Like many people who regularly find themselves out in nature, anglers are fond of rules – or perhaps ‘truths’ is a better term: reassuring certainties. There will be more about this whole topic in an upcoming article, but meanwhile, some of these ‘rules’ relate to water temperature. When the temperature rises to a certain number (which often conveniently coincides with a public holiday or the flowering of a particular plant!) the snapper start biting, kingfish can be caught, and the word is out for golden perch.

The problem is, the rules down in fish-land are rubbery at best, and no more so than for trout and water temperature.

After a few years of wet, mild summers, the spectre of warm water – and its negative effect on trout and trout fishing – has faded from our consciousness somewhat. However, if the forecasts for the coming months are to be believed (no guarantees there either!) warm water might be a ‘thing’ once again.

On the face of it, heat is bad for trout. They have evolved as mid to high latitude fish, and are only found closer to the equator where high altitude compensates. So, as an Australian flyfisher, from a practical viewpoint, I know that there are water temperature limits above which trout will cease to feed, and at an even higher temperature, they will die.

But sadly for those seeking certainty, those limits are all but impossible to neatly define. While some of the literature from New Zealand and the Northern Hemisphere suggests trout fishing (and even trout welfare) becomes problematic above 19C, in practice, higher temperatures are the norm on mainland Australian trout waters over summer. And as anyone regularly flyfishing over our summer months knows, the fishing at this time of year can be as good as it gets.

I’ve spent the better part of my angling life trying to unravel the trout/ temperature mess, and as they say, the more you know, the less you know! A few decades’ worth of detailed diary entries don’t help reconcile the supposed science with my experiences. In fact, at the extreme end, they occasionally show fishing mates and I catching trout in 25C water – even on dry flies. More typical are good catches, and good fishing, in water of 21 to 24C.

With the water temperature at 25C, trout still might have been possible, but Peter and I decided we were better off probing King River snags for cod.

Why the disparity between the textbooks and my on-the-water experience? One explanation is, with wild trout having now spent 150 years in the Australian environment, it’s quite possible there’s been some local adaptation to heat. Secondly, some overseas studies show that gradual exposure to warming water can allow a degree of acclimatisation to otherwise inhospitable temperatures.

Thermal refuges are another widely-accepted example of how trout can cope with warm water. The simplest of these is cooler water at depth – a concept familiar to anyone who’s swum in a deep dam, waterhole or lake over summer. Even deep water in a large pool on a stream can offer trout an escape from the heat. Then there are the colder tributaries (perhaps descending more rapidly than the main stream from high elevation, and/or more shaded) which are known places trout retreat to when the water becomes too warm. Less obviously to us, cold springs and seepage can create places in a stream where trout can find relief.

Shade is good for keeping water temperatures cooler.

But what of trout feeding and active in places where they seemingly have no right to be comfortable? One possibility which fisheries scientists have pointed out to me, is that being largely made up of water themselves, trout have considerable thermal mass. Having spent time cooling down their bodies in any of the thermal refuges above, trout can often retain that state for long enough to make a feeding sortie into water that is dangerously – even lethally – warm. That would explain trout taking cicadas off the top at Lake Dartmouth when the surface temperature is 26C!

So, how does all this play out when it comes to actually catching trout? The basic answer is that trout can be caught in water that’s much warmer than we might think it’s possible for them to feed in. A more complex issue for some anglers at least, is what is ethical – at least for catch-and-release fishing? Well, this is a slippery slope. Fishing for trout at all carries some risk of mortality. Very occasionally, in cold, clean water (including once at Little Pine Lagoon in September) I’ve seen apparently fit, healthy trout drop dead mid fight. If you want there to be zero risk of mortality to trout from flyfishing, then don’t fish!

As so often happens in our sport, flyfishing for trout in warm water comes down to shades of grey, not black & white. I guess if you’re looking for some sort of individual guidance, then Montana’s Hoot Owl restrictions offer something to work with – albeit from a place with a generally colder climate, and therefore where trout are probably less acclimated to heat than their Australian cousins. Typically, over summer, Montana’s Hoot Owl restrictions kick in if temperatures on some high value, popular rivers exceed 22.7C (73F) for 3 consecutive days. When Hoot Owl rules apply, you aren’t permitted to fish between 2pm and midnight.

In summer on Montana’s Madison River, Hoot Owl restrictions can sometimes see fishing closed between 2pm and midnight.

But personally, due to all the vagaries above, I’m not going to prescribe tidy water temperature guidelines for myself, let alone for you! I will say that when the water is warm, I’m always extra careful not to play fish for any longer than necessary (one of the side benefits of habitually using heavy tippet) and I’m careful not to remove a fish from the water for more than a few seconds, if at all.

In many respects, whether or not I fish warm water is self-regulated by opportunity. That is, if I find the water temperature at a chosen spot is getting up there, I’m likely to leave and find somewhere cooler, primarily because the actual fishing is likely to be better.

One option for a ‘safe’ photo on a hot day!

If there’s a takeaway from all this, it’s that the relationship between trout and warm water temperatures is complex – and we haven’t even touched on water quality, dissolved oxygen count, trout species, or size vs resilience.

Just keep all that in mind when deciding if a particular water is too warm to fish for trout.

Philip Weigall