The 'Ideal' Brook Trout Temperature Everyone Quotes Is Really a Range, and Size Matters

Ask around and you'll hear a specific number for what cold-water trout want in summer: something close to 15°C, the textbook "preferred temperature" for a species like brook trout. A new study surgically tagged adult brook trout with acoustic transmitters in three lakes and tracked exactly where they sat in the water column, day and night, through summer stratification, when warm surface water sits on top of cold water near the bottom. The brook trout temperature preference that came back from the data was messier than the textbook number suggests. Only a few individual fish actually spent most of their time at exactly 15°C. As a group, they worked a band between 13 and 17°C, avoided anything under 10°C or over 20°C, and how tightly an individual fish stuck to that band depended heavily on its size.

How They Tracked Fish You Can't See

Researchers used high-dimension acoustic arrays, essentially a grid of underwater receivers, in three lakes to follow free-ranging adult brook trout ranging from 26.2 to 54.5 centimeters in fork length. Each fish carried a surgically implanted acoustic tag, which let the team log where it sat in the water column through the day and night across a stratified lake, where warm surface layers and cold water near the bottom exist at the same time. That setup is what let them see something a single snapshot survey never would: how much an individual fish's position shifted hour to hour, not just where the population as a whole tended to cluster.

The Body-Size Twist

Here's the part that stands out. Even though individual fish varied a lot in which exact temperature they occupied at any given hour, the total thermal exposure each fish racked up over a month, measured in degree-hours, was consistent and inversely related to body size. Bigger brook trout accumulated fewer degree-hours than smaller ones. The math behind that relationship landed close to a threshold researchers associate with sublethal physiological stress. The researchers' read on it: adult brook trout may "actively manage metabolic rate as a function of body size" over stretches longer than a single day, not just react moment to moment to whatever water they're sitting in.

The Catch

That last point is a hypothesis, not a proven mechanism. The study found a consistent pattern, big fish running a tighter thermal budget than small fish, but it didn't test whether that's the trout actively choosing cooler water to manage its metabolism, or just a byproduct of bigger fish having less tolerance for warm water before hitting physical limits. Three lakes and a fork-length range topping out around 55 centimeters is also a specific slice of the brook trout world, not a guarantee every population behaves the same way.

What This Means on the Water

I'll be honest, brook trout aren't a fish I've targeted specifically. Most of my trout time is moving water in the Sierra Nevada, not stratified lakes holding brookies, so I don't have a personal story that fits this one directly. What I take from it as a general trout angler is pretty simple: the biggest fish in a lake are probably the pickiest about water temperature, not the toughest. That cuts against the instinct to assume a trophy fish can handle warmer water just because it's bigger and stronger. If you're fishing a stratified lake for brook trout in the heat of summer, this is one more reason a fish finder that reads temperature or oxygen layers earns its keep, and it lines up with a study on kokanee and rainbow trout I covered a few weeks back that found a similarly narrow summer depth window in a different lake on a different continent. Different mechanism, same basic lesson: "fish are deeper in summer" undersells how tight that window really is, especially for the biggest fish in the lake.

Citation

Stewart, E. M. C., Middel, T., & Ridgway, M. S. (2026). Brook trout thermal habitat selection in stratified lakes: diel patterns, body size, and the importance of thermal time. Canadian Journal of Zoology, 104. https://doi.org/10.1139/cjz-2025-0087