Dazed and confused: Rising C02 sends fish swimming towards predators

Carbon dioxide can make fish do some strange things

Many species have found themselves in harm's way as a result of the changing climate, but where the phenomenon may have a particularly profound effect is under the ocean. This is because rising concentrations of carbon dioxide in the atmosphere make the waters more acidic, a phenomenon that poses a real threat to the coral in Australia's Great Barrier Reef. Some of the ocean's fish may prove another victim to this chemical reaction, with new research revealing that more acidic waters throws their survival instincts out of whack, and can sometimes even cause them to swim towards predators.

Previous research has outlined the impact of the expected rise in ocean acidity on marine animals, consequences that include compromised respiratory function, aerobic performance and dulled senses such smell, sight and hearing. And with CO2 on the rise, currently at around 400 ppm and anticipated to nearly double by the end of the century, the outlook looks grim indeed.

But in truth, there are already marine environments that contain C02 levels much higher than we could achieve before the end of the century, more than ten times higher in fact. CO2 levels can build up over time in relatively stagnant fish farms through the decomposition of organic matter, dense fish populations and the fact that fish themselves expel carbon dioxide.

Despite these seemingly inhospitable conditions, only limited detrimental effects have been reported in fish living out their lives in the aquaculture industry. So what's the deal?

For scientists at the University of Exeter, there are a few possible explanations. These include the possibility that breeding species in these environments has caused them to evolve traits conducive to higher tolerance of C02, or that perhaps a natural ability to endure the higher C02 levels is what landed those species there in the first place.

Moreover, the fact that the fishes' every need is catered for in a farm, like food, shelter and protection from predatory species, means that the negative physical effects of higher C02 either go unnoticed or don't really matter, considering they only exist for our consumption. Sure they don't matter to us, but what about the poor, defenseless fish trying to go about its business in the C02-rich waters of 2100?

It is in this way that aquaculture environments can provide a glimpse into the future, and has the Exeter scientists describing them as "a giant long-term laboratory experiment." The researchers plan on using fish farms to further flesh out details on how heightened C02 levels might affect wild fish over multiple generations, and how the risks to their wellbeing could be managed.

Their early findings have reinforced earlier research in the area, but also revealed a surprise or two. The team found that higher amounts of C02 do indeed disrupt fishes' sense of smell, hearing and vision, something they attribute to the way the gas impacts how the brain processes signals from sensory organs. But they found that it did this in a way that caused them to at times swim towards the smell of a predator, rather than away from it. They would also ignore sounds that would usually scare them away from risky habitats.

The team says the findings of its research might not just help in future conservation strategies, but could also inform more productive and safer approaches to aquaculture, with previous research indicating that small increases in C02 can actually stimulate growth in some fish species.

"Our research will allow fish farmers to optimize conditions, and specifically CO2 levels, to improve the growth and health of their fish, profitability and the long-term sustainability of the industry," says Dr. Rod Wilson, climate-change marine biologist at Exeter University. "This is really important given that aquaculture is the only way we will increase seafood production to feed the growing human population, particularly given wild fish stocks are overexploited."

The research was published in the journal Global Change Biology.

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