No two bull trout populations live quite the same way. Some are resident, spending their whole lives in the single cold stream where they hatched, rarely topping two kilograms. Others are migratory and grow far larger: fluvial fish range the main river and run up tributaries to spawn, adfluvial fish rear in a lake and spawn in its feeder streams, and a few coastal populations are even anadromous, slipping out to salt water between rivers. The migratory life is what builds the giants — the largest bull trout on record stretched 103 cm and weighed 14.5 kg (32 lb).
Whatever their form, they are homebodies at spawning time. In autumn, as water temperatures fall, mature fish climb back to the exact cold headwater gravels where they were born, digging redds in specific sizes of clean stone. That fidelity is a strength and a vulnerability: cut a population off from its natal tributary and it cannot simply spawn somewhere else.
Their diet climbs the food web with them. Fry and juveniles start on zooplankton and streambed insects — chironomids especially — but bull trout turn predatory early and, once grown, feed heavily on other fish. A big bull trout is an ambush piscivore first and foremost, and everything about how you catch one follows from that.
Kootenay Lake — adfluvial bull trout grow large here, shadowing the kokanee schools that feed the lake's famous Gerrard rainbows.