RAINBOW TROUT
Oncorhynchus mykiss

Rainbow trout are a cool water species inhabiting freshwater creeks, dams, rivers and lakes. It thrives at 5-20ºC but there are mortalities once water temperature increases to 26-27ºC. However, the trout produced at PFRC are unique because they have adapted to withstand the higher temperatures of local conditions. Trout are also intolerant of low oxygen levels caused by heat and stagnation.

In the northern hemisphere some trout migrate between fresh water and saltwater for breeding but in Australia most are restricted to fresh water.

Rainbow trout were introduced to Western Australia in 1927 to provide recreational fishing in the south-west part of the State. They were originally released into streams between Albany and Gingin, but in most cases failed to establish self-sustaining populations due to the lack of suitable conditions.

The rainbow trout’s upper body is dark olive green to bluish – the sides are lighter in complexion and the belly is silver-white. The head and body are heavily speckled. There are often pink, red or orange markings along the head and flanks. Rainbows tend to have a more compressed body, when compared to browns. River fish and those on spawning migrations, tend to display more intense colouration.

Rainbow trout live for three to four years and can reach 5 kg. In nutrient-rich dams, trout can reach 2 kg in two years and 3 kg in three years. However, in nutrient-poor or overstocked dams, growth may be much slower, due to lack of space and resources.

Adult and yearling trout eat invertebrates (both terrestrial and aquatic), including insects, beetles and nymphs. They also eat small fish including redfin perch, an introduced species. Trout fry and fingerlings (trout that have grown to the size of a person’s finger) eat small invertebrates.

In order to spawn, trout need a bed of gravel or small stones, with well oxygenated water filtering up through the rocky bed. These features are usually found in mountainous upper reaches of rivers.

However, in south-west WA most rivers and streams have sandy, silty bottoms which are unsuitable for digging a through in which rainbows lay their eggs. Even when they do attempt to spawn, silt and a lack of well-oxygenated waterflow combine to smother the eggs.

Trout are artificially spawned at the Pemberton hatchery in June each year. Males and females are carefully removed from the broodstock pond and their abdomen is gently squeezed to release eggs and milt. These are then combined and after fertilisation, are washed and placed into incubators. Once the trout hatch, they absorb their yolk sac and are raised on a special diet.