Philip comes up with a trout fishing plan or three if it gets hot.

You arrive at one of your favourite stretches of trout stream, and it’s hard to contain your excitement as you recall the brilliant action this time last January. Once out of the air-conditioned car though, you quickly realise it’s too hot in the sun for waders, so you race to put on your wading boots alone, eager to make that first cast.

You hardly notice that you don’t notice the water as you step in. You’re too intent on the path of your foam hopper as it curls down that prime bubble-line, right next to a steep rocky bank. Last year, hoppers weren’t such a ‘thing’, yet you still caught plenty of nice trout on a Stimulator. This year, walking the riverbank sounds like walking on bubble-wrap, such are their numbers. Yep, the hopper is going to clean up.

Except an hour later, it hasn’t. In fact, not only have you not had a rise, you haven’t even seen a fish. About now, you start to worry about the water temperature, something which, in all the excitement, you’d either forgotten about or chosen to ignore.

Almost reluctantly, you pull the digital thermometer from your vest, and lower the probe into the water. You expect (or is that hope?) that, after cooking in a zip-up pocket, the numbers will dive like a rocket launch countdown on contact with the water. Yet they don’t, heading down only sluggishly until at 26C, there’s no amount of jiggling or moving to different bits of water that push the numbers any lower.

The reality sets in. Even allowing for the remarkable acclimatisation attributes of heat-hardened Australian trout, right now, this river is simply too warm for there to be any chance of catching one.

Move to improve

The above really happened to me some years ago during a heatwave on the Indi River. Fortunately though, there was a happy ending when a move to a small stream high in the nearby Australian Alps, found water 10 degrees cooler, and with plenty of actively-feeding trout – including some beauties.

In truth, I probably should have avoided the Indi altogether and headed up high straight away… if I’d only followed my head instead of my heart. I love the lower Indi, but heatwave conditions, including warm nights, are never good for this part of the river, which, despite its alpine surrounds, is below the altitude of some Melbourne suburbs. And a simple check online of the Biggara gauge on the MDBA website, would have confirmed the temperature truth.

Heatwave Havens

Hopefully, heat won’t be a big deal in south-eastern Australia this summer, but realistically, after three mild summers, we’re overdue for a hot one. Regardless, sooner or later heat will be something to factor into our trout fishing. To be simplistic about it (while keeping in mind that the detail is way more complex) trout prefer cool water to warm. So, when summer throws up a hot spell, it’s worth seeking out water that’s cooler.

Three main factors help with cool water: elevation (and speed of descent from elevation), shade, depth, and a cold source – such as springs or deepwater dam releases. Each on their own can help a lot, while in combination, brrrr!

The bottom line is, wherever you’re fishing when it’s hot, water which is cool enough will never be far away.

High up

I could write pages about how and why air temperature generally reduces with elevation – while also acknowledging that water is much more difficult to heat and cool than air. However, the simple and important takeaway is, high country streams and lakes tend to be significantly cooler than lowland waters in the same district.

In practical terms, during heatwaves, for enough elevation benefit alone (without other factors), I find I need to get higher than 1200 metres above sea level (m asl), even 1500m asl, to find substantially cooler water. An exception is when a stream descends very quickly from its high altitude origins – then, it can retain cold temperatures down to lower elevations than might superficially be expected.

Shade

This one is massive. The radiant heat of the summer sun means that the mere presence of good cloud cover to blunt this radiation on an otherwise hot day, can take a few important degrees off water temperatures (although there can be a flipside of cloud cover trapping heat in overnight).

Meanwhile, shade from overhead trees, or more rarely in Australia, steep-side gorges and slopes, is purely an upside. While exact numbers are hard to nail down, on the same stream, the difference between fully shaded versus open country, can be as much as several degrees. In hot weather, this can mean a trout fishing difference between healthy and hopeless. As flyfishers, we might get annoyed fishing that ‘overgrown’ water, where vegetation gets in the way of an easy cast. But on a hot day, that’s exactly where you want to be.

The investment that bodies like the Australian Trout Foundation make in streamside tree planting is partly in recognition of this benefit – not to make casting harder!

Depth

Deeper lakes, and sometimes deeper pools on streams, can provide significant areas of cooler water ‘insulated’ from the worst of the summer heat. In it’s simplest form, a reservoir of cold water can exist below a much warmer surface layer. The larger and deeper the lake, the greater the area of cold water, although this factor can have some value even when the water is only a few metres deep.

