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What’s happening to our trout streams?
  |  First Published: November 2014



Fisheries Victoria is responding to anglers' concerns about the state of trout fishing in the State's northeast streams, but how much of what's happening is inevitable and irreversible?

Anglers fishing for trout during the 2013/14 season reported poor fishing in Victoria's northeastern streams. Many suspect that the wholesale removal of streamside willows has contributed to poor environmental conditions and low trout numbers. Anglers in eastern Victoria and in Tasmania are also concerned at the apparent build-up of cormorant numbers.

Fisheries Victoria's immediate response has been to commission Arthur Rylah Institute scientists to survey trout populations in 4 streams. Of these streams, there are 3 flowing into Lake Eildon: the Howqua, Jamieson and Upper Goulburn rivers, and one flowing into the Ovens River: the King River, above and below Lake William Hovell.

Using electrofishing, the ARI researchers recorded historically low numbers of both brown and rainbow trout in the lower sections of all 4 streams. Water temperatures were high in most rivers, particularly in the lower sections.

High water temperatures linked with high summer air temperatures and low summer flows were considered to be the most likely causes of the low trout numbers in the lower reaches of these streams. The DEPI website report on this study notes that “water temperatures recorded during the survey were at or above the thermal tolerance of trout”. It also notes Bureau of Meteorology observations:

“Since 1996, North East Victoria has experienced on average the lowest stream flows on record” and “In the summer of 2013/14, there were record numbers of days where air temperature was above 25 degrees (at Mt Buller)”.

Water temperatures in some lowland streams reached 24°C in the mornings and up to 28°C in the afternoons during summer. Over the past 20 years, the number of summer days when water temperatures exceed 20°C has increased. Sustained water temperatures above 25°C are lethal to trout and, as cold water species, at above 19°C trout are likely to cease feeding and exit lowland reaches. In response, they are likely to head either up into cooler high altitude waters or down into the deeper cooler conditions in impoundments like Lake Eildon. Little wonder they are becoming hard to find and even harder to catch.

Looking ahead, warming temperatures will bring about many other changes that will adversely affect wild trout populations. These include reduced dissolved oxygen levels, reduced pH, low flows during critical winter spawning seasons and wholesale changes to the nature and availability of food species.

While highly suggestive, the ARI study cannot be said to provide a definitive link between poor trout numbers and climate change-linked elevated temperatures and low flows. Bear in mind the crippling state-wide drought that set in around 1995 and broke late in 2010 and the impacts it had on trout fisheries, particularly in western Victoria. While such events are not unprecedented here, extreme droughts are expected to become more frequent in future.

Taking a longer-term perspective, looking at what's happening with the trends in Australia's land and marine climate, the signs of climate change are clear and the scenario presented in these northeastern streams has been foreseeable for many years. Since the 1990s, DEPI recreational fisheries managers are among those to have foreseen the retreat of trout populations, and fisheries, across the State to cooler highland waters as the inevitable result of climate change.

Of course, other factors may affect trout populations in these northeastern streams. These include fishing pressure, cormorant predation, competition from carp, poor stream habitat and the removal of streamside willows. However, to date there is no evidence that any or all of these factors can account for the recent trends and condition of the trout fishery.

WHAT CAN NATIONAL CLIMATE TRENDS TELL US?

From the growing body of work on the implications of climate change on Australia's freshwater environments it's clear that the continuing and widespread warming pattern will result in a contraction in the range of trout.

CSIRO's projection of trends in air temperature (rising) and rainfall (declining) across much of Australia indicate a future of warmer waters and of declining stream flows and lake and impoundment levels. Combined with changes in vegetation, such as forest clearing, this future will include longer and more severe droughts. A 2008 study has shown that Victoria's autumn rainfall had already fallen by 40% since 1950. Other work has shown that, while rainfall in the Murray-Darling Basin fell by 11% since 1950, river flow declined by 55% compared to the long-term average. Warming air and land temperatures in future will render rainfall less effective in terms of land water resources, water quality and fish habitat.

Predicted reduced flood events and flows are also expected to threaten stream-side trees, including red gums, that provide shade and in-stream structure of importance to both trout and native fish.

