Shark behaviour changing due to climate change
There are regions in our oceans called “oxygen minimum zones”. They extend from about 200m below the surface to more than a kilometre down. They occur where abundances of sinking surface material cause a surge in decomposition processes, which consumes the available oxygen. David Sims, from the Marine Biological Association Laboratory in Plymouth, is interested in how larger predators, like sharks that routinely dive to over 1500 metres to feed on deep-dwelling squid and octopus, are coping with the fact that - propelled by climate change - these oxygen minimum zones are enlarging. This, he’s finding, is pushing the fish closer to the surface for more of the time, making them more likely to fall victim to human long-line fishing, as he explained to Chris Smith...
David - In this particular piece of work we did we were tracking blue sharks. And blue sharks can get up to about four metres long, and we attach electronic tags onto their fins, which can record not only where they are in the ocean - as they come towards the surface we can get direct satellite locations of where they are - but we can also use other tags which record the depth at which they're swimming. And so we tend to find that blue sharks spend most of their time in the top sort of 200 metre of the water column, but then during the day they undertake these deeper dives down to 400, 600 metres depth. And so the ones we tracked and tagged in the central Atlantic, so the Azores islands and further West towards the US and Canadian coast, we fitted those with electronic tags. And then using satellites, and on our laptops back in the laboratory, we were able to follow almost in near real time their movements. The blue sharks that we were tracking in normal oxygen waters were diving down to over 1500 metres to increase their foraging opportunities in deep water - squids and octopus. When they encountered this oxygen minimum zone those deep dives that they would have been doing just got shallower.
Chris - If they're not making those deep dives, does that mean the sharks are going hungry?
David - I don't think so because, in these oxygen minimum zones, what's interesting is perhaps not only the shark is limited by that low oxygen, but also that's the case for their prey species as well. Blue sharks are feeding on pelagic fishes - these are surface dwelling fishes, and they're probably also limited in diving. So it could be that above these oxygen minimum zones, prey is actually aggregated. And so it might be that these become sort of shark feeding hotspots, and the sharks might actually remain more in these areas than in others.
Chris - Does that mean that they're potentially in contention with other animals that are feeding on those prey or in contention with each other, or even in contention with us?
David - Obviously they're competing with other predators there such as tunas and swordfish and other sharks, of course, but one trick the blue shark might have up its sleeve is that one thing we did notice from this dive data in these oxygen minimum zones was that they did occasionally go very deep into potentially low oxygen water, very low oxygen. So it could be that the blue shark has an advantage in perhaps being able to hold its breath and do some deep dives, perhaps down to a thousand metres - those were some of the dives we mentioned, 750 metres. It could be that they're able to exploit prey which is hypoxia tolerant, in other words is tolerant to low oxygen conditions. And we know that many of the cephalopods, those squids and octopuses, there are species which are very tolerant to low oxygen. They have adaptations that allow them to remain in these low oxygen areas. So it could be the blue shark has an edge, but of course the other part of your question was, well, what about humans? How do they fit into the equation? Well, for that, I suppose we've got to think about the shooting fish in a barrel. If there's less water in the barrel, it's easier to shoot those fish. And that's what seems to be happening above these oxygen minimum zones. Blue sharks are becoming compressed into that shallower water. And so surface fisheries, such as longline fisheries, pose a much greater threat to the shark because their susceptibility to capture will be increased if they aggregate above these oxygen minimum zones,
Chris - It's a somewhat sombre message that you're conveying then. Obviously interesting observation that the sharks have changed their behaviour in this way, but it is bringing them into greater contention with human fisheries, therefore they're already vulnerable. They're going to become more vulnerable. What's the implication of what you found then? Is it basically curtains or is there something we can do about this?
David - There are things we can do. I think that there's room for optimism here because although we found in this study that the fishing effort and the fishing intensity was much higher above the oxygen minimum zone, and we found that catches were twice as high as they were in normal oxygenated waters. So there is a problem here in as much that more blue sharks are being caught than we would otherwise predict. The fishes know that they're perhaps easier to catch, and so they tend to go there. I think there is optimism because there can be of course management measures one can make to reduce the catches of sharks. These can be quotas for example, where sharks have to be put back if so many more is caught than is allowed. It could be that there are gear improvements. Ocean dwelling, ocean going sharks are notoriously susceptible to bycatch - in other words, being caught on these very long lines. These lines extend for about 100 kilometres each with about 1200 baited hooks. And there are thousands of these vessels around the ocean. So there's huge pressure. If gear can be improved to reduce the bycatch of sharks then that might be one way we reduce these catches. But I guess one thing that this study does point towards is the need for spatial management. It could be that as the oceans continue warming in the future due to climate change that we are going to have to think very seriously about mitigating the deoxygenation that we're seeing in the oceans, and which we'll continue by managing fisheries in those particular areas. So where we see that shark hotspot above that blob of low oxygen water, we're going to have to start to manage fisheries in that area itself, perhaps excluding them for certain times of the year when the sharks are there.