Ocean acidification: herring could benefit

Studies show complex effects of carbon dioxide on fish stocks

The larvae of many fish species are sensitive to ocean acidification - studies have already shown. Acidification is caused by large amounts of carbon dioxide (CO2), which enter the seawater from the atmosphere. This CO2 also affects the food supply for the larvae. Researchers at the GEOMAR Helmholtz Center for Ocean Research Kiel have been studying herring larvae to see how both effects can have a combined effect on young fish

As soon as they start life, it's about survival for young fish. They must learn to eat and escape enemies. At the same time, they are the most sensitive to environmental factors such as temperature, oxygen and the pH of the water at this stage of their lives. Exactly these factors are currently changing globally: temperatures are rising and oxygen is being lost to the oceans. In addition, more and more carbon dioxide (CO2) from the atmosphere gets into the seawater, where it forms carbonic acid and causes the pH to drop. But not only directly, but also indirectly, additional CO2 influences the chances of survival of fish larvae, because it can also change their food supply.

Researchers from Germany, Sweden and Norway led by GEOMAR have now used herring larvae to investigate how these two effects of ocean acidification combined can affect the survival and growth of juvenile fish. Recent experiments in the international journal Nature Ecology and Evolution showed that herring could benefit from an acidified food web. "Perhaps they will have an advantage over other, more sensitive species in a more acidic ocean of the future," says Dr. Michael Sswat from GEOMAR, first author of the study.

To test the response of the young herring to ocean acidification, the team allowed them to grow up in a complete food web under present and future CO2 conditions. For this purpose, it used the Kiel KOSMOS offshore mesocosms, which were anchored in 2013 for a long-term test in the Swedish Gullmarsfjord. "The mesocosms isolate 50 cubic meters of seawater with all the plankton organisms occurring in it, just like in a huge test tube," explains Prof. Dr. med. Ulf Riebesell from GEOMAR, co-author of the study. Five of the mesocosms were enriched with CO2 to simulate concentrations predicted for the end of the century. Five mesocosms were kept at current CO2 levels for comparison.

In mesocosms with elevated CO2 concentrations, natural algal blooms increased between February and June. "As a result, animal plankton also grew better, and the herring larvae then profited from this increased food supply," explains Dr. med. Michael Sswat. Six weeks after hatching, nearly 20 percent more herring larvae survived under future CO2 conditions. "This overall positive effect of ocean acidification on herring larvae was initially surprising, as previous studies have shown negative direct effects of acidification on larval survival for many other fish species," adds Dr. Catriona Clemmesen from GEOMAR, also co-author of the study.

One explanation for the surprising results was found in a parallel laboratory study, which showed that herring larvae are generally more resistant to pH changes. "Siblings of the herring larvae in the mesocosms were raised in the laboratory at comparable CO2 levels, without changes in the feed supply. Thus we were able to separate the direct effect of carbon dioxide on the herring larvae from the indirect influence via the food chain ", explains Dr. med. Sswat, who is also the first author of the laboratory study, which appeared in late January 2018 in the journal PLOS ONE.

The tolerance of herring larvae to pH changes could be due to the way of life of the fish. "Herring spawn mostly near the ground, where naturally high CO2 levels prevail. They are therefore probably already better adapted than other fish species such. the cod that spawns near the surface of the water, "explains Dr. Clemmesen.

How the survival of the fish larvae and thus whole stocks changes in the future depends on many factors. In addition to ocean acidification, rising temperature and overfishing are also changing marine communities around the world, and far from all of these consequences are foreseeable. "But changes in the ecosystem are likely. Therefore, there is a high risk that the direct and indirect consequences of unchecked CO2 emissions will have a negative impact on overall fish stocks, "concludes Ulf Riebesell.

Links to the studies: http://dx.doi.org/10.1038/s41559-018-0514-6 and https://doi.org/10.1371/journal.pone.0191947