One thing that makes being stuck within a few kilometres of home slightly easier to tolerate (I said slightly) is the brutal, unrelenting cold. For the last week or so, it just hasn’t let up. If we’ve had an hour of sunshine, I’d be surprised. Otherwise, the sky has been an icy grey blanket, regularly spitting rain, hail and even a bit of snow.

So, having tied up that whole pile of stick caddis I’d been meaning to get to, I’m prompted to reinvestigate how cold water – really cold water – affects trout.

A quick check of a couple of local lakes shows the current water temperature, at least near the surface, is 6C (give or take half a degree depending on exact location). For many years, I had it in my head that 6C was a key number for trout and cold water: warmer, and the trout would play; colder, and they shut down. I think I was told that by a respected fisheries scientist who I won’t name in case I’ve got the source or the detail wrong. Anyway, I held on to that number as fact, and it no doubt became self-fulfilling in my own experience, to the point where I would have either stopped fishing when I measured less than 6C, or stopped believing – which is pretty much the same thing.

There were a few exceptions, like a trout which ate a Red Tag on the Victoria River one May, when the edges were frozen and the water temperature measured 3C. Tasmania has also produced the odd trout in sub 6C water, such as a memorable morning at ice-fringed Lake Kay one September, and another September on February Plains surrounded by thick snow, with ice slurry on the windward shore. Still, the 6C rule seemed to hold more often than not.

The trip which caused a rethink was to the Snowy Mountains in early August 2018. The area lived up to its name, and Steve and I fished in snow (or close to it) for the whole trip. Despite the enormous thermal mass of lakes Eucumbene, Jindabyne and Tantangara, they were variously between 4C and 7C in the surface layer we fished (we were shore-based), and usually 6C or less. We caught or at least hooked trout everywhere, but the standout was a day at Tantangara.

After two days of heavy snow, a bluebird day dawned and encouraged us to brave the access road. We made it in – just. The sunny sky didn’t disguise the fact that the lake was frigid: on many shores, the slowly-rising water had submerged unmelted snow. I took a water temperature reading of 4C and resigned myself to a day of beautiful scenery, but few trout. For the first couple of hours, that’s how it played out. The visibility over the flooded grassy flats was very good, but nothing seen, nothing spooked. Casts into the deeper water were also fruitless. Then, at about 11.30am, Steve hooked into a good fish and landed it.

It was as if a switch had been flicked. Only minutes later, the same flooded flats which had been lifeless on the walk around the lake, were suddenly alive with trout.

What followed was some of the best fishing I’ve had in the Snowys. It seemed like there was an endless procession of trout swimming into any shallow, grassy bay. We hardly needed to move – if you hooked or spooked a fish, another would soon take its place. Fly choice wasn’t important so long as the pattern was noticed and didn’t get lost in the grass. I caught good trout (all browns) on everything from Claret Carrots to black Woolly Buggers. This wasn’t ‘passable’ fishing, trying to stir up comatose trout in deep water; it was as good as a prime Tasmanian wilderness lake on a perfect January day.

By the time I thought to check the water temperature again, it was 6C. I can safely assume though that it was less for some of that dream fishing.

Trying to objectively determine the lowest water temperature for worthwhile trout fishing turns out to be difficult. The literature is only too happy to discuss the optimal temperature range for trout and their upper temperature tolerance, though these vary a bit depending upon species, location and author. However, everyone seems surprisingly coy and vague about minimum temperatures.

One of the few definitive statements I can find is from noted fisheries professor Robert J Behnke. In his book Trout and Salmon of North America, Behnke, writing about the amazing tolerances of Arctic char, notes that “Arctic char enter the sea in June as sea ice is breaking up and the sea water temperature is below the freezing point of freshwater at minus 1C. They feed voraciously and opportunistically… and grow rapidly.” By comparison, he says that “At temperatures of about 4C and lower, brook trout, brown trout and rainbow trout will continue to feed at reduced levels, but they do not grow and will eventually lose weight.”

Meanwhile, in his co-authored book, The Artful Science of Trout Fishing, fisheries scientist Dr John Hayes from New Zealand makes some interesting cold water observations without being quite as prescriptive. He observes that spawning trout in the 5-6C winter water of Lake Alexandrina (near Tekapo) would wait until the water temperature hit its 10C mid-afternoon peak before running up the spawning creek. (This fits roughly with a wider view that trout generally are more active and feed harder in warming water than cooling water.)

Hayes also writes that trout in water below 8C won’t grow very fast, regardless of available food, and that most juvenile trout are hiding amongst the river rocks when water temperatures drop below 9C.

A common theme across various authors and reports, is an ‘optimal’ water temperature range of between 10C and 20C, with the parameters varying depending on species and who’s writing! Interestingly though, there’s sometimes a concession that ‘optimal’ isn’t necessarily optimal for feeding activity: for example, trout can feed hard at water temperatures well above what’s optimal – they just don’t convert that effort into body weight and condition.

Conversely, Hayes notes that he would often observe large numbers of brown trout cruising the margins of Lake Alexandrina in the chill of August, only to have these edge cruisers mostly disappear during the supposedly more ideal temperatures of November – frustratingly, just in time for the high country trout opening!

Conclusions? If the water isn’t frozen solid, there’s arguably hope. But it would be preferable if the water temperature was at least above 4C, ideally 6C, and on an upward trend. And if you can go fishing, do. That’s probably never been truer than right now, even if winter seems really set in.