25 July 2023

Great for the plate or simply a pest - is there a place for trout in our waterways?

| James Day
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ecologist monitoring blackfish

ACT Government aquatic ecologists inspect small two-spined blackfish as part of their monitoring program. Photo: Mark Jekabsons.

Trout fishing has always been contentious in the Australian angling community, with some fishers valuing them as a catch and others concerned about the harm they cause as an invasive species.

When they arrived here from the US and Europe in the late 1800s, trout’s popularity grew quickly due to their great taste and the challenge it takes to reel them in.

Unfortunately, that fighting spirit makes them a fierce competitor, and at times a predator, to native fish species.

The ACT Government treats them as an introduced species in their waterways, but there has been concern from University of Canberra (UC) ecologists that not enough is being done to control trout populations in the Cotter Reservoir.

READ MORE ACT Fishing Guide: where to go and what to avoid

According to Environment, Planning and Sustainable Development Directorate (EPSDD) aquatic ecologist Matthew Beitzel, trout usually coexist with native species but when the environment is in a poor state they have been known to eat them.

The fish especially of concern are the Macquarie perch and two-spined blackfish, and a group of fish known as galaxias. Mr Beitzel says galaxias are small freshwater fish, found in cooler areas. Threatened species of galaxias are found in NSW and Victoria, but not in the Territory.

However, since the completion of the Enlarged Cotter Reservoir in 2010, UC ecologists have noted in their annual monitoring reports the damage made by brown and rainbow trout on the dam and the upstream section of the Cotter River.

Man holding rainbow trout

Pictured is a rainbow trout, which, with its brown counterpart, particularly benefits from the operation of large dams. The downstream release of cold water arriving out of the lower levels of the water body gives them a suitable habitat to rest and breed. Photo: ACT EPSDD.

While last year’s native fish impact report was corrupted by the large amounts of water released from the Cotter Dam during the monitoring period, the 2022 report and its predecessor recommended installing systems that could limit trout populations.

This is due to their belief that the size and numbers of these fish would grow over time, which in the crowded environment would motivate them to start eating the native river species.

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From EPSDD assessments of fish guts taken from the Cotter River, Mr Beitzel says trout are known to have eaten the blackfish but officials haven’t detected any cases of the same happening to Macquarie perch younglings.

Mr Beitzel says the ACT Government deals with trout like other invasive species in its animal and plant pest management plans. Instead of their numbers, they reduce the damage they cause.

However, he acknowledges the Cotter catchment is the closest trout fishing location to Canberra and is important to fishers.

Earthmoving vehicles on hill overlooking river

Earthworks being done on the Enlarged Cotter Dam project. Photo: Mark Jekabsons.

Many consider the waterways around Canberra to be unique in the Australian fishing world, due to their ability to host both trout and native species.

ACT Fishos vice-president Glen Malam says the warmer lakes and rivers closer to Canberra are particularly suited to native fish, “while the cold alpine lakes up in the Snowy Mountains scheme are a great environment for trout”.

Near Tumut, this distinction can be seen a few metres apart, he says.

“You’ve got the very deep Lake Talbingo, which is quite good for trout. Then immediately below it is Jounama Pondage, which is part of the pumping cycle of the Snowy scheme and where you can get native fish,” Mr Malam says.

He believes this provides the ACT with an opportunity for everyone to be satisfied. Populate the colder creeks and rivers with trout, and leave the warmer waterways for native fish.

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There seems to be a lot of talk about suppression of the trout population in the Cottor Reservoir by insertion of traps. With such a beautiful dam built right on Canberra’s door step, why not let the tax paying fishermen and fisherwomen onto the dam to naturally remove these so called pest’s…?After all, isn’t this why we have to pay for fishing licences, to restock fisheries, to create more recreation for anglers?
Seems to me there is an abundance of nice trout in the system just waiting to be put on someones dinner plate…. maybe the politicians should be thinking about angler’s and recreational activities so it gets people outdoors more often in today’s society rather than wasting money on traps.

