While they are only native to the west coast streams from northern Mexico to Alaska and into Siberia, 100-plus years of domestication have made them an easy fish to culture. The year 1870 saw early rainbow trout propagation ongoing in two most unusual settings, the basement of the San Francisco City Hall and the University of California Berkeley campus. One of the first national fish hatcheries, at McCloud River, California, was built for brooders in the 1880s, to get eggs for shipment back East. Rainbow trout culture has been a success, if success is measured in terms of fishing opportunity. Today, rainbow are found in nearly every state, even in the deep South where dam tailwaters create cold water habitats.
In nature, rainbows start thinking about procreation from February to June, depending upon elevation and latitude. Migrations to spawning habitat are highly variable. Some fish move only a few hundred feet to spawn; others make grand migrations. On the extreme, one fish tagged in Montana moved nearly 300 miles to spawn.
It takes three years for rainbow trout to become sexually mature. After their first spawn, they may go on an hiatus and spawn at two-year intervals, two, maybe three times more. The number of repeat spawners though is minimal, less than 30 percent. Rainbow trout usually live no longer than 11 years, and the females usually outlive the males.
Clean gravels in streams are a requisite to a successful spawn. Rarely do rainbows spawn in lakes, and if they do, there is minimal success. Females clean gravels in riffles of sediment where she drops her eggs over a 20 square-foot area. A rainbow trout of five pounds may lay up to 6,000 eggs. Oxygen-bearing water pours over the incubating eggs lying in between the gravels. The eggs incubate for about three weeks.
Upon emerging from the redd, or nest, they drift into a nursery area (slow-flowing backwaters) where they eat tiny plankton and crustaceans. The young fish are quite vulnerable to predation from above and below the water. Herons and kingfishers strike from above; other trout, including rainbows strike from below. Crayfish, snakes, and even spiders prey on young rainbows.
Those that escape a predator's death grip grow rapidly that first year. This is one of nature's great adaptations to assure success -- the quicker the fish grows, the less vulnerable it will be to predators. Even still, very few fingerling trout survive to reproduce, perhaps only two fish per 10,000 eggs laid. By the time a rainbow trout reaches two years of age, it may be up to six or seven inches long. Growth slows after that, and on average, rainbows may put on about two inches per year thereafter. Of course there are exceptions, as evidenced by the large sizes rainbows can reach. Twenty-pounders have been hauled in from time to time, but they are certainly not the norm.
As the young mature, they move to midstream, where like their parents, they lie in wait behind current breaks for drifting food. It's a great strategy for maximizing food intake while expending as little energy as possible -letting the groceries come to them.
Unlike other trout, rainbows are a little less secretive in their feeding habits. They will feed in broad daylight, in open water, on insects, fish, frogs, and even small mammals. Mammals are reserved for only the bigger rainbow trout. But fish figure prominently in a rainbow's diet, and they develop a taste for meat earlier than other fish-eating trout, like the brown. Rainbows have an eclectic appetite. Dietary studies have turned up the unusual in rainbow trout stomachs -- blackberries, algae, pine needles, chicken feathers, baby birds, and lizards.
In summer, stoneflies and mayflies, both in the adult and nymph stages are sought by rainbow trout. Insects may be what gets them through winter, even in ice conditions, rainbows eat aquatic insects. Rainbows have a tendency to eat less from the surface than brookies or browns.
Rainbows do best in clean flowing water that ranges from 38 to 70 degrees. Waters too warm will put them off the feed in summer. Warm waters also inhibits growth.
--Springer is a fisheries scientist in New Mexico.
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