<P> Resident freshwater rainbow trout adults average between 1 and 5 lb (0.5 and 2.3 kg) in riverine environments, while lake - dwelling and anadromous forms may reach 20 lb (9 kg). Coloration varies widely between regions and subspecies . Adult freshwater forms are generally blue - green or olive green with heavy black spotting over the length of the body . Adult fish have a broad reddish stripe along the lateral line, from gills to the tail, which is most pronounced in breeding males . The caudal fin is squarish and only mildly forked . Lake - dwelling and anadromous forms are usually more silvery in color with the reddish stripe almost completely gone . Juvenile rainbow trout display parr marks (dark vertical bars) typical of most salmonid juveniles . In some redband and golden trout forms parr marks are typically retained into adulthood . Some coastal rainbow trout (O. m. irideus) and Columbia River redband trout (O. m. gairdneri) populations and cutbow hybrids may also display reddish or pink throat markings similar to cutthroat trout . In many regions, hatchery - bred trout can be distinguished from native trout via fin clips . Fin clipping the adipose fin is a management tool used to identify hatchery - reared fish . </P> <P> Rainbow trout, including steelhead forms, generally spawn in early to late spring (January to June in the Northern Hemisphere and September to November in the Southern Hemisphere) when water temperatures reach at least 42 to 44 ° F (6 to 7 ° C). The maximum recorded lifespan for a rainbow trout is 11 years . </P> <P> Freshwater resident rainbow trout usually inhabit and spawn in small to moderately large, well oxygenated, shallow rivers with gravel bottoms . They are native to the alluvial or freestone streams that are typical tributaries of the Pacific basin, but introduced rainbow trout have established wild, self - sustaining populations in other river types such as bedrock and spring creeks . Lake resident rainbow trout are usually found in moderately deep, cool lakes with adequate shallows and vegetation to support production of sufficient food sources . Lake populations generally require access to gravelly bottomed streams to be self - sustaining . </P> <P> Spawning sites are usually a bed of fine gravel in a riffle above a pool . A female trout clears a redd in the gravel by turning on her side and beating her tail up and down . Female rainbow trout usually produce 2000 to 3000 4 - to - 5 - millimetre (0.16 to 0.20 in) eggs per kilogram of weight . During spawning, the eggs fall into spaces between the gravel, and immediately the female begins digging at the upstream edge of the nest, covering the eggs with the displaced gravel . As eggs are released by the female, a male moves alongside and deposits milt (sperm) over the eggs to fertilize them . The eggs usually hatch in about four to seven weeks although the time of hatching varies greatly with region and habitat . Newly hatched trout are called sac fry or alevin . In approximately two weeks, the yolk sac is completely consumed and fry commence feeding mainly on zooplankton . The growth rate of rainbow trout is variable with area, habitat, life history and quality and quantity of food . As fry grow, they begin to develop "parr" marks or dark vertical bars on their sides . In this juvenile stage, immature trout are often called "parr" because of the marks . These small juvenile trout are sometimes called fingerlings because they are approximately the size of a human finger . In streams where rainbow trout are stocked for sport fishing but no natural reproduction occurs, some of the stocked trout may survive and grow or "carryover" for several seasons before they are caught or perish . </P>

How many species of rainbow trout are there