This U.S. study, managing similar alien trout impacts on threatened native fish, in a similar sized stream, via a trout trap, should be a key reference in this management debate:

Healy et al. (2020). Remarkable response of native fishes to invasive trout suppression varies with trout density, temperature, and annual hydrology. Canadian Journal of Fisheries and Aquatic Sciences, 77. https://cdnsciencepub.com/doi/10.1139/cjfas-2020-0028

Recovery of imperiled fishes can be achieved through suppression of invasives, but outcomes may vary with environmental conditions. We studied the response of imperiled desert fishes to an invasive brown (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) suppression program in a Colorado River tributary, with natural flow and longitudinal variation in thermal characteristics. We investigated trends in fish populations related to suppression and tested hypotheses about the impacts of salmonid densities, hydrologic variation, and spatial–thermal gradients on the distribution and abundance of native fish species using zero-inflated generalized linear mixed effects models. Between 2012 and 2018, salmonids declined 89%, and native fishes increased dramatically (∼480%) once trout suppression surpassed ∼60%. Temperature and trout density were consistently retained in the top models predicting the abundance and distribution of native fishes. The greatest increases occurred in warmer reaches and in years with spring flooding. Surprisingly, given the evolution of native fishes in disturbance-prone systems, intense, monsoon-driven flooding limited native fish recruitment. Applied concertedly, invasive species suppression and efforts to mimic natural flow and thermal regimes may allow rapid and widespread native fish recovery.

Some clarifications for the article:

(1) alien trout *have* been recorded predating on Macquarie perch in the dam itself

(2) the trout trap would not eradicate trout from the Cotter River, but simply reduce their number so Macquarie perch and two-spined blackfish populations have less predation pressure upon them and a better chance of surviving

(3) as fish larvae and fish juveniles digest so quickly that alien trout predation on fish is often hard to detect from visual inspection — but DNA assays of stomach contents (for traces of the prey species) can always detect it

(4) such alien trout predation on Macquarie perch larvae and juveniles is undoubtedly happening in the river upstream of the dam but there has been insufficient effort to detect and quantify it

(5) copious amounts of alien trout predation of two-spined blackfish both in all three Cotter dams and all stretches of the Cotter river have been documented.

Excuses trying to blanket-blame habitat degradation and dams instead for the catastrophic decline and multiple river system extinctions of Macquarie perch and trout cod are extremely disinegenous and now robustly disproven by the Sustainable Rivers Audits 1 & 2, other fish surveys, as well as other variety of other scientific and historical research. Convenient but false stereotypes about “natives in warm water, trout in cold water” and “trout and native fish coexistence” are also now robustly disproven.

As for the Enlarged Cotter Reservoir, alien trout are completely out of control in there and impacting endangered Macquarie perch and two-spined blackfish severely. The trout trap, which was an adjunct to the EPBC approval conditions for the dam enlargement, must be built. Urgently.

I can’t believe there’s even a debate about installing it. Of course we must install it. Even if that means less trout for fishers fishing in there. Honestly, too bad. Go trout fishing somewhere else. As fishermen, we need to get reasonable and stop being so selfish. With such an outrageous number of streams and dams in this region filled with invasive trout (at enormous ecological cost) and available for trout fishing, we can afford to give up a handful of streams for Macquarie perch conservation and other threatened native fish conservation. I can’t believe it’s even a debate.

As someone who is a keen trout fisher (of ~30 years), but an even keener conservationist:

There will always be “a place in Australia for trout” … if only because they are an invasive species deliberately entrenched in mountain streams across a huge arc of south-eastern Australia, that we can never be rid of.

But this doesn’t excuse all aspects of the status quo.

It’s completely unacceptable that alien predatory trout are in every single sizeable montane/upland stream in south-eastern Australia with none reserved for native fish … especially as former native fish inhabitants such as Macquarie perch and trout cod are now on the brink of complete extinction.

