Ocean heatwaves do not appear to have a major impact on bottom-dwelling fish, including commercially important ones such as plaice, cod, or the Pacific pollack traded as Alaska pollack. This is what an international research team reports after analyzing 248 ocean heat waves from 1993 to 2019 in the northern hemisphere from the subtropics to the Arctic. Accordingly, these often had a minor impact on fish stocks and were much weaker than natural fluctuations.
This year, the surface temperature of the oceans has reached a record high with a global average of 21.1 degrees Celsius. According to preliminary data from the US platform “Climate Reanalyzer”, in the North Atlantic it was more than 25 degrees Celsius at the end of August. Such phenomena are likely to increase because much of global warming is absorbed by the oceans.
Heat waves in the sea – based on unusually high temperatures lasting at least five days – are associated with many negative consequences for ecosystems, writes the group led by Alexa Fredston from the University of California at Santa Cruz in the journal Nature. They cite coral bleaching, the death of kelp forests or reef fish in shallow coastal waters as examples. However, it is unclear how such events would affect the fish that live on the northern hemisphere’s continental shelves – where many large fishing fleets operate.
Compared with the natural fluctuation, the influence of the heat was not large
To check this, the team analyzed scientific surveys from fisheries authorities in the North Atlantic and the Northeast Pacific – i.e. from North America and Europe. It was mainly about fish that were caught in 20 to 450 meter deep waters. The comparison of temperature data and catches provided examples of how heat waves in the sea can affect fish populations – both positively and negatively.
For example, a 2014-2016 heat wave in the Gulf of Alaska known as The Blob was associated with a 22 percent drop in fish biomass. On the other hand, a 2012 heat wave in the Northwest Atlantic was followed by a 70 percent increase in biomass. While these effects were substantial, the group writes, they were not particularly large compared to the natural range of variation.
Take the North Sea as an example: after a heat wave in 2008, the biomass there dropped by only 6 percent. In 2011, on the other hand, it increased by 97 percent without a heat wave being involved. “Contrary to our expectations, in surveys associated with ocean heat waves, the mean changes in biomass were close to zero,” the research team writes. No clear trend could be identified for individual sea areas, latitudes or certain sea depths either.
There may well be larger effects in individual cases, but that cannot be generalized. “Marine heat waves may have impacted bottom-dwelling fish communities in past decades, but when they did, the impact was small,” it says. “It is possible that very extreme ocean heat waves will cross a tipping point in the future beyond which adverse ecological effects will occur, but we have not seen such a tipping point for the most recent historical period.”
in one Nature– Comment writes Mark Payne from the Danish Meteorological Institute in Copenhagen, the study raises questions: “Taken in isolation, one might conclude that this study rejects the idea that ocean heat waves are an ecologically significant phenomenon,” he writes. “That would be a mistake.”
Studies have shown major consequences for marine mammals, seabirds, corals or fish living near the water surface. In contrast, the current study only refers to fish species that live at depths of up to 500 meters. In view of the fact that heat waves are likely to increase in the sea, it is now necessary to clarify why such phenomena affect some groups of animals and others less.