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The Rainbow Trout
General description:
Rainbow trout possess the well-known streamlined salmonid form, though body shape and coloration vary widely and reflect habitat, age, sex, and degree of maturity. The body shape may range from slender to thick. The back may shade from blue-green to olive. There is a reddish-pink band along each side about the midline that may range from faint to radiant. The lower sides are usually silver, fading to pure white beneath. Small black spots are present over the back above the lateral line, as well as on the upper fins and tail. In some locations, the black spots of adults may extend well below the lateral line and even cover the entire lower side. Rainbow trout are positively identified by the 8 to 12 rays in the anal fin, a mouth that does not extend past the back of the eye, and the lack of teeth at the base of the tongue. River or stream residents normally display the most intense pink stripe coloration and heaviest spotting followed by rainbows from lake and lake-stream systems. Spawning trout are characterized by generally darker coloration.
Life history of the Trout in it's own habitat:
During late winter or early spring, when water temperatures are on the rise, the maturing adult rainbows usually seek out the shallow gravel riffles or a suitable clearwater stream. Spawning takes place from late March through early July, depending upon the specific location and the severity of the winter. The female uses her tail to prepare a nest, 4 to 12 inches deep and 10 to 15 inches in diameter. From 200 to 8,000 eggs are deposited in the nest, fertilized by a male, and covered with gravel. Hatching normally takes place from a few weeks to as much as four months after spawning, depending upon the water temperature. A few more weeks may be required for the tiny fry to emerge from the gravel. Upon emergence, the small trout assemble in groups and seek shelter along the stream margins or protected lake shore, feeding on crustaceans, plant material, and aquatic insects and their larvae. Rainbow trout rear in similar habitat for the first two or three years then move into the larger water of lakes and streams and turn more to a diet of fish, salmon carcasses, eggs, and even small mammals. Age of onset of sexual maturity varies markedly between individuals due primarily to such factors as population density, productivity of the aquatic environment, and genetic makeup. In the wild, male and female spawners as young as ages 3 and 5, respectively, have been found, but a majority of both sexes mature at age 6 to 7. Spawning frequency ranges from annually to once each three years. Rainbow up to age 11 have been observed spawning.
Trout farming
The rainbow is a very fast growing trout on a farm, and in waters where a suitable food supplu exists. It has a short life cycle, living for no more than a maximum of five years, and there is evidence that during this limited period it can attain weights of up to 60lbs in a few lakes of its native continent. It is less well suited to most waters in the UK, although waters such as Avington and Dever springs have developed methods of supplying Rainbows of over 30lb in weight.
The trout farming operation is simple enough. The egss, or ova, are stripped manually from the hen fish and fertalised with milt stripped manually from the cock fish, after which they are placed in trays in the hatchery and supplied with a plentiful flow of pur, well oxygenated water. Incubation time depends upon water temperature, but hatch normally occurs thirty days, or a little more. The fry emerege from the egg with the yolk still attached to their body, and for a few days they merely lie quiescent at the bottom of the tank, absorbing the yolk. Eventually they begin to swim up off the bottom, and from then onwards will take a high protein manufactured food which promotes fast growth to occur. Depending upon the water temperature growth rates can be such that the trout attains a length of 12 inches and a weight of 12-14 ounces within a year of hatching. Normally the breeding operation takes place in late autumn or winter, so the trout reach the normal minimum length for stocking purposes by the following winter or, at worst, early spring. Trout kept on the farm abd fed regularly with high protein foods, can comfotably reach weights of up to 20lbs before the end of their life cycle. The better farmers hold back their fastest growing trout and use these for breeding purposes, but many are sold for stocking at four years of age to provide the attraction to the angler of a chance to catch a monster. Rainbows introduced into a reservoir or lake are often confused by the transfer from the farm ponds and tend to remain in a large group for a day or two. I found this experience common to Llyn Fawr lake. Eventually the larger group breaks down into smaller units until all the newly stocked trout are spread throughout the water. In the early days they may be unaccustomed to taking the food forms that live in the water since they have been fed almost by the clock since hatching. They soon settle down to natural feeding and quite soon especially in the warmer months they will often rise and take naturals from the surface probably quite the same as when they were in the stock ponds. Rainbows are hungry fish, prepared to feed more or less continually on almost anything available to them. They are great absorbers of daphinia, and will move up and down the daphnia clouds feeding virtually non stop on this easily digested, high protein life form. They will also eat the nymphal, larval and pupal forms of aquatic flies as well as the flies themselves, freshwater shrimps, water beetles and bugs of all types, tadpoles and small frogs, terrestrial flies that find their way onto the water, but one of the most important foods for continued growth is small fry. These can be minnows, sticlebacks and small coarse fish, Trout are also not averse to easting large quantities of acquatic snails. In a lrage and fertile water, rainboes that are not caught in their first year of introduction become progressively more difficult to catch, and are fully capable of reaching weights in excess of ten pounds, by which time it is probable that the major proportion of their diet consists of other fish, including newly stocked 12 inch rainbows.
This page was last updated on Tuesday, April 3, 2001
