Last week I made my annual pilgrimage to Four Mile Hole in Kakadu National Park. I’ll get onto the fishing later in the column but firstly I wanted to report on something I found absolutely amazing: the almost total eradication of the noxious floating aquatic plant – salvinia. Frankly, it blew me away; there were snags that we fished that for years you couldn’t get a lure to. If you’re a regular reader of this column, then you’ll probably guess why the dreaded salvinia disappeared to the extent that it has. The reason can only be that the arch enemy of salvinia, the salvinia weevil Cyrtobagous salviniae, has gobbled up most of it. This dates back to October 2017 when there was a monumental release of ravenous Cyrtobagous salviniae weevils in the iconic Four Mile Hole. Prior to that, there had been an ongoing saga of considerable proportions and, for the previous four years, I’d flagged a few times the dire situation at Four Mile which had been literally blanketed by the noxious South American plant. Following very poor wet seasons, in 2016 it was so bad that Kakadu park management had no option other than to close the lagoon for the whole year. To its credit, park management reopened Four Mile in May 2017 following a record wet season which had flushed an estimated 90 per cent of the salvinia out of the lagoon and down to tidal waters where it undoubtedly perished. But it steadily grew back and was destined to blossom across the lagoon again if another good wet season didn’t eventuate. However, the light at the end of the tunnel came thanks to the Gundjeihmi Aboriginal Corporation in Jabiru and its proactive Djurrubu Rangers. The corporation had been looking for a program for its female rangers and Senior Field Ranger, Margaret Rawlinson, and hubby, Head Ranger Matt Rawlinson, encouraged the Djurrubu Rangers to commence a large-scale weevil-breeding program. “Somebody had to do it,” Matt told me back then. “The whole program’s important because, if the salvinia takes over the waterways, there’ll be no fishing, and so many people love coming to Kakadu to fish. “Without support and help, there will be no fishing – we need to get rid of the salvinia,” Matt said. Djurrubu Rangers had set up eight 12,000L tanks, each holding 3000-4000 weevils. “They have a 28-day breeding cycle and we release once a month timed to capitalise on this cycle when the eggs are about to hatch,” Matt explained. Between 1000 and 1500 weevils were released into Four Mile Hole, which is many, many times more than previous releases. Interestingly, they were released with living clumps of salvinia that were raked into existing salvinia patches in the lagoon; although you could see the darker salvinia poured into the water was not nearly as healthy looking as the thriving plants already on the water. Also present for that monumental release was Kadadu Head Ranger for South Alligator, Kharn Spokes, and NT Weed Management’s scientists Louis Elliott and Chris Parker. It was great to see a unified approach to attacking the salvinia. Apparently, following the Four Mile release, further releases took place there, which would also explain why the salvinia issue is over, at least for the time being. What makes it even more incredible is that there was hardly a wet season this year, which would normally mean that the salvinia would not get flushed away and would flourish. It just goes to show that, by putting significant resources towards individual waterways and actually “nuking” them with the salvinia weevil, they can be cleared up significantly. On that note, I understand the equally-iconic Yellow Waters billabong is inundated with salvinia, and probably needs a visit by a few thousand weevils. As to the fishing last week, we managed to find a couple of bigger barra: 90cm and 86cm respectively. There were also plenty of rats, and all fish were hungry. The road was back to its corrugated normality and boat launching and retrieving required a strategic approach.
Crystal Neal (left) and Melanie Ottaway (right) help Noosa’s Sian Cassidy celebrate the capture of her Four Mile Hole 86cm barra which was her very first one.
Crystal Neal's 90cm barra nearly tore the rod out of the holder on the troll while she was flicking up the front, but she got to it pretty smartly.
Sian Cassidy with one of the “rats” that comprise the main barra population in Four Mile Hole at the moment.