Bull trout reach sexual maturity at between four and seven years of age and are known to live as long as 12 years. They spawn in the fall after temperatures drop below 48 degrees Fahrenheit, in streams with abundant cold, unpolluted water, clean gravel and cobble substrate, and gentle stream slopes. Many spawning areas are associated with cold water springs or areas where stream flow is influenced by groundwater. Bull trout eggs require a long incubation period compared to other salmon and trout, hatching in late winter or early spring. Fry may remain in the stream bed for up to three weeks before emerging.
Juvenile fish retain their fondness for the stream bottom and are often found at or near it.
Bull trout may live near areas where they were spawned or migrate from small streams to larger streams and rivers or from streams to lakes, reservoirs or salt water. Smaller resident fish remain near the areas where they were spawned while larger, migratory, fish will move considerable distances to spawn when habitat conditions allow. For instance, bull trout in Montana's Flathead Lake have been known to migrate up to 250 km to spawn. Migration is important to maintaining healthy bull trout populations. Bull trout are vulnerable to many of the same threats that have reduced salmon populations in the Snake River Basin.