Spatio-temporal dynamics and behavioural ecology of a “demersal” fish population as detected using research survey pelagic trawl catches: the Eastern Baltic Sea cod (Gadus morhua)

Casini, Michèle; Tian, Heng‐Ci; Hansson, Martin; Grygiel, Włodzimierz; Strods, Guntars; Statkus, Romas; Sepp, Elor; Gröhsler, Tomas; Orio, Alessandro; Larson, Niklas

doi:10.1093/icesjms/fsz016

Cited by 5 publications

(6 citation statements)

References 28 publications

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“…However, the recovering EBC stock did not re-colonize their former north-easterly territories (i.e. Gotland basin; [ 54 ]). The spreading of areas with hypoxic or even anoxic conditions in the Gotland basin resulted in a large-scale loss of historic spawning grounds and hence a massive decrease in cod abundance in the northern area [ 34 ].…”

Section: Discussionmentioning

confidence: 99%

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Living apart together: Long-term coexistence of Baltic cod stocks associated with depth-specific habitat use

Schade¹,

Weist²,

Dierking

et al. 2022

PLoS ONE

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Coexistence of fish populations (= stocks) of the same species is a common phenomenon. In the Baltic Sea, two genetically divergent stocks of Atlantic cod (Gadus morhua), Western Baltic cod (WBC) and Eastern Baltic cod (EBC), coexist in the Arkona Sea. Although the relative proportions of WBC and EBC in this area are considered in the current stock assessments, the mixing dynamics and ecological mechanisms underlying coexistence are not well understood. In this study, a genetically validated otolith shape analysis was used to develop the most comprehensive time series of annual stock mixing data (1977–2019) for WBC and EBC. Spatio-temporal mixing analysis confirmed that the two stocks coexist in the Arkona Sea, albeit with fluctuating mixing proportions over the 43-year observation period. Depth-stratified analysis revealed a strong correlation between capture depth and stock mixing patterns, with high proportions of WBC in shallower waters (48–61% in <20m) and increasing proportions of EBC in deeper waters (50–86% in 40-70m). Consistent depth-specific mixing patterns indicate stable differences in depth distribution and habitat use of WBC and EBC that may thus underlie the long-term coexistence of the two stocks in the Arkona Sea. These differences were also reflected in significantly different proportions of WBC and EBC in fisheries applying passive gears in shallower waters (more WBC) and active gears in deeper waters (more EBC). This highlights the potential for fishing gear-specific exploitation of different stocks, and calls for stronger consideration of capture depth and gear type in stock assessments. This novel evidence provides the basis for improved approaches to research, monitoring and management of Baltic cod stocks.

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Section: Discussionmentioning

confidence: 99%

“…basin; [54]). The spreading of areas with hypoxic or even anoxic conditions in the Gotland basin resulted in a large-scale loss of historic spawning grounds and hence a massive decrease in cod abundance in the northern area [34].…”

Section: Plos Onementioning

confidence: 99%

Living apart together: Long-term coexistence of Baltic cod stocks associated with depth-specific habitat use

Schade¹,

Weist²,

Dierking

et al. 2022

PLoS ONE

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“…The average weight of a 40-cm long cod has dropped from 900 to 600 g from the early 1990s to 2018. This is of biological concern in terms of affecting population reproductive potential [3] and mortality [4], but also changing trophic interactions [5,6]. Additionally, the increase of slender cod has been detrimental for the fisheries industry that complained about increased catches of scrawny individuals with little or no commercial value.…”

Section: Introductionmentioning

confidence: 99%

Otolith chemistry indicates recent worsened Baltic cod condition is linked to hypoxia exposure

Limburg

Casini

2019

Biol. Lett.

