Acid threatens almighty cod

For cod facing the effects of climate change, it’s not the heat but the acid that threatens. 

Ocean acidification – caused by atmospheric carbon dioxide entering the water and lowering its pH value – means there’s a narrower thermal window for the success of two species of northern cod, meaning that one of the world’s largest fisheries will be threatened if global temperature increases are not limited to 1.5 degrees Celsius, a new study finds. 

The researchers, led by Flemming Dahlke of the Alfred Wegener Institute in Germany, find that embryos of cod are able to survive only within a limited temperature range, and that the range lessens in more acidic conditions. 

The researchers predict that Atlantic cod (Gadus morhuai) would be unable to spawn below the Arctic Circle by 2100 under a “business as usual” emissions scenario. The polar cod (Boreogadus saida) faces an even more perilous future, since it not only needs colder water, but also depends on ocean ice for its habitat. 

The study is yet another in a slew that suggest that if global average temperatures continue to rise unabated, entire ecosystems will approach collapse within the century.{%recommended 5123%}

“In light of embryonic intolerance to ocean warming and acidification, we show that with unabated greenhouse gas emissions, large areas presently used for spawning will become less suitable for recruitment of Atlantic cod and Polar cod, possibly leading to cascading impacts on Arctic food webs and associated ecosystem services,” the authors write in the journal Science Advances.  

The consequences of cod decline do not stop at the fishery itself. 

Polar cod is “an essential food item for many marine birds and mammals”, the authors note, while the Norwegian Atlantic cod fishery generates $1.1 billion annually. 

“Estimating changes in spawning habitat suitability for these focal species therefore has high socioecological relevance,” the authors say. 

Changing the habitat for a major fishery would also make regulation more complicated. It will become harder to manage stocks under current regulations when the fish have changed locations and reproductive behaviours. 

“Potential displacements of commercially important fish stocks across management boundaries and exclusive economic zones pose major challenges not only to national fishermen and conservationists, but also to international bodies and regulations, which intend to avoid overexploitation, resource conflicts, and the degradation of pristine ecosystems in the Arctic,” the authors write. 

Still, there is hope. Dahlke and colleagues say that under an aggressive decarbonisation plan, cod fisheries could avoid serious impacts. 

“Our results also emphasise that mitigation measures, as pledged by the Paris Agreement, can ameliorate climate change effects on both species,” the authors report. 

That’s especially heartening considering that the Arctic waters are expected to see higher impacts from climate change, through ocean warming, acidification, and sea-ice loss, than most other areas.

Realistically, though, the global community will have to act quickly to avoid 1.5 degree Celsius warming. 

“Given that current CO₂ emissions trajectories yield a 1% chance of limiting global warming to 1.5°C above preindustrial levels, our results call for immediate emission cuts following scenarios compatible with warming by 1.5°C to avert irreversible ecosystem damage in the Arctic and elsewhere,” the scientists conclude.