GLOBAL OUTLOOK FOR TROUT FISHERIES

DEPI's Trout Response Plan (see below) acknowledges that wild trout stream fisheries in Tasmania, NSW and New Zealand have also recently reported poor results. In fact, trout numbers are falling in many countries around the world and climate change is emerging as a major cause. Whether present naturally or through introduction, trout numbers around the world are falling and in some regions their decline is described as ‘dramatic’.

In the USA, from the Appalachians to California, it is said that many natural trout populations could disappear as suitable habitat shrinks due to the projected continuation of warming trends. One study proclaimed that “Global warming is the greatest threat to the survival of trout in America's interior west and that 50% of trout habitat in the Rocky Mountains region could be lost by 2100. A study of 20 of the US major trout rivers and streams has found a significant long-term pattern of warming.

The story is similar in Europe where climate change effects are superimposed on pollution, water extraction, fishing pressure and other factors. Habitat suitable for trout has already been greatly reduced in Spain and Portugal and is projected to disappear totally by 2100. In one of the UK's most popular waters in Wales, trout numbers have suffered as the result of warmer waters and reduced flows. Fears are held over the effects of further warming in coming decades.

FISHERIES VICTORIA'S RESPONSE

Results of the ARI survey of 4 northeastern streams were presented to trout anglers at Snobs Creek and Mansfield in April 2014. In June, DEPI released “a response plan to investigate the decline in wild trout recreational fisheries in Victoria” referred to as the ‘Wild Trout Fisheries Management Program’. If funded, this program will examine a broad range of environmental, trout biology, fishing pressure and related issues. It will also look at the impact of reduced trout stocking in Lake Eildon on the inflowing streams and at the adequacy of current trout fishing regulations.

As well as assessing the status of trout populations in 12 ‘priority wild trout rivers’, DEPI scientists will examine trout movements in the Delatite River, fishing pressure in the Howqua River and the effectiveness of stocking to enhance stream populations. Details of these and other facets of the program will be posted on DEPI's website. Funding over 3 years will include $565 000 from the Recreational Fishing Licence trust account and $325 000 from other sources.

One of DEPI's key challenges will be communicating the results and the longer term implications of these investigations to trout anglers. This applies particularly to the question “Will trout stocking help the wild brown trout river fisheries recover?” Previous trout fishery research showed that, in all but a couple of western Victorian trout streams resident ‘wild’ trout populations are self-sustaining at levels up to the carrying capacity of the water conditions prevailing at the time. These long-adapted trout populations have demonstrated their ability to rebound in the aftermath of events like bush fires and droughts. This is one of the qualities that have led most trout anglers to prefer that wild trout are left undisturbed by what the Australian Trout Foundation Inc has recently referred to as ‘pets’ that should be confined to stocked lakes. On the basis of that earlier work, annual trout stocking ceased in all but a handful of streams back in the 1980s.

Despite Fisheries Victoria’s actions to improve understanding of the underlying causes of recent declines in trout fishing, climate change impacts are all pervasive. Key performance indicators of this program will include the extent to which anglers are engaged and informed and whether they adapt their future fishing expectations in the light of the results.

WHAT MORE CAN BE DONE?

The main message from broad international statements on current trends and future projections is that climate change will intensify its impacts on trout habitat, populations and fisheries. As cold-water species, the prospects for trout are bound to the shrinking regions, which provide healthy, cold and free flowing stream conditions. Short of global success in halting and reversing the effects of climate change, this is inevitable.

However, local and overseas work shows that, in the medium term - our lifetime - there are a number of actions that can go some way towards securing our stream trout fisheries. Unquestionably, the work that DEPI is now undertaking, in conjunction with anglers, will have positive results in this direction.

Already, there is enough completed work to show the importance of protecting and encouraging the growth of shading streamside trees as a means of moderating the effects of extreme daytime summer air temperatures on stream temperatures. Swedish studies have shown the importance of providing shallow flowing stream margins that provide young trout with relief from competition from older trout for habitat and food. Taking a fresh look at established trout fishing regulations and attitudes to stocking will be important parts of preserving the longevity and range of our trout fisheries.

A WORD OF CAUTION

The same factors that are adding pressure to trout populations are also having impacts on native fish and other indigenous species, many of which are already threatened or endangered. More than ever, the remediation of trout populations and habitats will have to be assessed as part of a whole-ecosystem approach to fisheries and environmental management.

Source: Bureau of Meteorology

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