Fishing-wise, you can either get your wet fly down into this cooler zone, or less often, benefit from trout temporarily leaving it to do a snatch and grab raid near the warm surface. A classic example is trout feeding on an evening mudeye migration. Travelling up from the cooler depths, trout can retain a safe body temperature for long enough to eat some big food, before retreating to cool down… and repeat.

Another striking example was witnessed by a fisheries scientist mate working at Lake Dartmouth one January. Despite a near lethal water surface temperature of 27C, trout were rocketing up from the depths to grab cicadas, before rocketing back down again. Bank anglers can benefit from this sort of behaviour by fishing steep, deep shores when the hoppers are on.

Streams with a strong flow will benefit much less from the cold water at depth phenomena due to mixing. Ironically though, streams with a typically low summer flow and frequent deep pools, such as some Monaro and south-west Victoria examples, will stay cool at depth.

Cold source

Groundwater/ springs, and artificial dam releases (tailwaters) can both chill stream flows. Major input from the former is rare in Australia, although there are small areas on some streams where visibly flowing cold springs enter and provide a cold water refuge for trout trying to escape the heat. On occasion, I’ve found trout stacked up in these places, although I reckon these vulnerable fish are best left alone until cooler conditions return. There are also places where cold water enters from groundwater well below a stream’s surface. Only visible with thermal imaging, once again these spots are probably more valuable as trout refuges than for fishing.

Artificial dam releases are a whole different story. If the offtake is deep enough (see Depth above), they can provide tens of kilometres of high quality cold water trout habitat on rivers like the Goulburn, Mitta, Tumut, Swampy Plain/ upper Murray; and to a less dramatic extent, on rivers such as the Snowy below Jindabyne, and the Kiewa below Mt Beauty. There are also numerous ‘mini’ tailwaters beneath dozens of other dams. Releases from  these can be more erratic and therefore less reliable, but if you’re in the area anyway and it’s hot, a check won’t hurt.

Places to avoid

During hot periods, it obviously pays to avoid streams exposed to full sun for much of the day, unless they’re very high up, or chilled by dam releases as described above.

Lots of exposed rock (particularly dark rock) in or bordering a stream, will conduct a lot of heat into the water on sunny days. Dark, tannin-stained water heats up more quickly than clear water on both lakes and streams.

For lakes, shallow waters are best avoided when it’s hot: stick to deep, and preferably for bank anglers, lakes with steep shores where you can easily access those depths. And lakes and streams on the coastal side of the Great Divide, or on top of it, can benefit from the cooling effect of onshore winds that don’t make it further inland. In other words, these waters can get a respite from the heat, which inland trout waters miss. This effect is especially pronounced in central and western Victoria and the Monaro.

Other tips

To repeat the point from earlier, it’s worth remembering that water has a high thermal mass: it requires a lot of energy to heat, but equally, it is difficult to cool quickly – as you’ll discover if you want to chill those beers on ice asap!

So, to avoid warm water more generally, keep in mind that it’s duration of heat that’s most problematic. A single hot day is less of a concern than a series of hot days. Warm nights aren’t good either, because they deny much of the opportunity for nocturnal cooling. Similarly, if you’re looking for a break in the heat cycle, a few hours after the passage of a cold front won’t do it. You want that cooler (and preferably cloudy) weather setting in for a few days.

And the mass of water to be heated (or cooled) is an important factor too. The worst heatwaves are going to struggle to have a detrimental impact on a tailwater like the Mitta when it’s sourcing 11C water at a rate of 8000 ML/d. But it won’t take long to impact that small, paddock-lined creek or shallow lake.

Also remember that full summer sun is a powerful water heater, even if the air temperature isn’t too bad. Give me warm and cloudy instead of cool and sunny – at least until late summer, when the strength and duration of sunlight begins to wane.

Finally, buy a decent, portable thermometer and use it. Guessing water temperature by feel doesn’t really work: the relative air temperature will skew your opinion. (For example, even 25C water can feel cool on a 40C day.)

Other species?

Something else to consider if heat plays havoc with your plans, is to target species that are more heat tolerant. Murray cod, golden perch and bass are widespread, at home in the heat, and never far from trout fishing areas (in fact often, they overlap). And saltwater fishing tends to be business as usual as things warm up.

Whatever way you go, remember that good hot weather flyfishing is absolutely doable – you just need to plan your trip accordingly… and put those beers on ice early!