It’s even more unacceptable that most of these streams are *still* stocked with alien trout every year … including into endangered Macquarie perch habitats (e.g. Upper Murrumbidgee).

Alien predatory trout, and in many systems, constant stockings of them, are responsible for the disastrous conservation situation of Macquarie perch and trout cod and two-spined blackfish to a very, very large degree. And for many Galaxias species, solely responsible.

It is completely to be expected that:

(1) deliberately making an alien predatory species utterly dominant in native species habitats
(2) constantly topping up populations of that alien predatory species (and their strong natural breeding) with yet more individuals via constant releases
(3) will cause long term declines and multiple population collapses and localised extinctions in those native species.

This process has been going on across south-eastern Australia for far far too long and it finally has to stop.

Mr Lobstermash3:52 pm 24 Jul 23

The ACT Government does not treat trout like pests. In fact, they are subject to size and bag limits, have a closed season to protect spawning trout from June to October, and do not have the same express prohibition on transfer between waterways like carp, redfin and mosquito fish.

While trout are not (legally) stocked in the ACT, populations of trout in the Cotter system are also going to be high due to the excellent water quality, prohibition of fishing in Cotter Dam and the protection of fish that migrate from the dam into the river to spawn.

Just to provide balance, the so-called native recreational fishing in the local ACT dams is as alien as the trout. Murray cod and golden perch can’t spawn in dams and wouldn’t be there if it weren’t for stocking programs. The dams are also infested with pest fish and plagued by poor water quality.

The rivers provide the most authentic native fishing experience. However, historical records show they’re not, and never will be again, like they used to be.

I love fishing in the ACT and surrounding waterways, but let’s not pretend that any of it is native and natural. To encourage people to get out in nature and enjoy a healthy past-time, we should strive to have healthy dams and waterways, teeming with fish that people want to catch. Just quietly, I wouldn’t mind the local lakes being clean enough to hold trout. Or to help reduce the population of trout in Cotter Dam.

The changes in both the geomorphology and the fish fauna of the ACT Murrumbidgee is far more radical than most people can imagine. It’s quite sad.

As you say, protecting alien trout in the ACT with size limits, bag limits and closed seasons is not treating them as a pest. On the contrary, it’s pampering them — a damaging invasive alien species — in an indefensible way. It’s cultural cringe legacy and it should end.

It gets even worse when you note that alien trout get a full fishing closure to “protect” their spawning season, while our poor native Murray cod get a Clayton’s fishing closure during their spawning season, which still allows fishing … and which immediately leads to a lot of detrimental accidental and deliberate impacts and interference to their spawning. You’d think it’d be the other way around!

It’s worth noting Murray cod can and do spawn in impoundments, especially if appropriate spawning sites are available (rock and timber). It’s the relative lack of survival of the larvae that’s the issue, for fascinating but as yet unrecorded reasons.

Rob McGuigan3:42 pm 24 Jul 23

The Trout like the European Carp are introduced species that out compete native fish species. Eradication would be preferable but it’s almost impossible to achieve with a successful introduced soecies. In the Touts case it’s an introduced and successful predatory species so that makes it even more difficult to eradicate.

Mr Lobstermash12:41 pm 26 Jul 23

Unlike carp and redfin, trout are quite susceptible to poor water quality, prolonged turbidity and high temperatures. Not that you’d want to impose those conditions on our primary drinking supply (AKA the Cotter system), but they (more or less) keep trout out of our slice of the Murrumbidgee and eradicated them from LBG.

Ironically, between the early 1900s and the late 1960s, the entire ACT/NSW stretch of the Murrumbidgee River, and Burrinjuck Dam, were dominated by alien rainbow trout. It’s only in the late 1960s they started waning and dropping out. A rising thermal regime was the main reason. This is little known or remembered these days.

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