An Estuary Health Check
Perth sits very close to the coast, right on the estuaries of its two rivers, and where seawater meets freshwater, the environment is extremely dynamic for the fish that live in the estuary, and ultimately, for the people who want to catch them. A team of researchers from Murdoch University is studying one of the most prized and relied upon estuary catches, black bream. They're tagging and tracking the fish to find out what happens to them as the amount of freshwater decreases. As years of drought continue across Australia, this work could show how we can expect these environments and the animals that live in them to respond. I joined the team on a scientific fishing trip in the Perth suburbs...
Victoria - So, I'm standing on the boat ramp, looking at the beautiful wide Canning River, looking like a mill pond at the moment. It's a gorgeous afternoon and I'm with Joel Williams from Murdoch University. What exactly are we going to do today?
Joel - So today, we're going to take the boat out. We're going to go about 500 meters upstream in the Canning River and we're going to do some seine netting to collect black bream.
Victoria - Why black bream in particular? Why is this species of such interest to your research?
Joel - Well, such unique species - black breams are a member of the Sparidae family. It's unique in that it completes its entire lifecycle within an estuary, and it's also, a highly popular fish with recreational anglers, they like to target and eat them. Today, we're going to be going out tagging some new fish. We haven't tagged that many fish yet because it's the start of the programme, but there's every chance we may get to tag fish.
Victoria - Okay, so let's take the boat out and get going.
Joel - Very well. That's the first time it's ever not started.
Victoria - Take two.
Joel - During the day, they're probably sitting either in the deeper waters, deep than 2 meters so they're hanging around snags or trees or reeds and then during dusk, they come out into the sand flats and into the shallows to start feeding. That's obviously when the best time to go fishing is, that's where we're at now.
Victoria - It's a perfect landing. Okay, Joel, so we've stopped by the bridge and there is a tiny little beach. So what's special about here that you think it might be good to catch fish?
Joel - So, this is one of our seasonal sampling sites that we come to each season. We were here early in the week. We know we caught some fish here, so hopefully, we're going to catch some fish again. What we have is a 40-meter seine net which is a net that we start in the shore and then using the boat, we take the net out as far as we can and then do a big arc, two people hauling that in and hopefully, we'll have some bream that we'll be able to tag. Okay, so let's pull it up. As you can see, we've got some fish.
Victoria - How many do you normally get in a net this size?
Joel - The last time, I think we got about sixty bream and probably about thirty sea mullet as well.
Victoria - It's the mullet that are jumping. We can see that here.
Joel - Its the mullet that are jumping. The smart ones are jumping over the net.
Victoria - So, what have we got?
Joel - So, that's a black bream here. We've also got the sea mullet which some people love to eat as well, and then we've got a toad fish, what's making the hissing sound.
Victoria - The big flat ones are the black breams.
Joel - The ones that are your stereotypical fish shape and silver colour, that fish shape, they're the black bream. The one slender ones, they're the sea mullet then the toad fish or they look similar to the puffer fish that you may have seen on Finding Nemo - the ones that puff up. These toad fish do the same too. They puff up as well and make funny sounds. Basically, what we're trying to find out is how these fish are using the estuaries. The Swan Canning Estuary is quite a large estuarine system. By tagging the fish, the fish gets captured either by us or by recreational fishermen. We can tell whether that fish is moved over time.
Victoria - And what do you find out by their movement?
Joel - So, we're particularly interested because estuaries are - the end users are freshwater flow, so they're particularly vulnerable to climatic events such as drought, flooding, particularly climate change. So, Australia has gone through a large period of drought especially in western Australia here. It's incredibly dry. We've been getting dramatically less freshwater flowing into our systems. So, the whole biophysical structure of the estuary is changed. The salinity is changed, the dissolved oxygen has changed, and so basically, we want to know how these fish are moving throughout the systems under these changed conditions. It can also helps us predict how future climate change may impact the distribution and how these fish are swimming around the estuaries.
