SE Australian bushfires and our local streams

As the big fires are extinguished, Cameron and Katie assess what’s happened – and what’s likely.

During the 2019/2020 Australian bushfire season, blazes have been widespread. Over 43 freshwater catchments in south-eastern Australia have been impacted. North-east Victoria and south-east NSW have been among the hardest hit; home to some of Australia’s best trout and native fisheries. We also call these areas home; both from a guiding viewpoint and as a place to live.

Social media and instant access to sensationalist ‘news’ may have painted a dismal picture of the fate of our local fisheries. It might even seem as if your favourite fishing spots are no longer an option. But amongst the losses, there are plenty of reasons to hope as we discuss the likely and expected impacts of the fires.

Burnt and unburnt areas

Catchments most impacted by recent fires include those located in the Alpine National Park (parts of the Cobungra, Bundara, Victoria rivers), Buffalo (Buffalo, Dandongadale), Kosciusko National Park South (Upper Murray/Indi and some feeder streams) and the Upper Murray catchment (Upper Murray, Indi, Nariel, Cudgewa, Thowgla Creek). However it should also be noted that many streams and catchments haven’t been burnt. As just a sample, catchments basically unaffected include the Goulburn catchment (including all Eildon feeders except a small section of the Jamieson), the Kiewa catchment, Ovens and feeder streams upstream of Porepunkah, King River catchment, Upper Mitta Mitta catchment above Anglers Rest, Mitta Mitta catchment below Dartmouth Dam (including Snowy Creek), the Swampy Plain (including Geehi), upper Tooma, the Snowy and Thredbo, and many others.

Although these fisheries have been largely unaffected by the fires directly, the local communities have so far endured a season of anglers (and other visitors) staying away while fearing the worst.

The Emergency Victoria Incidents and Warnings map showing the broadscale extent of the fires as at 13 February 2020 (http://emergency.vic.gov.au/). On closer inspection, it has been apparent that many sections of bush, farmland, streams and riparian vegetation shown as burned on this map have been spared. While some areas have indeed burned very hot, others have burnt at slower rates or not at all and it is these areas that can significantly help our fisheries recover.

The burnt landscape following the Corryong fires.

Fire is nothing new to south-eastern Australia’s fisheries. Although hard to comprehend right now, it must be remembered that we have experienced similar and even larger fires, at least within the trout fishing range of south-eastern Australia. The 2002/2003 and 2006/2007 fires burnt out even more stream trout fisheries, yet these quickly recovered with minimal, if any, human intervention.

One of many fine streams in north-east Victoria that remains unburnt from the current fires. The fallen and scared trees are from the 2007 fires. This area, like many, looked like ground zero immediately after the fires, but has recovered well. (Photo: David Anderson, http://www.twigwater.com/)

Smoke and Fire

For weeks in January, we battled with thick smoke from surrounding bushfires. Heavy and persistent smoke restricted us to only short fishing sessions with poor visibility, or none at all when it became too thick to go outside. Towns such as Corryong were directly impacted by fire, while surrounding towns were constantly under varying levels of emergency warnings (Watch and Act to Emergency Evacuations). This was the beginning of the tourism industry feeling the pinch of missing out on the long-awaited summer holiday business. As fires started to move through the landscape, road closures meant much of the area could not be accessed – even if a given town or stream was actually unburnt.

Smoke from the Mount Buffalo / Abbeyard fires.

Fortunately, most main roads in the region are now open, with only the more remote areas still restricted.

Immediate effects

The most apparent impact from the bushfires in the short term is the sediment load, ash and debris washing into waterways following the first heavy rainfall. Some may remember the sediment slug that came down the Buckland River into the Ovens following the 2003 fires. This sediment slug saw trout populations reduce substantially and dead fish, including some natives, were recorded as far downstream as Myrtleford. This has recently happened in the Upper Murray catchment and is very reminiscent of past events.

The main catalyst for these sudden post-fire fish kills is when localised storms drop a large amount of rain quickly. In the case of the Upper Murray event, 40mm of rain fell in half an hour. Although the heaviest falls were quite localised, this resulted in a sudden, concentrated, heavy ash and debris load entering the river from smaller feeder streams. As bad as they look in the following pictures, these types of fish kills are typically restricted to specific sections of streams or rivers, and don’t generally cause a complete fishery collapse.

Ash and debris from Horse Creek, Upper Murray, carried down following heavy rainfall.