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Deoxygenation worldwide is increasing in aquatic systems with implications for organisms' biology, communities and ecosystems. Eastern Baltic cod has experienced a strong decline in mean body condition (i.e. weight at a specific length) over the past 20 years with effects on the fishery relying on this resource. The decrease in cod condition has been tentatively linked in the literature to increased hypoxic areas potentially affecting habitat range, but also to benthic prey and/or cod physiology directly. To date, no studies have been performed to test these mechanisms. Using otolith trace element microchemistry and hypoxia-responding metrics based on manganese (Mn) and magnesium (Mg), we investigated the relationship between fish body condition at capture and exposure to hypoxia. Cod individuals collected after 2000 with low body condition had a higher level of Mn/Mg in the last year of life, indicating higher exposure to hypoxic waters than cod with high body condition. Moreover, lifetime exposure to hypoxia was even more strongly correlated to body condition, suggesting that condition may reflect long-term hypoxia status. These results were irrespective of fish age or sex. This implies that as Baltic cod visit poor-oxygen waters, perhaps searching for benthic food, they compromise their own performance. This study specifically sheds light on the mechanisms leading to the low condition of cod and generally points to the impact of deoxygenation on ecosystems and fisheries.

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“…The inclusion of some level of oxygendriven change in feeding, whether through a decline in the biomass of benthic invertebrates (B) and in cod oc cupancy of benthic habitats (O), through a decline in feeding activity, assimilation efficiency, and egg survival (P), or all three of these, all appear in superior models (Appendix S5; Figure S11). The former two mechanisms are supported by observations of a decline in cod feeding on benthos (Neuenfeldt et al 2020) and in benthic occupancy of cod (Casini et al 2019). The latter is supported by observations of reduced appetite in hypoxic conditions (Pichavant et al 2001; Chabot andClaireaux 2008), as well as reduced assimilation efficiency (Wang et al 2009) and egg survival (Köster et al 2005) of cod.…”

Section: Discussionmentioning

confidence: 77%

Declining food availability and habitat shifts drive community responses to marine hypoxia

Duskey

Casini

Limburg

et al. 2023

Preprint

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Worsening marine hypoxia has had severe negative consequences for fish communities across the globe. While individual and populationlevel impacts of deoxygenation have been identified, it is unknown how they interact to drive changes in food webs. To address this, we incorporated several major impacts of hypoxia, including declines in benthic re sources, habitat shifts, increasing mortality, and changes to rates of feeding, assimilation, and reproductive efficiency, into an existing size spectrum food web modeling framework. We used this structure to ask the following questions: which of these direct effects are most critical to capturing population and community dynamics in a representative hypoxic system, how do they interact to result in community responses to deoxygenation, and what are the potential consequences of these effects in the context of accelerating deoxygenation? We tested the effect of different combinations of oxygendependent processes, driven by observed oxygen levels, on the food web model’s ability to explain time series of observed somatic growth, diets, biomass, and fishery yields of commercially relevant species in the Baltic Sea. Model results suggest that food availability is most critical to capturing observed dynamics. There is also some evidence for oxygendependent habitat use and physological rates as drivers of observed dynamics. Deoxygenation results in declining growth both of benthic and benthopelagic fish species, as the latter are unable to compensate for the loss of benthic resources by consuming more pelagic fish and resources. Analysis of scenarios of ideal, declining, and degraded oxygen conditions show that deoxygenation results in a decline in somatic growth of predators, an altered habitat occupancy resulting in changing species interactions, and a shift in energy flow to benthopelagic predators from benthic to pelagic resources. This may have important implications for management as oxygen declines or improves.

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Spatio-temporal dynamics and behavioural ecology of a “demersal” fish population as detected using research survey pelagic trawl catches: the Eastern Baltic Sea cod (Gadus morhua)

Cited by 5 publications

References 28 publications

Living apart together: Long-term coexistence of Baltic cod stocks associated with depth-specific habitat use

Living apart together: Long-term coexistence of Baltic cod stocks associated with depth-specific habitat use

Otolith chemistry indicates recent worsened Baltic cod condition is linked to hypoxia exposure

Declining food availability and habitat shifts drive community responses to marine hypoxia

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