Bear, McMahon & Zale found westslope cutthroat trout and rainbow trout to have similar growth rates and similar optimum growth temperatures, but incipient lethal temperatures 4.7 °C apart — 19.6 °C against 24.3 °C (PDF). Bull trout survival was at least 98 % from 8 °C through 18 °C and zero at 22 °C and above over 60 days (Selong et al. 2001). Brook trout growth becomes negative around 24 °C, with the upper limit for positive growth estimated at 23.4 °C (JEB 2017).

The lethal temperature is the wrong number for almost every question an angler or a manager actually asks, because a fish stops growing long before it starts dying. Bull trout are the cleanest demonstration: feeding falls off above 16 °C and stops entirely at 22 °C, while death does not arrive until 20.9 °C sustained. A bull trout river running 18 °C through August is not killing its fish — it is starving them, and the fishery hollows out quietly over years while every survey still records the species as present.

This has three consequences. A thermal standard set at the lethal limit protects presence, not fishing — it preserves a population that no longer grows. "The fish survived" is not evidence the water was fine, which is the flaw in judging a hot summer by whether there was a visible die-off. And the fishable window is narrower than the survivable one, so the honest summer question is not "will this kill them" but "are they eating".

Because the growth optima converge and the lethal limits diverge, warming water does not reduce all trout equally — it re-sorts them. The band between 19.6 °C and 24.3 °C is water that rainbow trout can hold and westslope cutthroat trout cannot, which makes that five-degree window the mechanism by which rainbow trout take over cutthroat streams from the bottom of the watershed upward. Displacement and hybridization into cutbow then run together. The same logic explains why bull trout, with the lowest feeding ceiling of the four, retreat furthest upstream as a system warms and why bull trout distribution is the most sensitive available indicator of thermal change. See fishery feedback loops for how this ratchets.

Thermal tolerance gets the attention, but spawn season assigns each species a different failure mode, because it determines what season the eggs sit in the gravel through.

Spring spawners — rainbow trout and westslope cutthroat trout — incubate through freshet, so their recruitment risk is scouring flow. A single large, badly timed peak can remove a year-class outright. Their eggs never see winter.

Fall spawners — bull trout and brook trout — incubate through low, stable winter flows and so largely escape scour, but they pay for it with a long incubation in the coldest season, where the risks are anchor ice, dewatering and freezing of shallow redds. Bull trout additionally require groundwater upwelling at their spawning sites, which ties their recruitment directly to the vertical connectivity described in fishery health systems.

Practically: after a violent freshet, worry about rainbow and cutthroat year-classes. After a hard, low-water winter, worry about bull trout and brook trout.

The regional 20 °C afternoon threshold used as a hoot-owl trigger (see reading river flows) lines up with these numbers rather than being arbitrary. At 20 °C, westslope cutthroat trout are already past their incipient lethal temperature, bull trout are four degrees past the point where feeding falls off, and rainbow trout — the most tolerant of the group — are within about four degrees of theirs. Add the oxygen effect, which runs the wrong way at exactly the same time, and 20 °C is roughly where a released fish's recovery margin disappears for every species in the region simultaneously. The rule is set for the weakest species present, which in most Kootenay water means cutthroat or bull trout, not rainbow.

Kokanee are a cold-water pelagic species whose summer habitat is squeezed from above by warming surface water and from below by oxygen depletion — the temperature–oxygen squeeze is documented as a general constraint on cold-water fish refuge use (Freshwater Biology 2024) — but specific kokanee growth optima and lethal limits are not sourced in this vault and should not be inferred from the trout figures above. #open

The warm-water species in the region — largemouth bass, northern pike, walleye, yellow perch — invert the entire table: for these species the summer thermal ceiling is not the binding constraint, and winter and dissolved oxygen usually are. The five-gate model still applies unchanged; only the direction of the temperature term flips. Species-specific figures are not yet sourced here. #open