Like all members of the salmon family, the rainbow trout looks particularly well designed. Its slim, streamlined body is perfectly suited to a predatory life in fast flowing rivers and streams. The back is grey-green, the sides silvery and the belly white. Rainbows usually have a purplish stripe (rather like a petrol spill on a wet pavement) along their sides and it is this which gives them their name.
Follow the feeding
Rainbow trout eat almost anything in or on the water, at any depth – from nymphs and molluscs on the bottom, to insects falling on the surface -just like brown trout. Rainbows behave most like their cousins in running water, waiting in the eddies behind boulders for food to be swept down to them.
Like brownies, they become more predatory as they get older. At only a couple of pounds in weight, they attack shoals of minnows and sticklebacks, and by the time they reach double figures, they are eating any fish smaller than themselves. In still waters they stick together, often swimming at great speeds and feeding right on the surface -quite a sight. They also feed like mackerel, rounding up prey fish in the shallows and then attacking them. In Britain most rainbow trout are raised in farms, so their diet consists entirely of trout pellets. Some anglers have reputedly made huge catches of young fish on flies tied to resemble these pellets (though these tales may be exaggerated).
Life in a cold climate
In the wild, rainbow trout spawn in redds (shallow pits) dug in gravel by the female -rather like salmon do. The eggs hatch out in two to three months, depending on temperature, and the immature fish have a yolk sac to keep them going for the first few weeks of life. At this stage they are known as alevins and they spend their time in the gravel where they hatched.
When the yolk sac is used up, the fry feed on plankton and quickly gain weight – they can double their weight in three weeks. Farmed rainbows are raised from eggs and milt (the males’ sperm) stripped from adults, and end up either on a fishmonger’s slab or in a fishery when they weigh anything from l-20 lb (0.4-9. lkg).
Rainbows have been introduced to Britain from the USA and are therefore used to slightly higher water temperatures and drier summers. Because of this, they begin to spawn in early spring – the time when the levels of their native rivers are high enough. This may explain the rapid growth rate of rainbows – the parrs (fry) must be large enough to survive a winter after only about five months. European trout spawn in autumn, the eggs hatching after the winter. This gives the young all of the spring and summer to feed up for winter.
Whatever the exact reason, rainbows rarely breed in the wild in Britain. They are widely stocked because they are easy to farm, and their tolerance of higher temperatures helps them survive in small ponds which previously held only coarse fish. However, they spawn in few of these waters and the populations are actually self-sustaining in only one or two, including Blagdon Reservoir in Somerset and the River Wye in Derbyshire.