Rainbow trout (Oncorhynchus mykiss) were first introduced to New Zealand in the early 1880s. They are descended mainly from Californian steelheads – rainbow trout that migrate to sea and spend most of their lives there. However, New Zealand rainbow trout do not migrate to sea.
‘It was a whopper’
Anglers have long been notorious for their exaggerations. In 1888 a writer noted: ‘There are several kinds of trout liars. The liar of weights, who never catches more than half a dozen trout a day, but they can weigh anywhere from 8 lb to 10 lb. Then there is the liar of numbers, who always catches so many dozens in an hour and 28 minutes. And there is the liar of places, who knows hidden pools, dark and still, in the secret places of the rocks that are just boiling over with trout … and you fish in them for eight mortal hours without a nibble.’ 1
Rainbow trout are less widespread than brown trout. There has been virtually no natural dispersal.
Ova imported in 1883 by the Auckland Acclimatisation Society survived, and fry were released into local waterways. In 1892 the species was liberated in Lake Rotorua, and in 1897 in Lake Taupō.
Rainbow trout can tolerate higher water temperatures than brown trout: they are found in warmer waters such as the Kai Iwi lakes in Northland. They also occur in a few rivers.
Rivers: limited success
Stocks were also released into many rivers, but they mostly disappeared. Nearly all the rivers where they did establish themselves flow into large lakes – in the central North Island, and the southern South Island.
It is not understood why rainbows have failed to become established in many waterways. Adding to the mystery is their presence in a few rivers that do not drain into lakes, such as the Pelorus and Rai in the South Island, and the Mōhaka in the North Island. These have very stable riverbeds, which may be a factor.
Soon after they were brought to New Zealand, rainbow trout grew very large in Lake Taupō, but then declined in size. The average weight of trout in one angler’s bag was 10½ pounds (4.8 kilograms) in 1911, but by 1918 it had dwindled to 4 pounds (1.8 kilograms). The early bonanza was probably due to the trout feeding on the then plentiful native fish.
Rainbow trout eat more and grow faster than brown trout. In rivers they feed in fast water, using energy to stay in the same place while feeding on drifting invertebrates. In lakes, rainbows tend to live in deeper water than brown trout and often feed on different prey – usually small fish, such as smelt in Lake Taupō.
This may explain why rainbow trout are bold feeders, more easily caught than the wary brown trout.
Rainbow trout may reach 750 millimetres and more than 10 kilograms in New Zealand. Fish of 600 millimetres and 2–3 kilograms are often caught, and fish weighing 4–5 kg are not uncommon.
Most rainbows tend to live for four or five years, although individuals up to 11 years old have been recorded.
- Several hundred to several thousand eggs are laid in a small hole by the female and fertilised by the male.
- After 1–3 months the eggs hatch into alevins (fry with yolk sacs attached). These live in the gravel, feeding from their yolk sac.
- They then emerge as fry, about 25 millimetres long. By late summer they have reached 50–70 millimetres.
- As juveniles and spawning adults they live in streams, where they are exposed to predators on the banks.
- Adults usually run upstream from a lake to spawn in late winter and early spring, in headwater streams with gravel beds. Not all rainbow trout survive spawning.
Much of the central North Island winter fishing is centred on rainbow trout running upstream from Lake Taupō to spawn in tributaries such as the famed Tongariro River.