Deoxygenation impacts on Baltic Sea cod: Dramatic declines in ecosystem services of an iconic keystone predator

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.

Automated summary

The intensified expansion of the Baltic Sea's hypoxic zone has been proposed as one reason for the current poor status of cod in the Baltic Sea, with repercussions throughout the food web and on ecosystem services. The study found that the extent of minimally suitable areas for cod is the most important predictor of decreased cod maximum length, and simulations show the potential for Baltic cod to increase its maximum length if hypoxic areal extent is reduced. The findings highlight the importance of reducing hypoxic areas in order to maintain ecosystem services affected by the decrease of cod maximum length.