A Murray cod that has been suffocated by a fine ash layer coating its gills.

In the worst case, over time, sediment loads increase as the once vegetated banks erode. In the weeks following heavy rain, stream flows increase as the burnt soils repel water. Ash combined with sediment and debris can rapidly deplete available oxygen, often to levels below what fish and some of their food sources (some macroinvertebrate species) can tolerate. The high sediment and ash loads can suffocate aquatic animals with a fine layer of silt that coats their gills. The fine silt can also fill in pools and enter the spaces between cobbles where macroinvertebrates live. There can be reduced light penetration into the now turbid and ash-laden water, impairing visual predators such as trout and making photosynthesis by aquatic plants more difficult.

On a more positive note, research by David Freudenberger and Richard Norris on the Eucumbene and Thredbo Rivers following the vast 2003 Alpine Fires, showed that immediately following the fires, macroinvertebrate populations in fastwater streams temporarily changed to include very small caddis, worms and midge larvae which briefly took advantage of the siltier, more nutrient-rich environment – species which are still good trout food. Over subsequent months, flushes of cleaner water gradually restored habitat for more typical fastwater species such as mayfly and stonefly.

Black ash travelling downstream. This stream didn’t burn at this location but is still receiving the black ash from upstream impacted areas.

Long term recovery – what to expect

In the months following a fire, increased nutrient loads, combined with warmer temperatures, can potentially lead to blue-green algae blooms in the lower, warmer reaches of rivers, which in their own right, can occasionally lead to fish kills. (In 2016, 1700km of the Murray River saw a blue-green algal bloom, though not fire related.) Currently, with a lot of the Upper Murray catchment above Lake Hume burnt, it’s possible there may be an algal bloom that could flow downstream of the lake and through the Murray River.

Again, depending upon how and when rain falls, in the worst case, sediment loads can persist for several years following a fire. With the lack of vegetation to provide shade, as well as sediment loads filling in pools and deeper sections, stream temperature can increase over time. In addition, after an initial increase in stream flows following heavy rainfall events, flows can actually reduce somewhat in the long term. This is because in the upper catchments at least, regenerating younger bushland uses (and loses) more water through evapotranspiration when compared with the older trees replaced.

What does this all mean for our fisheries?

Predicting population recovery of fisheries in burnt areas is a challenge as there is very little research in Australia on this topic. What we do know though, based on our own experiences and those of anglers who keep accurate records, is fish populations often recover; and sometimes with surprising speed. This is especially so with trout, being a fast-growing, fast-breeding species. Meanwhile, those fish of any species which survive such events, are more likely to possess the right genetic make-up to survive future events.

As already touched on, the most obvious fire impacts on our fisheries, are the immediate fish kills. Large numbers of fish of all sizes and species are often recorded. We have now documented this in a section of the Upper Murray with many Murray cod (including some a metre plus in size aged at least 20 years), golden perch, trout cod, Murray crayfish and carp washed up on gravel bars or caught on in-stream structure – a sad and undeniable loss when it comes to the large cod especially. Other smaller kills have been reported within our area in streams such as the lower Cudgewa and Nariel Creeks. Inspections of other streams after the first rain events from burnt areas have not yet revealed fish kills and this can be put down to the rainfall not being as intense. Streams are flowing turbid but have not reached the tipping point of others.

Cameron assesses the damage and finds a dead 110cm Murray cod, one of the fish he regularly targets.

As roads reopen into the more remote sections, we’ll be able to do a more comprehensive assessment. Looking at the state of the streams lower down, it is likely that trout populations (both brown and rainbows) have been impacted in the burnt sections, but to what extent it’s too early to tell.

Shifting habitats

By nature, fish are highly mobile in response to prevailing conditions, especially trout. We know this from studying their movements in response to increasing water temperatures and previous bushfire events. In 2013, for example, the Ovens River headwaters received a large bushfire. When this fire was finally extinguished by heavy rain, the river received an ash-laden flow which severely impacted water quality and caused some fish kills. Many people completely wrote-off the river for fishing after the initial reports.

What we witnessed told a different story and one of resilience. As the ash-laden water travelled down the river, many fish escaped and sought refuge in feeder streams. As soon as the flows stabilised and cleared, brown and rainbow trout returned to the main river and were providing good fishing within a month. These fish went on to to successfully spawn and repopulate the river.