Victoria - So, what happens now?
Joel - So basically today, we're going to use what we refer to as T-bar tags. We have a tagging gun which is actually the same tagging gun that retailers use to put price tags on clothes. That annoying little plastic thing that you try and bite off with your teeth, basically, that's the same thing we're inserting into the fish. It makes a really good anchor into the muscles of the fish. The tags contain a unique fish number and our phone number, so if somebody catches the fish, they can call that number. They can tell us the ID of that fish and if that fisher man releases the fish again and it gets tagged, we can follow where it's moved. So first thing, we measure the fish because if the fish gets re-captured, we want to measure it again to see how much it's grown. So, this fish 300 (mm), take the fish with a firm grip so that doesn't kick, and then this is the dorsal fin here. What we do is we stick the needle with the tagging gun just underneath the scales and we actually remove one scale and then we go back in. just like giving an injection, we go in in a 45-degree angle, insert it and you can feel a go between the thin rays, pull the trigger, and then remove the needle, and then the tag.
Victoria - And that's it. They have just got that little ID number on the...
Joel - I can feel it's secure there and as least protrusive position as possible just underneath the fin there and the fish should swim away happily ever after.
Victoria - Have you gathered information from this particularly species that shown you how they are responding to environmental change and particularly to climate change and the drought that's been going for so long in Australia?
Joel - Yes, I've actually originally came from Melbourne University where I recently completed my PhD and as a part of that project, we did a similar study using what we call acoustic telemetry, acoustic tags. That's where we surgically implant a transmitter that's about the size of a AAA battery and then we placed receivers throughout the estuary. And so, we can basically know exactly where those fish are in any given time throughout the year. What I found is that freshwater flow was so low due to the drought that saline water had moved a long way upstream and so had the fish.
Victoria - So, apart from changing where these fish are and then maybe changing where the recreational fishermen want to catch them, what other larger effects could that be having on the ecology of this estuary?
Joel - The fish especially during the spawning season are moving upstream. Upstream, the river channel is a lot smaller which means that the area of spawning habitat is potentially reduced. Previously, there may have been spawning in the estuary basins where you have this basically large lakes which increase the size of the spawning area.
Victoria - So, they're essentially being funnelled by the water conditions, changing and pushing them upstream into a smaller area.
Joel - Exactly. There's other impacts such as, with less freshwater flow, the amount of dissolved oxygen which is crucial for fish to survive, particularly at early stages such as the egg and larval stage, if we don't get freshwater flow then we have these deeper pools of water that become really anoxic come quite toxic to living.
Victoria - You're looking particularly at the bream in this study, but are they sort of a signal species for what's happening at a larger environmental scale and to other species?
Joel - Absolutely. In southern Australian estuaries, they're the ideal fish, but there are many other species such as the King George whiting, cobbler, tailer, snapper that all move into these estuaries. These are all fish that we like to eat. They're good table fish and these are particularly fish that are vulnerable to the effects of climate change in anthropogenic or human disturbances.
Victoria - We've been here. We tagged a good few fish and you're now going to get the fishermen involved and hopefully, you get a lot of these tags and really get some movement patterns. What do you want to do with all of that data? What's the next step in this study?
Joel - So, the Swan Canning river s=which we visited today is only one of our estuaries that we're interested in. We actually have 7 estuaries spread out through western Australia, from north of Perth, all the way down to the southwest corner. All these estuaries have different biophysical structures and they're all going to be impacted be climate change and drought in different ways. So, what we're aiming to do is collate all these data and start making predictions of how factors such as salinity, dissolved oxygen, estuary types and how climate change is going to impact on the biology and ecology of these fish. This data can now be used by fisheries managers and catch men managers to help manage this fish species and protect that into the future.
Victoria - So, I guess you've got a lot more bream to tag in order to generate that big data set, so I better let you go.
Joel - No worries. Thank you very much.
Victoria - Joel Williams there from Murdoch University.