Cod egg survival stays high with limited warming, but plummets when the temperature rises a few degrees Celsius in their current spawning grounds.
Polar bears and coral reefs are obvious victims of climate change. But a warming world will also challenge many other animals. For example, cod. The fish play a critical role in marine ecosystems and human economies. Climate change could devastate Atlantic and polar cod.
A team of scientists wanted to find out how rising sea temperatures and ocean acidification might affect these fish populations.
“We went out with our research vessel into the Barents Sea, and caught Atlantic cod and polar cod, and brought them back alive into a research facility in Northern Norway.” Flemming Dahlke, a marine biologist at the Alfred Wegener Institute in Germany.
“And in this facility, we raised the eggs and larvae under different temperature and ocean acidification, mimicking the conditions expected for the next decades to come.”
The researchers placed the eggs and larvae in various carbon dioxide concentrations, as well as three different temperature scenarios: one, the “business as usual” scenario, where humans continue to emit greenhouse gases as current rates and ocean waters increase 3 to 4 degrees Celsius. They also tested an intermediate scenario of a 2-degree Celsius rise, and a “best-case” scenario, where water temperatures rise only 1 degree. They then recorded egg survival rates and larval development.
“The results were pretty clear. We found that with ocean acidification, the eggs become more sensitive to temperature extremes. So this means that their thermal tolerance range narrows. And that we found for both species. And we also found that they use more energy to regulate against the stress of temperature and ocean acidification.”
The study is in the journal Science Advances. [Flemming T. Dahlke et al., Northern cod species face spawning habitat losses if global warming exceeds 1.5 °C]
“And when we used these data and combined them with the climate projections, we find that with the worst-case scenario there will be a decrease in egg survival chances of about 50 percent in many of the important spawning areas of those species.”
Although these populations might be able to adapt by shifting their spawning grounds northward.
“The question is if the species will find also suitable feeding conditions in those alternative spawning areas. So it could be that the temperature is right, but they won’t find enough food up in the north.”
Big changes in cod populations will likely have ripple effects: cod are food for many other animals, like birds and seals. And countries like Norway depend on the income that cod fisheries haul in. But the new research also offers a bit of good news.
“On the positive side, we find that in the scenario where we successfully limit global warming to 1.5 degrees, both species would be able to successfully reproduce in those areas where they spawn currently.”
[The above text is a transcript of this podcast.]