Fishing: Birth of really big ones

Yearling trout from the Ngongotaha hatchery are released into one of the Rotorua lakes. Photo / Duncan Brown

Bellbird song echoes through the bush as you take the track from the carpark at the Landing, where the road meets Lake Tarawera, a 20-minute drive from Rotorua.

It is only a few minutes' easy walk, and the path is well trodden by the heavy boots of anglers wearing body waders, their long fly rods trailing behind so they don't become snagged in the low branches.

In April, May and June there will be a line of half a dozen such anglers, standing knee-deep across the Te Wairoa Stream as it tumbles into the green lake water, the current slipping down over the shelf of sand and gravel, which has been carried downstream from the hills.

The stream is a small but important one. It is born where the waters of Lake Rotokakahi, also called the Green Lake, flow out in a corner of the lake, rushing through a culvert under the road and then meandering along by the road until it reaches the Buried Village, where tourists marvel at the huge trout lying in the clear water. Then it tumbles over a waterfall before the final, short journey down to Tarawera.

What makes this stream so important is its gravel, the nuptial bed for the large rainbow trout that live in the Green Lake. Every winter they enter the stream, swim downstream and then turn around and lie together as they go through the ancient spawning ritual.

The baby trout born here will swim upstream to the lake, where they will grow and repeat the cycle. Because it is such a small, short stream there are not many trout in the lake, and the abundance of rich food sees them grow to huge proportions. But fishing in this lake is restricted to the few owners who live around Whakarewarewa, for it is one of two Maori-owned private lakes - the other is Lake Rotoaira, near Turangi.

Some tiny trout are probably washed downstream and finish up in Tarawera.

But the same cycle happens in the lower part of the stream, and large trout run up from Tarawera during the winter as far as the waterfall, which is a natural barrier. These fish are intercepted at a trap a few metres from the stream mouth.

Every day Fish and Game officers check the trap, measure the trout and then release them above the trap to continue their spawning journey.

There is another reason the trap and its big fish are so important to the whole Rotorua fishery, for the officers will select big, healthy males and females and squeeze the ova and milt from their fat bellies. The fertilised ova are then taken to the trout hatchery at Ngongotaha where a new generation of trout will be born.

This process is the key to the success of the trout fishery in the district, which is famous throughout the world. Of the ova laid in the gravel beds of the Te Wairoa and other streams, only about two in every thousand will survive to return as adults three years later.

But the eggs which are hatched in tanks in the controlled, oxygen-rich water of the stream which runs through the hatchery will have a much better survival rate.

The tiny hatchlings, called alevins, live off the nutrients in the egg sac for the first two months, then grow quickly on a diet of salmon protein. In a year, at about 20cm long, they are liberated in one of the fishing lakes in the district.

About 80,000 such fingerlings are put into these lakes, while another 40,000 or so are taken as far north as Dargaville and south to Wellington where other Fish and Game officers stock their own trout waters.

The big Rotorua lakes - Tarawera, Rotoiti and Okataina - have limited spawning streams and so their populations can be controlled through liberations in autumn and in spring. The yearling's fins are clipped diferently so that when caught they can be identified and their growth rates monitored.

They have survived the perilous first year in the wild and will grow into adults in two years. These are the trout which anglers look forward to catching at the season opening on October 1, when most will be 2-year-olds and average 2.2kg. By the following winter they will have reached 3.5kg or more, with some tipping the scales at 4.5kg.

Lake Taupo, with 800km of prime spawning beds, regenerates naturally but the trout grow to only half the size of their Rotorua cousins in the same period.