Ocean acidification could impact Atlantic cod populations more severely than previously thought

Research over the past few years has shown that ocean acidification impacts Atlantic cod at the earliest stages of their lives, while they are still eggs and larvae. Scientists hoped that those cod who managed to reach maturity might be helping the species adapt to the conditions brought on by global climate change. But new research appears to have dashed those hopes.

Earth’s oceans absorb as much as one-third of the excess carbon dioxide pumped into the atmosphere by human activities. When that CO2 reacts with water, it forms carbonic acid, lowers the pH level of marine waters, and thus makes the oceans more acidic. While marine species can shift their ranges to avoid the warmer water temperatures driven by climate change, there is no way for them to escape ocean acidification.

For a 2016 study, Martina Stiasny of the GEOMAR Helmholtz Centre for Ocean Research, a research institute in Kiel, Germany, led a research team that conducted experiments with Atlantic cod (Gadus morhua) caught from two wild populations in the Western Baltic and the Barents Sea. The researchers determined that, at the ocean acidification levels expected by the end of the century if we do nothing to draw down C02 emissions, twice as many cod larvae will die within their first 25 days, causing the number of cod who reach maturity and reproduce to drop by 8 and 24 percent for the Western Baltic and Barents Sea populations, respectively.

Stiasny was among the scientists who’d hoped that those cod larvae that did make it to adulthood would help make their population more resilient. “So far, we liked to believe that at least the larvae that survived would be able to deal with these conditions, and could have, across generations, allowed the species to adapt,” she said in a statement.

But a new study Stiasny led, which was published in the journal Global Change Biology last month, found that surviving cod larvae suffer significant organ damage and developmental delays that could cause problems throughout their lifetimes.

In addition to manipulating CO2 levels to study the effects of acidification on larval development, Stiasny and team also limited the amount of food available to the cod larvae involved in their experiments so they could study how the two stressors might interact. They found that larvae fed without restriction exhibited little difference in growth and skeleton formation whether they were subjected to elevated CO2 levels or not. Larvae subjected to “energy limitation,” meaning their access to food was restricted, in addition to elevated CO2 levels were found to be significantly larger and to have better-developed skeletal structures than those subjected to energy limitation but normal CO2 levels. However, critically important organs like the liver were impaired in the cod subjected to elevated CO2 levels, and they had comparatively smaller gills, as well.

“It is therefore likely that individual larvae that had survived acidification treatments will suffer from impairments later during ontogenesis,” the authors write in the study.

Gill development in mature cod that were subject to ocean acidification in early life stages is especially worrying: “Compared to the body size, they are underdeveloped,” Catriona Clemmesen, a study co-author and head of the larval ecology group at GEOMAR, said in a statement. Gills are not only vital for oxygen uptake in fish, but also for adjusting their internal pH.

Stiasny led another study, published last year, that found that acclimation of the parental generation to ocean acidification can compensate somewhat for reduced larval survival rates, but only when food is readily available, meaning prey concentrations in the wild would have to be quite high — perhaps unrealistically high. “These ideal situations are very unlikely to be encountered by the larvae in nature,” Clemmesen said. In a more realistic food availability scenario, exposing the parental generation to acidification only leads to even worse health outcomes for cod larvae.

“Our results are of particular importance, since the Atlantic cod is one of the most important commercial fish species worldwide,” Stiasny said. “It therefore not only supports a large fishing industry but is furthermore an important source of protein for many people. Dwindling populations would have far reaching consequences not only for the environment and marine ecosystems, but also for the fishermen, the industry and human nutrition.”

CITATIONS

• Stiasny, M. H., Sswat, M., Mittermayer, F. H., Falk‐Petersen, I. B., Schnell, N. K., Puvanendran, V., … & Clemmesen, C. (2019). Divergent responses of Atlantic cod to ocean acidification and food limitation. Global Change Biology, 25(3), 839-849. doi:10.1111/gcb.14554

• Stiasny, M. H., Mittermayer, F. H., Göttler, G., Bridges, C. R., Falk-Petersen, I. B., Puvanendran, V., … & Clemmesen, C. (2018). Effects of parental acclimation and energy limitation in response to high CO2 exposure in Atlantic cod. Scientific reports, 8(1), 8348. doi:10.1038/s41598-018-26711-y

• Stiasny, M. H., Mittermayer, F. H., Sswat, M., Voss, R., Jutfelt, F., Chierici, M., … & Clemmesen, C. (2016). Ocean acidification effects on Atlantic cod larval survival and recruitment to the fished population. PLoS One, 11(8), e0155448. doi:10.1371/journal.pone.0155448

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