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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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Deoxygenation impacts on Baltic Sea cod: Dramatic declines in ecosystem services of an iconic keystone predator
Abstract: The intensified expansion of the Baltic Sea’s hypoxic zone has been proposed as one reason for the current poor status of cod ( Gadus morhua) in the Baltic Sea, with repercussions throughout the food web and on ecosystem services. We examined the links between increased hypoxic areas and the decline in maximum length of Baltic cod, a demographic proxy for services generation. We analysed the effect of different predictors on maximum length of Baltic cod during 1978–2014 using a generalized additive model. The extent of minimally suitable areas for cod (oxygen concentration ≥ 1 ml l−1) is the most important predictor of decreased cod maximum length. We also show, with simulations, the potential for Baltic cod to increase its maximum length if hypoxic areal extent is reduced to levels comparable to the beginning of the 1990s. We discuss our findings in relation to ecosystem services affected by the decrease of cod maximum length.
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- Award ID(s):
- 1923965
- PAR ID:
- 10278362
- Date Published:
- Journal Name:
- Ambio
- ISSN:
- 0044-7447
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)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.more » « less
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null (Ed.)Accurate age data are essential for reliable fish stock assessment. Yet many stocks suffer from inconsistencies in age interpretation. A new approach to obtain age makes use of the chemical composition of otoliths. This study validates the periodicity of recurrent patterns in 25 Mg, 31 P, 34 K, 55 Mn, 63 Cu, 64 Zn, 66 Zn, 85 Rb, 88 Sr, 138 Ba, and 208 Pb in Baltic cod (Gadus morhua) otoliths from tag–recapture and known-age samples. Otolith P concentrations showed the highest consistency in seasonality over the years, with minima co-occurring with otolith winter zones in the known-age otoliths and in late winter – early spring when water temperatures are coldest in tagged cod . The timing of minima differs between stocks, occurring around February in western Baltic cod and 1 month later in eastern Baltic cod; seasonal maxima are also stock-specific, occurring in August and October, respectively. The amplitude in P is larger in faster-growing western compared with eastern Baltic cod. Seasonal patterns with minima in winter – late spring were also evident in Mg and Mn, but less consistent over time and fish size than P. Chronological patterns in P, and to a lesser extent Mg and Mn, may have the potential to supplement traditional age estimation or to guide the visual identification of translucent and opaque otolith patterns used in traditional age estimation.more » « less
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Abstract Anthropogenic deoxygenation of the Baltic Sea caused major declines in demersal and benthic habitat quality with consequent impacts on biodiversity and ecosystem services. Using Baltic cod otolith chemical proxies of hypoxia, salinity, and fish metabolic status and growth, we tracked changes from baseline conditions in the late Neolithic (4500 BP) and early twentieth century to the present, in order to understand how recent, accelerating climate change has affected this key species. Otolith hypoxia proxies (Mn:Mg) increased with expanding anoxic water volumes, but decreased with increasing salinity indexed by otolith Sr:Ca. Metabolic status proxied by otolith Mg:Ca and reconstructed growth were positively related to dissolved oxygen percent saturation, with particularly severe declines since 2010. This long-term record of otolith indicators provides further evidence of a profound state change in oxygen for the worse, in one of the world’s largest inland seas. Spreading hypoxia due to climate warming will likely impair fish populations globally and evidence can be tracked with otolith chemical biomarkers.more » « less
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ABSTRACT ObjectiveThe purpose of this study was to understand the possible provenance of a unique population of Atlantic Cod Gadus morhua that is found in the low-salinity Åland Sea region of the northern Baltic Sea. This population consists of large, healthy individuals, in contrast to the Atlantic Cod in the rest of the Baltic Sea. MethodsWe used laser ablation inductively coupled plasma mass spectrometry to measure levels of boron (as B:Ca) in the otoliths of Atlantic Cod in regions throughout the Baltic Sea. We examined both lifetime chronologies and concentrations in the core region that corresponds to birth and early life. ResultsWe found that B:Ca concentrations were 31 to 348 times higher in the otoliths of cod that occupy the Åland Sea, including in the core region. These concentrations were much higher than expected given that boron is linearly, positively proportional to salinity, which is higher in the southern Baltic Sea, and other populations displayed very low concentrations by comparison. ConclusionsBased on the otolith B:Ca as a unique marker, we suggest that the cod that are sampled in the Åland Sea may be a separate population from those that inhabit the rest of the Baltic Sea. This would not prevent it from mixing with other populations but could point to a separate spawning area. The source of the elevated boron is currently unknown, but the widespread occurrence in cod otoliths from the Åland Sea indicates an extensive nonpoint source.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/s13280-021-01572-4
