Rainbow trout (Oncorhynchus mykiss) are good to eat and a popular target for freshwater anglers.
They belong to the family Salmonidae, which originally comes from North America but has been introduced for aquaculture and recreational fishing to every continent except Antactica.
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.
In response, various groups of people, mainly volunteers, built a hatchery in Pemberton to breed trout to stock waterways.
In 1971, we assumed responsibility for the breeding and stocking program at the hatchery, which is now called the Pemberton Freshwater Research Centre (PFRC).
The species is also produced through aquaculture operations and has a farm gate value of more than $100,000 a year in WA.
The rainbow trout’s upper body is dark olive green to bluish – the sides are lighter 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.
Rainbow trout live for three to four years and can reach 5 kg. In nutrition-rich dams, trout can reach two kg in two years and three kg in three years. However, in nutrient-poor or overstocked dams, growth may be much slower.
Habitat
Rainbow trout is a coldwater 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 salt water for breeding but in Australia most are restricted to fresh water. However, trout that are acclimatised to sea water at a young age may be reared in marine conditions until maturity. This offers potential for stock enhancement and aquaculture in saline water in WA.
Lifecycle
To spawn, trout need a bed of gravel or small stones, with oxygenated sub-surface water filtering up through the rocky bed. These are usually found in mountainous upper reaches of rivers and tributaries.
However, most rivers and streams in south-west WA have sandy, silty bottoms unsuitable for digging a ‘redd’ (trough), in which trout lay their eggs. Even if the trout do attempt to mate, silt and lack of well-oxygenated waterflow combine to smother the eggs and larvae.
In suitable natural environments, the female trout digs a deep redd by lying on her side and flapping her tail to dislodge gravel. She releases eggs, which mix with milt (sperm) from a closely accompanying male and become lodged in the redd. The female then dislodges more gravel to fill in the redd.
After hatching, the larvae, called ‘alevins’, remain in the redd, absorbing the yolk sac. At this stage they look like tadpoles. Within two months, they become young fish (fry) and emerge to feed.
In WA rainbow trout are ready to breed at about two years of age and females can produce up to 2000 eggs per kilogram of body weight.
Trout are artificially spawned at the PFRC in June each year. Males and females are carefully removed from the broodstock pond and their abdomen gently squeezed to release eggs and milt. The eggs and milt are then combined, and after fertilisation, are washed and placed into incubators.
Once the alevins hatch they absorb their yolk sac and are then raised on a special diet.
Fry and yearling trout (eight to 12 months old) are stocked into south-west waterways each year.
Ex-broodstock trout are also stocked. While these larger fish are stocked for recreational angling, they also play an important role in controlling redfin perch, a feral species.
Diet
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 that multiplies in number, eats out the food supply and becomes stunted in size as a consequence. Trout fry and fingerlings (trout that have grown to the size of a person’s finger) eat small invertebrates.
Predators
Adult trout are the highest-order aquatic predators. However, redfin perch prey heavily on trout fry. Cormorants and other fish-eating birds can prey on young trout and even adult trout if the waterway is bare of cover such as aquatic weeds, logs and rocks the fish can hide under.
Illustration © R. Swainston/www.anima.net.au