Rising carbon dioxide levels might not just cause increased temperatures worldwide. It might also hamper the abilities of fish to find their friends, in other words fish from the same species might not be able to group together and fend off predators.
Lauren Nadler, from James Cook University in Australia, exposed tankfuls of reef fish to the sorts of CO2 levels we expect to see in about a century and found that they lost their ability to recognise their shoal-mates, as she explained to Chris Smith...
Lauren - Previous studies in solitary fishes have found that elevated carbon dioxide interferes with the functioning of neurons in the brain. Basically, neurons work by sensing concentrations of compounds in the blood surrounding them. And so, the changes in the ocean water are affecting the concentrations of those compounds in the blood. And so, it's changing when different neurons are being turned on and off. This disruption affects many parts of the body and particularly sensory abilities like sight and smell, which are both really important for recognising familiar fish and the ability of fish to swim away quickly from a threatening stimulus. So, what would be a predator. And they really aren't doing that very well. So, they're reacting slower or if at all. So, it's just more likely that they're going to get eaten.
Chris - So, if there were a very high level of CO2 in the atmosphere from whatever source, this could get into the water that the fish live in and in turn change the acid balance of the water which in turn would impact on the fish.
Lauren - So basically, humans are releasing a lot of carbon dioxide into the atmosphere. As the concentration of carbon dioxide in the atmosphere goes up, the ocean is going to absorb a lot of that. So that increased carbon dioxide is going to affect the chemistry of the water which is what is that keyword 'ocean acidification' that people discuss.
Chris - So, how were you trying to study this?
Lauren - I collected schools of fish on coral reefs. I brought them back into the lab and then I changed the chemistry of the seawater to mimic what we expect to see in the near future. And then we put the schools into this altered seawater, which mimics what we will see by the end of the century. And we had them stay in that seawater for a period of about a week's time. Studies have shown that it's a good amount of time to look at the effects of carbon dioxide on behaviours. Now, after we had let them hangout in this altered seawater for a week, we tested them in something called a 'choice test'. So, we gave them the choice between a school composed of their friends and a school composed of strangers. They were actually able to distinguish and preferred to school with their friends under normal conditions. But when they had been experiencing this elevated carbon dioxide level, they weren't able to distinguish anymore which school was their friends and which school were the strangers.
Chris - So, if we extrapolate what you've seen in the tank to the ocean proper, what would be the implications of this then and what sorts of fish would be impacted?
Lauren - It's actually over half of all species in the world's oceans school at some point during their lives, including many economically important fish species. And so, what we'd need to further understand is how the tradeoffs of group living are going to change if the benefits of living with friends is lost? So, I think there could be a range of effects, but I think we really need to do more research to find out what those effects might be.
Chris - Did you study a different range of fish species or could this just be one particular kind of fish that does this?
Lauren - I just looked at one species, but these effects on the sensory abilities. So, the ability to see and smell and the ability to make decisions properly. That's been done in a range of species. So, what we're finding is that the effects are really felt throughout a range of species. So, it'll be really interesting to do more work in the future and look at a range of species and see how species-specific these trends are, so if some species are more tolerant than others.
Chris - Talking of being tolerant, do you think this is something which fish can adapt to because we're not going to see the levels go from where we are today to the levels we expect in a hundred years time overnight? It's going to take until the end of the century to get there. So, your experiment is a little bit artificial because you have literally put the fish in at the deep end, straight in at these very high carbon dioxide concentrations. They would have up to 100 years to get used to this wouldn't they, normally.
Lauren - That's a great question. That's actually a topic that's been really looked into at this time. Previous studies recently have found that some traits can be acclimated to in as little as two generations. So, with some of these behavioural and physiological impacts that I've been discussing, you know, improved in a really short period of time. However, the ability to improve these traits over the generations has been absent in some of the traits. So, it's really important to do more work in terms of this longer term acclimation, to understand how fish populations may change in the future from these environmental changes.