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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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Changes in population depth distribution and oxygen stratification are involved in the current low condition of the eastern Baltic Sea cod (<i>Gadus morhua</i>)
Abstract. During the past 20 years, hypoxic areas have expanded rapidly in theBaltic Sea, which has become one of the largest marine “dead zones” in theworld. At the same time, the most important commercial fish population ofthe region, the eastern Baltic cod, has experienced a drastic reduction inmean body condition, but the processes behind the relation betweendeoxygenation and condition remain elusive. Here we use extensive long-termmonitoring data on cod biology and distribution as well as on hydrologicalvariations to investigate the processes that relate deoxygenation and codcondition during the autumn season. Our results show that the depthdistribution of cod has increased during the past 4 decades at the sametime of the expansion, and shallowing, of waters with oxygen concentrationsdetrimental to cod performance. This has resulted in a progressivelyincreasing spatial overlap between the cod population and low-oxygenatedwaters after the mid-1990s. This spatial overlap and the actual oxygenconcentration experienced by cod therein statistically explained a largeproportion of the changes in cod condition over the years. These resultscomplement previous analyses on fish otolith microchemistry that alsorevealed that since the mid-1990s, cod individuals with low condition wereexposed to low-oxygen waters during their life. This study helps to shedlight on the processes that have led to a decline of the eastern Baltic codbody condition, which can aid the management of this population currently indistress. Further studies should focus on understanding why the codpopulation has moved to deeper waters in autumn and on analyzing the overlapwith low-oxygen waters in other seasons to quantify the potential effects ofthe variations in physical properties on cod biology throughout the year.
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- Award ID(s):
- 1923965
- PAR ID:
- 10272352
- Date Published:
- Journal Name:
- Biogeosciences
- Volume:
- 18
- Issue:
- 4
- ISSN:
- 1726-4189
- Page Range / eLocation ID:
- 1321 to 1331
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/bg-18-1321-2021
