Sawfish finding themselves in hot water
Sawfish are strange rays, which look like flat sharks with long chainsaws for noses. These oddities live in the ocean, but one species spends its youth in Australian rivers. As temperatures are on the up, the water they have to live in is already getting hotter, so how are they coping? Adrian Glise and Karissa Lear from Murdoch University have been probing the survival prospects of these fish, as Chris Smith found out.
Adrian - What we’re trying to understand really is how changes in climate in the region and changes in land use practice through agriculture, how that’s going to affect essentially the quality of the habitat for them, so they’re a critically endangered animal. Fitzroy is probably one of the last important nurseries for them the species, so really trying to figure out what threats they’re facing and how we can change our behaviour as humans to safeguard their habitats is really what we’re after.
Chris - How are you doing that?
Karissa - We’re using accelerometers, which is the same technology as in your smartphone or fitbit to measure the movement and activity patterns of these fish. So in correlation to temperature to see if they become more active or less active if the temperature rises or falls.
Chris - Obviously sharks don’t have smartphones, so how are you doing that?
Karissa - The accelerometers we use, we can either attach them externally to the dorsal fin of the fish or we can surgically implant them into their body cavity. The accelerometers we use also measure temperature. That’s another factor so same tags.
Chris - How do you get the tags on these things?
Adrian - There’s two ways really. We have two types of tags that we’re using. One tag will stay on the animal for a very long time and will transmit it’s data so what we do with those is we implant them into the animal itself. We do that because it doesn’t cause any drag and it can stay in the animal for a very long time. The other tag we’re using is an external data logger. We put that on the animal, we drill a little hole through the fin and attach it to the fin and then when we catch an animal again we can basically retrieve that unit and download the data.
Chris - You said “catch an animal.” Are you having to catch these things - how big are they?
Karissa - The ones in the river can get up to about two and a half metres.
Chris - How do you catch one of those?
Karissa - We mostly use gill nets. The sawfish with teeth all the wary down the rostra, they get caught pretty easily just with their nose in the gill net. We can pull them up to the boat and onto shore.
Chris - So what do you do you travel up to the Kinderly, you go and find a promising looking bend in the river where you think these things hang out and just put nets in there?
Karissa - Yes. Essentially.
Chris - How many of you are there to pull in a fish that’s two and a half metres and have got very short bill teeth - how do you do that?
Karissa - We usually have about three of us on the boat.
Adrian - We also have a lot of help from indigenous ranger groups up there, so that local knowledge for us is very important. The guys take us to the places where their grandparents and they’ve caught these animals for a long time. So we’re not going completely blind but, at the same time, they’re very remote places. It basically involves us flying to Derby first and then at Derby we get into our four wheel drive. We put a boat on the roof and then it’s 150 kilometres into the bush essentially. Without some of that knowledge of the locals really I think we’d have a pretty hard time finding them in the first place.
Chris - Talking of what you’re finding, have you got data yet?
Karissa - We have; we’ve just started to get data back.
Chris - What’s the trend that’s emerging? What are you seeing?
Karissa - A lot of the fish look like they’re doing a diurnal pattern with temperature. During the day they’ll rest in very cool places so that their energy expenditure is low and then during the night when it’s colder outside, they can hunt in the warmer water, which increases their muscle activities, so they can actually hunt more easily in warmer water.
Chris - And if one extrapolates that to what we think is happening with climate change and things like that Adrian, is this data promising? Does this suggest that this is quite a tolerant species or not?
Adrian - Well, actually, that’s a really interesting question. Last year we have a very, very poor wet season which means we had very little water (09.35) . We actually saw very, very high temperatures last year. So the activity, or the potential for activity increases over temperature gradually, but then it usually gets to a certain point in temperature on which it declines rapidly and we’re just starting to see that tipping point in some of that data. So I think that the (09.58) in a sense that they can quite happily sit in water at 33 degrees celsius but, I don’t think there’s much wiggle room above that really. And I think what we’re going to start to see is we’ll start to see us going over those tipping points more often in the future.
Chris - Do we not think though Karissa that the animals could just move because if that becomes an inhospitable, impropitious place for them to hand out, presumably they’ll be other places potentially down the coast a bit that might be a little bit cooler where they can still survive?
Karissa - That might be a good option for pelagic species in the ocean that have a lot of different spaces that they can move into but these river systems, there’s only a couple of rivers in northern Australia and these animals need to use that habitat for their nursery. So they don’t really have a choice where to live in those first few years of their life.
Chris - So they are pretty tied to the environment?
Adrian - Yeah. I mean at the end of the day - it’s where your mum drops you off isn’t it? If you rmum drops you off in the mouth of this particular river you’re going to swim up that stream of fresh water. And also, don’t forget, especially those estuarine areas where they’re born there are a lot of saltwater crocodiles in there and all you really want to do as a tiny little sawfish, and you get born around 70cms, you try and get into the fresh, you try and get into the shallow areas. Because, I think, in the estuary you’re not going to have a particularly good time so I think you’re very tied to where you’re born.
Chris - What do you think the long term prospect is then?
Adrian - It’s tough to say. I think as it gets hotter the animals are definitely going to struggle more, but I think what’s going to be very important is that we maintain a lot of water in the river. Because as long as there’s a lot of water and we have those really deep holes in the river and those essentially offer cool water refuges, and I think they’re going to become more and more important as the environment get’s hotter and hotter. And as long as that’s the case I think they’ll probably be OK.