Rising carbon dioxide (CO2) levels in the oceans are affecting the brains and central nervous systems of fish, literally driving them crazy, it was reported this week.
A team of scientists said about 2.3-billion tonnes of human CO2 emissions dissolved in the world’s oceans every year, causing changes in the chemical environment of the water in which fish and other species lived. Increased carbon concentrations interfere with fishes’ neurochemistry, confusing them and affecting their ability to hear, smell, turn and evade predators.
“All fish could, in principle, be affected, but the ones at most risk are probably those with high metabolic rates that need to expose a large gill surface area to the water to get the oxygen they need. They will then also be more exposed to the water CO2 level,” Professor Göran E Nilsson told the Mail & Guardian.
“Fish in the Southern African oceans will be at the same risk as in other oceans. Those that normally inhabit a stable environment with stable CO2 levels, like pelagic sea fishes [those living in the upper levels of the oceans], are probably more at risk as they may lack the proper capacities to adapt to rising CO2. This could include species like tuna and small coral reef fishes.”
Professor Horst Kaiser, of the department of ichthyology and fisheries science at Rhodes University, was not aware of any South African researchers working on this topic, but said “it seems reasonable to assume that the findings done by our Australian colleagues would also be valid for South African waters. In fact, the fish species they worked on occur in our region”.
The problem could have a serious impact on the precarious state of South African fisheries, which provide a livelihood for at least 143 000 people. Nearly 50% of local marine resources are already fully exploited and 15% are overexploited.
The study, which was led by Nilsson of the department of molecular biosciences at the University of Oslo, and Professor Philip Munday from the James Cook University in Australia, was conducted at the Lizard Island research station on the Great Barrier Reef. The results were published the journal Nature Climate Change.
Past research shows that high CO2 in the water affects sense of smell
Munday and his colleagues had previously shown that the sense of smell of young coral reef fish was harmed by higher CO2 in the water, as they found it harder to find a reef to settle on and to detect the smell of a predator fish. Although fish normally avoid the smell of a predator, those exposed to CO2 become attracted to it.
Other experiments showed that the fish also lost their natural instinct to turn left or right, an important factor in schooling behaviour. “This makes them more vulnerable, as lone fish are easily eaten by predators,” Munday said.
The team then examined whether fishes’ sense of hearing — used to locate and home in on reefs at night and avoid them during the day — was affected. “The answer is, yes, it was. They were confused and no longer avoided reef sounds during the day. Being attracted to reefs during daylight would make them easy meat for predators.”
Their latest research showed that high levels of CO2 stimulate a receptor in the fish brain called GABA-A, leading to a reversal in its normal function and over-excitement of some nerve signals.
Although most animals with brains have GABA-A receptors, the team said the effects of elevated CO2 were likely to be felt most by those living in water because they normally have lower CO2 blood levels. The main impact is likely to be felt by some crustaceans and by most fish, especially those that used a lot of oxygen.
“We’ve found that elevated CO2 in the oceans can directly interfere with fish neurotransmitter function, which poses a direct and previously unknown threat to sea life,” Munday and Nilsson said.
South Africa’s department of fisheries announced this week it would not extend the recreational West Coast rock lobster season “in the best interest of this species that was almost depleted not long ago”.
Other species classified as collapsed include some of the most popular seafoods such as kabeljou and geelbek, according to a report published by the World Wide Fund for Nature South Africa last November.
“The status of many of South Africa’s linefish species is particularly worrying, with almost 70% of the commercial species considered collapsed,” the report said.
The South African commercial fisheries industry is worth about R5-billion annually. In the impoverished Eastern Cape region, R500-million in foreign revenue is generated by squid fishing every year.