Re-thinking the “ecological envelope” of Eastern Baltic cod ( Gadus morhua ): conditions for productivity, reproduction, and feeding over time

• Published in: ICES Journal of Marine Science
• Main Authors: Svedäng, Henrik, Savchuk, Oleg, Villnäs, Anna, Norkko, Alf, Gustafsson, Bo G, Wikström, Sofia A, Humborg, Christoph
• Other Authors: Kuparinen, Anna, Stockholm University Baltic Sea Centre, Swedish Agency for Marine and Water Management, Swedish Environmental Protection Agency, Sophie von Julin Foundation, Academy of Finland
• Format: Article in Journal/Newspaper
• Language: English
• Published: Oxford University Press (OUP) 2022
• Subjects: Ecology; Aquatic Science; Ecology, Evolution, Behavior and Systematics; Oceanography; Gadus morhua
• Online Access: http://dx.doi.org/10.1093/icesjms/fsac017; https://academic.oup.com/icesjms/article-pdf/79/3/689/46880044/fsac017.pdf

Abstract Hypoxia is presently seen as the principal driver behind the decline of the former dominating Eastern Baltic cod stock (EBC; Gadus morhua). It has been proposed that both worsening conditions for reproduction and lower individual growth, condition, and survival are linked to hypoxia. Here, we elucidate the ecological envelope of EBC in terms of salinity stratification, oxygen content, and benthic animal biomasses, and how it has affected EBC productivity over time. The spawning conditions started deteriorating in the Gotland Deep in the 1950s due to oxygen depletion. In contrast, in the Bornholm Basin, hydrographic conditions have remained unchanged over the last 60 years. Indeed, the current extent of both well-oxygenated areas and the frequency of hypoxia events do not differ substantially from periods with high EBC productivity in the 1970s–1980s. Furthermore, oxygenated and therefore potentially suitable feeding areas are abundant in all parts of the Baltic Sea, and our novel analysis provides no evidence of a reduction in benthic food sources for EBC over the last 30 years. We find that while reproduction failure is intricately linked to hydrographic dynamics, a relationship between the spread of hypoxia and the decline in EBC productivity during the last decades cannot be substantiated.