A new study shows that ocean acidification is having a much more serious effect on the larvae and juvenile Atlantic cod than previously thought. Coupled with rising sea temperatures and overfishing the species is now under serious threat.

Ocean acidification is one of the most serious problems effecting the marine world today. It is caused by increased carbon dioxide levels in the atmosphere due to human activities. The CO 2 reacts with water to form carbonic acid, which lowers the pH of the oceans making them more acidic. Until recently the main concerns about ocean acidification were its effects on corals, crustaceans and molluscs. The acidification causes calcium carbonate to become weaker in the animals shells and skeletons. But now it has been shown to effect the development of juvenile fish species as well.

New research from the Helmholtz Centre for Ocean Research in Germany shows serious negative effects from ocean acidification on Atlantic cod, in particular early life stages such as eggs and larvae. During these stages they are very susceptible to acidification as eggs can be dissolved and skeletal development impeded. As a result a large proportion of eggs and larvae will never develop into fully grown fish. This reduces the recruitment of new individuals to the population, which is already in decline.

Until now it had previously been hoped that those individuals that survived would be stronger and pass on successful genes to their offspring which would eliminate the problem in future generations. However the research also showed that a majority of the fish that survived had developed serious growth defects. These included smaller livers and undeveloped gills that were comparatively too small for the fish. This resulted in premature deaths, occasionally due to suffocation, or an increased chance of predation. So even the individuals that survive infancy are still under threat.

It’s also not just acidification that is negatively effecting Atlantic cod. Rising ocean temperatures, also caused by increased CO 2 and other greenhouse gases, is effecting their ability to successfully reproduce. Recent research by the Arctic Monitoring and Assessment Programme predicts that Arctic cod will need to migrate further north to find the colder breeding grounds they prefer. Places like the Barents Sea will become increasingly important as a breeding ground for the species. But the same research predicts that it will also become too warm if current trends continue.

The two issues stem from the same problem as levels of CO 2 pollution have exponentially increased in the last century. But when they are combined they can become increasingly deadly. The same paper states that in certain areas, where high levels of increasing temperatures and ocean acidification overlap, the mortality rate of the juveniles can be as high as 75%.

A decrease in the numbers of Atlantic cod surviving to maturity is very worrying because the population is already under threat from historic overfishing. Cod is an important food source in Europe, in particular in the UK and Scandinavia, as well as being a vital industry for some communities. If fishing trends continue and ocean acidification and its effects become worse, the species is unlikely to ever recover in the future.

What is perhaps more worrying is that if ocean acidification are impacting Atlantic cod the chances are other fish species will also be affected in the future. This is a stark reminder that ocean acidification is becoming a serious global threat to a wide variety of marine creatures.

To check out more of Harry’s work, visit his site Marine Madness or follow him on twitter @harryjpbaker