Moored to a stick at slack tide on the Adelaide, a strong and steady blip is puncturing static through the speakers of a VEMCO VR100 acoustic receiver. It’s picking up an acoustic transmitter attached to juvenile male sawfish PML007 (or James, after the modern classic antihero), who is emitting a 78kHz sonic signal every second. We’ve been bobbing here languidly for a half hour or so. Throw in some unbeatably pleasant dry season weather, and the drama that played out only a few hours ago really does seem very far away.
Rewind to 6:30AM. “This’ll be a fairly cruisy day,” NERP/RIEL PhD candidate Kate Buckley is saying as we motor out at daybreak. The mist rising from the water makes it look like a ghostly prairie in the early morning light and a very tranquil setting for a spot of sawfishing. “We’ll just be checking nets, and recording and releasing the bycatch. Unless we catch a sawfish,” she adds: “In which case all hell will break loose.”
We head up Marrakai Creek to string up the gill nets, hurling one anchor out onto the riverbank, where it promptly sinks until it’s mostly submerged by thick mud slathered all over the Adelaide. The nets are held in position by another anchor dropped into deeper water. Owing to brown sediment so thick it coagulates and swirls like a lava lamp, this anchor – along with everything else in the Adelaide River system – is invisible. There’s a large tub on the bow to hold any elasmobranchs (sharks and rays) that turn up in the nets, as Kate will be recording these and measuring their dimensions. We putter about in the NT Fisheries Research Vessel “Porsche” checking the nets and shooting the breeze… And suddenly, unexpectedly, we catch ourselves a sawfish.
Immediately it’s all (four) hands on deck: Fill the tub with river water as Kate frees the fish. Then we need to haul anchor, heave nets – to shortly discover that in a freak coincidence another young sawfish, a bigger male, has turned up in the second net. The opportunity is too good to pass up so he’s also placed in the tub. The rest of the nets are stowed and Kate makes calls while the fresh water in the container is kept fresh by bucketing water from the river in and out of it (while this writer tries not to think how much she must resemble a bit of meat dunking a la Adelaide River Queen Jumping Croc Cruises!).
Kate gets confirmation that Dion Wedd, Territory Wildlife Park curator, is on his way to the boat ramp with an oxygenated tank to assume care of the smaller female sawfish. A marine scientist, Kate took three years from her job as head aquarist at the Territory Wildlife Park to study. However she has maintained close ties with the institution. The Territory Wildlife Park has a long history of keeping and displaying captive sawfish, which directly feeds into Kate’s research on the conservation benefits and ecological impacts of displaying threatened elasmobranchs in public aquaria. And freshwater sawfish are certainly a good candidate for conservation research: the future of the species has been of such concern that Pristis pristis were recently upgraded from category CITES II to CITES I – trade control ranks that include rhinoceros, red pandas and other species looking down the barrel of extinction in the wild.
(Today’s little female will enter the park’s collection to join fellow juveniles ‘John Deere’ and ‘Husqvarna’, also caught in recent months for Kate’s project. In keeping with the garden maintenance company theme this latest addition is named ‘Makita’.)
Back upstream to Marrakai Creek to tag and release James. Tagging involves attaching two separate VEMCO transmitters: one coded so the 20 stationary receivers moored along the Adelaide River system can pick up James’ movements specifically and record his whereabouts over the next few months; and one that will be tuned into closely for the next 48 or more hours by tag teams tracking in 12-hour shifts. Kate has two hydrophones handy to pick up this transmitter wherever it is signalling in the Adelaide’s viscous swathe of brown: a unidirectional hydrophone that can ‘hear’ the transmitter within a 300-odd metre radius; and a directional hydrophone that enables the tracker to pin-point the exact location of the sawfish in the river.
“Looking at the strength or loudness of the signal received by the receiver, you can work out where the signal is coming from,” Kate explains. “The strength of the signal indicates proximity, allowing you to close in on the fish until you are virtually on top of it. This is how we are able to manually track the fish down the river, recording its whereabouts at 15 minute intervals.”
A month ago Kate was tracking female sawfish PML006 when one day she fell off the radar. Two whole days were spent in the Adelaide River system, including several choked and unnamed side creeks, literally fishing around for PML006’s acoustic signal with the unidirectional hydrophone to no avail.
Kate is still unsure what happened to PML006 – whether she fell prey to a croc, or had faulty transmitters, or simply evaded the hydrophone’s detection. “Until this project, no one has manually tracked this particular species for longer. Very little is known about the ecology of juvenile freshwater sawfish and this tracking is providing lots of new information.”
James, however, proves to have a strong and steady presence and Kate works out we will have listened to about 40,000 ‘blip’s over the course of our shift. Seemingly unfazed by the morning’s surgery, James exhibits what Kate now considers typical juvenile sawfish behaviour – remaining more or less stationary in the slow parts of the river and then taking off with the turn of the tide, riding the current at a pace that we occasionally struggle to keep up with.
As night falls the temperature drops noticeably. Barking owls and dingos call to each other from somewhere on the banks while crocodiles slither and splash in the darkness and in the absence of a moon Kate makes do with the stars’ reflection on the water to help mark how far the boat is from the blacked out banks. 11:30 marks the end of our 17 hours on the water, and the less experienced boaters among us quickly learn how illusory certain collision with certain banks are in the roving spotlight, as we zoom at maximum speed back to the boat ramp where Dion and NT Fisheries’ shark scientist Grant Johnson are waiting to relieve us.
So marks the end of the first of five back to back tracking shifts following James around the Adelaide River System. Kate will track a few more wild sawfish, mixing it up with tracking sawfish released back into the wild from the Territory Wildlife Park in order to compare their behaviour and measure any impact a stint in captivity has on sawfish’s chances of survival in the wild. In a few months she will start downloading data on sawfish movements from the stationary acoustic receivers along the Adelaide and is due to soon start work on the other areas of her research, including tracking the endangered speartooth shark (Glyphis glyphis), and assessing the level of success public aquaria have in communicating conservation messages on both Pristis pristis and Glyphis glyphis.
First posted on RIEL blog