Effect of Marine Hypoxia on Baltic Sea Cod Gadus morhua: Evidence From Otolith Chemical Proxies

Abstract

The Baltic Sea contains the world's largest anthropogenic deoxygenated zone, with increasing episodes and areal extent of hypoxia/anoxia. Atlantic cod in the Baltic has suffered a loss in condition which has been attributed mainly to hypoxia. Otoliths, the aragonitic structures that form part of the hearing/balance system in fishes, accumulate Mn in the presence of hypoxia and other reducing environments. Otoliths grow over the lifetime of fishes, and thus lifelong records of hypoxia exposure exist for each individual fish. However, otolith Mn/Ca ratios are also sensitive to growth effects. We tested a new proxy to at least partially account for growth: Mn/Mg, since Mg levels reflect metabolic activity but not hypoxia. This and other elemental proxies were parsed annually from the otoliths to reconstruct lifetime histories of mean, maximum, and cumulative values of this proxy as well as others (Sr/Ca) that inform us about salinity conditions. We analyzed cod from five different time periods: Neolithic (4500 YBP, a normoxic baseline), 1980s, 1990s, 2000s, and 2010s-under different hypoxia intensities, assessing fish growth and condition in relation to hypoxia experience recorded by otolith proxies. Fish growth decreased with increasing hypoxia exposure; condition at capture (measured by Fulton's K index) showed a strongly positive relation to growth indexed by magnesium (Mg/Ca). We conclude that cod otolith chemistry proxies not only inform about the hypoxia, growth, and metabolic status of cod, retrospectively throughout life, but also reflect the worsening situation for cod in the Baltic.

The Legacy of Hypoxia: Tracking Carryover Effects of Low Oxygen Exposure in a Demersal Fish Using Geochemical Tracers

“…For example, unlike the Atlantic Croaker, Atlantic Cod inhabiting the Baltic Sea, the largest hypoxic zone in the world, have experienced drastic declines in body condition, productivity, and health (Casini et al. ; Limburg and Casini ). Therefore, examining the responses of multiple species to hypoxia will provide a more holistic understanding of the consequences of hypoxia for fishes and the trophic interactions they mediate.…”

“…). Finally, Mn incorporation into otoliths may be sensitive to ontogenetic differences in growth rates across age‐classes (Limburg and Casini ); thus, restricting geochemical assessments of exposure to the first year of life is a conservative approach that avoids the potential confounding effects of asymptotic growth with age.…”

“…; Mohan and Walther ; Altenritter et al. ; Limburg and Casini ). Under hypoxic reducing conditions and where Mn is present in the sediments (Holmes ), particulate Mn oxides are reduced to Mn 2+ (Laslett ) and escape the sediments in a dissolved form (Lewis and Luther ; Statham et al.…”

Sawfish, Read in Tooth and Saw: rostral teeth as endogenous chemical records of movement and life-history in a critically endangered species

“…Sawfish are known to inhabit shallow, nearshore habitats and mangrove swamps which could potentially experience hypoxic conditions. The peaks in Mn/Ca exhibited in both the adult and embryo rostral teeth ( Figure 3D) rise above the threshold set by Limburg and Casini (2018). This suggests the possibility that sawfish may be experiencing some level of hypoxic stress.…”

“…Spikes in Mn/Ca have been shown to be an indicator of exposure to hypoxia in multiple fish species (Altenritter et al 2018, Limburg andCasini 2018). This is due to a switch in redox state which releases Mg into the water column from the substrate under hypoxic conditions (Thorrold and Shuttleworth 2000).…”