how these dynamics occurring in the ecological system affect global trade of cod. We find that the global export is fully dominated by NEA cod catches. Applying Structural Equation Modelling, we discover that the high biomass level of NEA cod has positive effects on catches and exports and leads to lower global market prices. However, zooming in on individual stocks and the countries exploiting them using correlation networks, we find heterogeneous responses of other countries, where catches for some stocks increase and others decrease in response to lower global prices. Our results highlight how changes on one fishery may have important repercussion on stocks in different ecosystems, as well as on societies reliant on them.

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.