Feral Fish Infestation in Bennett Brook

The local Perth freshwater fish fraternity was recently rocked by the discovery of a non-native fish species in one of the Swan River's major tributaries: Bennett Brook. Two of my former colleagues at Murdoch University (where I earned my Honours degree in fish biology) Dr. David Morgan and Dr. Stephen Beatty encountered this cichlid species while doing routine aquatic sampling in Bennett Brook in the Perth suburb of Lockridge three weeks ago. It's sparked somewhat of a miniature media frenzy as well. Check out these links here, here and here. I like the way they've completely sensationalised the story by using terms like "cane toads of the waterways" and "mutated feral fish". Where they got the notion of a mutation I have no idea but I guess it's all designed to grab the public's attention which is a good thing because we certainly don't want to see a repeat of this kind of scenario in any more of our waterways.

At the moment it's a little unclear exactly what fish we're dealing with. Photos have been sent to one of the pre-eminent cichlid experts in Sweden and he's revealed that it's a South American species, native to the Amazon River basin currently known as Geophagus brasiliensis. There's a fair amount of taxonomic confusion regarding this species though as this name has been applied to what could be up to ten different species!! So we're tentatively calling it "Geophagus brasiliensis" but it will certainly be given a different name when someone decides to do a taxonomic revision of this group.

Another mystery is how this fish became established in the wild here in Perth, although it seems the most likely cause was an irresponsible, or perhaps even an intentional release of unwanted aquarium pets by a local aquarium hobbyist. Is this a case of eco-terrorism? Probably not, otherwise I'm sure every drainage in every suburb of the city would be over-run by now. Why people would intentionally release an exotic fish into a local stream rather than just neck the bloody things or sell them back to the aquarium shop (yes that's right people, most reputable aquarium shops will buy back unwanted fish, especially pricey ones like cichlids) is beyond my comprehension. Now because of somebody's foolishness we're faced with a miniature scale conservation threat. Exotic fish like these "mutant" cichlids compete with native fishes for food and habitat and even prey on juvenile native fishes and larvae.

There are many examples of the proliferation of introduced species in Australian waterways and their deleterious impacts on our native fish fauna. Take the Mosquitofish for example, it is now the most common and widespread freshwater fish on the continent and it doesn't even effectively control mosquitos which is why it was introduced in the first place. Look at Carp in the Murray-Darling, their bottom feeding habits are churning up that river into a mud-bowl that's gradually becoming less and less inhabitable for native fishes. And most worryingly in terms of the cichlid outbreak here in Perth is the example of the rise to prominence of another cichlid species (i.e. Tilapia) in the Pilbara region of WA. These fish were first released into the Gascoyne River about 25 years ago, not by the government but by irresponsible members of the general public (sound familiar?). They've since spread to a handful of other river systems in the Pilbara and in just a quarter of a century have become the most abundant fish species in the Gascoyne, displacing native fishes that have lived there for millenia. Sad indeed and it just goes to show how opportunistic and adaptable cichlids can be to the aquatic conditions found here in Australia.

Dr. Stephen Beatty from Murdoch University uses electrofishing equipment to collect the feral cichlids

I spent the latter half of last week in the Bennett Brook area assisting the Murdoch team catching as many of these exotic cichlids as we could. Let me tell you the numbers aren't small either. The bad news is that this species has formed a wild breeding population in a very short period of time and is now more numerous than native fishes in the Lockridge area. Fisheries reports that they prey on native fish and invertebrates as well. The original release must have taken place within the past two years because Dr. Morgan didn't find it during his extensive surveys of the area in 2004. The good news however, is that they haven't spread very far yet and are still a long way from reaching the Swan River. It is hoped that they have no tolerance for brackish water (mixture of salt and fresh), although to be sure, it's imperative that salinity tolerance experiments are conducted ASAP to determine if there is any chance at all of this species spreading to other parts of the greater Swan-Avon catchment under its own steam....a grim prospect indeed for the already beleaguered river system.

My gut feeling from what I've seen so far of this story is that any thought of complete eradication of these pests from the wild is probably a lost cause. They're too well established now in what is quite an extensive drainage system. It seems that the best we can hope for is to try and prevent them from spreading further up and down Bennett Brook and to try and localise them in Lockridge by use of strategically positioned fish traps like the one in the photo below.














A fyke net is set up in Bennett Brook to block the further spread of the introduced cichlid

Let's hope the general public take heed from this debacle and learn to do the right thing when it comes to disposing their aquarium fish (or as a good friend of mine would say: "grow up"). The rivers of the South-West can ill afford another Bennett Brook.

UPDATE.... Certain portions of the general public are indeed spineless, the fyke net pictured above was stolen despite having a warning sign posted onto it which read "Do Not Disturb - Fisheries Research In Progress." In the words of Chris Cooper's character from American Beauty: "This country's goin' straight to hell."