The 2003 fire impacts on the Buckland River saw many fish escaping upstream into more favourable habitat. As conditions improved, trout began to return back down the system. The opposite can occur where the top of catchments burn. Fish can undertake downstream movements when faced with rapidly declining water quality, allowing them to enter connecting streams and larger lakes if available. Looking at fire burn maps in detail, many affected streams have refuge habitats (either upstream, downstream or feeder streams) available in areas that have not burnt or have had burns smaller in area compared to their drainage catchment.

The fish are still here and patiently waiting for the return of anglers.

Fish spawning

The ability of natives and trout to successfully spawn and recruit can be impacted in the short-term (and occasionally long-term) by fire runoff. Sediment and ash loads that persist months and even years after the fires can potentially impact the developmental and earlier life stages of fish as these are most vulnerable to poor water quality and toxicants.

Native fish such as Murray cod, trout cod, two-spined blackfish and river blackfish are nest spawners during the warmer months of the year (October – January), which is usually Australia’s bushfire season. If heavy sediment and ash persists into breeding seasons, it can have the ability to smother and kill eggs before they hatch.

Brown and rainbow trout and native Macquarie perch are gravel spawners. Browns usually spawn in late autumn into early July, whereas rainbows usually spawn on high waters during late August to mid-October. Freshly laid eggs could be smothered by fine silt and ash if it still persists during the cooler months, which can lead to eggs dying because of a lack of oxygen. Rainbows may have more success in spawning as there should be more clean water many months after the bushfire season.

Another concern is preferred spawning sites, such as trout ‘redds’ may be covered by fine sediment and ash. Typically, the gravel areas where redds are located are in the tail of a pool, and these areas can be hit hard from erosion and sedimentation. The mid to lower reaches of some of our rivers can be important sections for trout spawning and with lower current velocities, can be more susceptible to long-lasting impacts from sedimentation. Here, the water current has slowed enough for heavier particles (such as sand) to settle out and fill in gaps between larger gravel. Areas which appear to have been filled in by sand can often be attributed to unstable catchment areas still recovering many years post bushfire.

What can we do?

Habitat

As rain and successful firefighting efforts shift our focus away from the first priority of extinguishing the fires, it’s important to start thinking about the future and how to protect our waterways. To maintain the resilience of our fisheries, one of the first things we can do is restore vegetation along waterways. Even narrow vegetation corridors can improve sediment and erosion control to prevent debris being washed in, and buffer temperature extremes through shading. By limiting the impacts of future sedimentation, we can improve the ability of our fish to successfully spawn.

Some stream frontages are going to need a helping hand.

Where fish populations have historically been based on stocked fish, with no successful natural recruitment, there are already calls to restock fish that were lost post-fires. Before we undertake a stocking program though, we need to objectively gauge the health of the recovering rivers and surviving fish populations. In the worst cases, streams altered by fires may not immediately have available food sources or suitable habitat to support re-introduced populations of fish. In such cases, time to recover or management interventions such as replanting vegetation, may be more important.

On the other hand, on ‘wild’ streams where fish have previously demonstrated the ability to reproduce and survive the impacts of fire, their offspring are the most likely to survive future environmental stresses – and not only bushfire. Hatchery-reared eggs, larvae and /or fingerlings might not have the ‘tough’ genetic make-up to survive or be adapted to life in such systems.

Local communities and looking forward

Our advice is, as soon as road or track access reopens, visit your favourite fishing spots. You might be surprised by the resilience of a given stream. Hopefully you won’t be disappointed by the damage – but there is only one way to find out. Put your rod down and take a look around. Look under stones – check for ash layers and macroinvertebrate populations, take note of the riparian vegetation – is it still around? If you go to your favourite fishing spot and there are no fish, that doesn’t mean they aren’t there. Drive upstream or downstream, spend some more time in the local area to understand the altered state of your stream, and don’t always go off other people’s fishing reports. And of course stay up to date with local conditions and warnings, play it safe!

As the fires go out and the smoke drifts away, there are many beautiful streams like this in north- east Victoria which remain unburnt.

Make sure you visit the local communities and as the hashtag says, take a #emptyesky. Support the locals in these areas – they are the guardians of our favourite streams. And finally, consider helping Landcare, Greening Australia, ATF (or equivalent organisations) with their post-fire revegetation efforts. As much as we all know that overhanging trees are great at stealing our flies, we need them to shade the streams, feed the macroinvertebrates (which then feed the fish!) and stabilise the banks to minimise lasting impacts from bushfires.

Signs of hope. The environment is already starting to recover.