Fall is here and winter fast approaches, which symbolizes death to some people. But for some fish species, fall is the beginning of the life cycle. Brown and brook trout, to name two, are fall spawners. By mid-October they have reached their spawning peak.
All stream dwelling trout spawn in a similar fashion, whether rainbows in the spring or browns and brookies in the fall. The female picks a site to build a nest, or redd, in suitable gravel. Then she lies on her side and swims along the bottom, moving gravel with her tail. This excavates a depression and cleans sediment from the depression.
While she prepares the redd, males gather nearby and compete for the right to be next to her. Think high school dance. When she has completed her work, she deposits her eggs in the redd and the successful male fertilizes those eggs. Finally, she covers the eggs by moving the gravel back over the depression, then returns to deeper water to spend the winter.
Spring spawning seems obvious. Eggs develop into fingerlings through the summer when food is abundant and warmer water temperature means fish grow faster. So why spawn in the fall?
All young members of the trout and salmon family (salmonids), emerge in the spring, however fall spawning results in earlier emergence of young than spring spawning. That is if those brown trout or brook trout eggs survive the winter.
Trout egg development relies on water temperature and oxygen content. However, small streams can freeze up in the winter or water levels may drop too low. It's a gamble: earlier emerging fish get to food resources quicker, but if ice scours the stream bed or if the stream goes dry in the winter the eggs die.
Perhaps it's best to look at where brook and brown trout evolved, since neither is native to Montana. Brown trout originally came from western Asia and Europe, including the British Isles. The most common strain of brown trout in the United States came from Lock Laven, Scotland. Brookies evolved in eastern North America in the Appalachian Mountains from Georgia north to eastern Canada and west to Hudson Bay and Minnesota. Both species came to Montana in the late 1800's.
The northeast, especially New England, is very wet compared to north central Montana. Annual rainfall in Boston, for example, is 43 inches a year. Great Falls averages about 15 inches of precipitation annually. That means brook trout evolved in a wet climate, where the streams may be lower in the fall, but probably contain enough water and oxygen to overwinter brookie eggs. Likewise brown trout came from a wetter climate than Montana: Northern Europe precipitation runs from 20 to 40 inches a year and parts of Great Britain reach 80 inches a year.
With brown trout more at home in valley streams and rivers that receive run off, it's easy to see how they found a niche open to a fall spawning fish. Each wild animal species has its natural niche, its place in nature that allows it to evolve and adapt. For brown and brook trout that place is a fall spawner.