A medium sized and laterally-compressed fish with a slightly forked tail. Maximum weight 10 kg; usually 1–4 kg. Rainbow trout are very similar in body shape to Brown trout, but usually have a prominent pink stripe down the sides, fine black spots on the body and tail (Brown trout don’t have spots on the tail), and the adipose fin is opaque with a black border. Rainbow trout also lack the reddish-orange spots on the sides found in Brown Trout. The back is usually greenish or steely-blue and the belly white.
Some individuals from lakes are silvery and lack the pink stripe. Juveniles can have prominent dark blotches on the sides (as can Brown trout), called ‘parr marks’, but these disappear as the fish get larger.
The Rainbow trout prefers cool, upland streams and lakes. It has a slightly higher thermal tolerance than Brown trout and water temperatures above about 27°C cause high mortality. Other aspects of the species’ ecology are very similar to Brown trout, with the diet containing freshwater insect larvae, crustaceans, snails, small fish and wind-blown terrestrial insects. There is a tendency for Rainbow trout to feed at the water surface more than Brown trout. Rainbow trout become increasingly piscivorous as they grow, with the diet of Rainbow trout >300 mm in the Canberra region having increasing proportions of fish.
Individuals mature at 2–3 years of age and spawn from July to October. The female constructs a nest (‘redd’) in gravel, where the slightly adhesive, demersal eggs are deposited. The eggs are large (4–5 mm diameter) and hatch in 3–12 weeks, depending on water temperature. Rainbow trout are popular targets for recreational angling, although anglers seem to prefer the more elusive Brown trout. Rainbow trout (and Brown trout) are often found with the parasitic copepod Lernaea cyprinacea (anchor worm) attached, particularly around the fins.
During times of heat stress, the species is prone to heavy infestation from this parasite, with large red sores from secondary infection obvious.
As with Brown trout, Rainbow trout have had a serious impact on the distribution and abundance of the native galaxiid species in south-eastern Australia, with many threatened galaxias species now confined to small sections of headwater streams above barriers to trout invasion. Nine of 14 newly described galaxiid species have been recently listed as critically endangered, with all of these species threatened by trout.
Significantly, it is highly likely that trout have caused the extinction of several additional galaxias species before they could be discovered. In the Basin trout have seriously reduced the abundance and distribution of many galaxiids including Mountain galaxias, the threatened Barred galaxias and Stocky galaxias. Rainbow (and Brown trout) prey on other threatened native fish species such as Two-spined blackfish and Southern pygmy perch and are suspected of having deleterious impacts on Trout cod and Macquarie perch.
Trout species also impact on a number of threatened frog species such as the Spotted tree frog (Litoria spenceri) and are prolific predators of spiny crayfish (Euastacus spp.) in the uplands. The subtle impacts of trout on native species have not been well studied in Australia, but international research has demonstrated impacts on native fish mesohabitat use, foraging success, social behaviour, and spatial organisation. Trout are known to affect habitat use by Mountain galaxias.
There is a misconception that Rainbow trout have less impact on native fish, with some outdated fisheries policies therefore allowing stocking of Rainbow but not Brown trout. Both are listed in the 100 world's worst invasive alien species by the IUCN. Brown trout are probably more piscivorous than Rainbows, but both trout species prey on and/or have significant dietary overlap with small native fish and crayfish (individuals or species).
Rainbow trout are native to the western coastal drainages of North America and were first introduced to Australia in 1894, as fertilised ova from New Zealand where the species had been introduced from California. After rearing they were released into the upper Murrumbidgee River in NSW. Further introductions from New Zealand were to Vic (1898) and Tas (1897 or 1898) with subsequent introductions to Qld (1896), SA (1903) and WA (1930). The species is usually found throughout the montane catchments in which it occurs and can occupy even the smallest headwater streams.
Rainbow trout are widely distributed in the cooler upland streams of the Basin in Vic, NSW and the ACT, as well as a small number of streams in SA and the Condamine-Balonne in Qld. Fisheries agencies in the Basin have substantial stocking programs for this and other trout species, as they are a valued recreational angling target.
During the Millennium Drought trout abundance declined significantly as small streams dried and widespread bushfires affected many catchments. Following the breaking of the drought in 2010, the distribution of Rainbow trout again expanded to fill their previous range. Vic and NSW fisheries agencies release approximately 2.5 million Rainbow trout each year; private hatcheries also make releases. Rainbow trout are more ‘catchable’ than Brown trout, making them popular in stocking programs. A total of 2326 individuals were collected across 13 river valleys in the Sustainable Rivers Audit (2004–2013), with 59% and 36% caught in upland and montane elevational zones respectively. 981 individuals have been collected in the MDB Fish Survey (2014/15–2021/22) across 11 river valleys.
Cadwallader 1996; Clements 1988; Davies & McDowall 1996; Ebner et al. 2007a; Gillespie 2001; Healey et al. 2020; Jackson et al. 2004; Jackson & Williams 1980; Jarvis et al. 2019; Lintermans 2000, 2013b; Lintermans et al. 2020; Lowe et al. 2000; McDowall 2003, 2006; Raadik 2014; Tilzey 1976.
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