Fishing: Circle of life as trout spawning runs get under way

NZ Herald

16 Jun, 2017 05:00 PM5 mins to read

A pair of rainbow trout spawn in a stream at Lake Tarawera. Photo / Geoff Thomas

A pair of rainbow trout spawn in a stream at Lake Tarawera.

Picture / Geoff Thomas

Rivers and streams running into lakes around the country will be holding trout much larger than the usual resident population for the next few months. For the annual spawning runs have started, following heavy rainfall and a drop in temperature which trigger the runs.

The trout will be 3-year-old fish which have matured sexually, and follow the inbuilt instinct which overcomes their natural fear of shallow water as they leave the security of the lake and wriggle frantically over the sandy lip at a stream mouth.

This activity always takes place at night, when the fish feel more secure.

In New Zealand our trout populations are almost entirely rainbow or brown trout, with isolated pockets of American brook trout and lake trout.

The most famous runs of spawning trout which provide world-class fly fishing are in the Rotorua and Taupo lakes, and every night fly fishermen will be lining up at stream mouths as they cast their flies out to where the trout are circling close to shore.

The fish are not feeding, having built up fat over summer and autumn to carry them through the rigours of spawning. But they will occasionally snap at a fly, and this is what the anglers are waiting for. Or, anglers will be lined up along the banks of rivers like the Tongariro and Waitahanui Rivers at Taupo, and the Ngongotaha Stream in Rotorua.

For the trout it is a hazardous experience as trout are far more vulnerable to predators in a shallow stream, but unlike salmon they do not die but return to the lake, exhausted and in poor condition. Some will regain body fat and spawn for a second time, as rainbows can live to five years and brown trout to eight years, which is why they are generally larger.

For the hatchlings life is very dangerous, and few survive to make it to the lake where they grow quickly. A female rainbow trout may carry 3000 eggs, or ova, with larger specimens producing more.

But of every thousand eggs laid in the gravel bed of a stream an average of only two will return as mature adults to repeat the process three years later. As they grow the baby trout, first called fry, then fingerlings, are eaten by birds like shags and by eels and other fish.

All of the trout and salmon in this country were introduced, mainly from North America in the late 1880s. For there are no members of the salmonid family native to the Southern Hemisphere.

The salmon, which enter the rivers of Alaska and Canada in vast numbers, have a different life story. They, too, must run a gauntlet of predators which range from grizzly bears to wolves, seagulls and eagles. In fact, the great salmon runs in the cold northern countries from Scandinavia to Russia to North America are a vital element in the ecosystem, providing sustenance to all of the animals and birds which rely on the fish to build up reserves of fat to carry them through the harsh winters.

And the adult salmon do it only once. Their life story is a fascinating one. The baby salmon migrate downstream over their first year of life and enter the sea as smolts where they live for two years, growing quickly on rich food like herrings and krill.

Then, like the trout, they mature sexually and while trout will return to the stream in which they were hatched which is a short distance from the lake, salmon will travel thousands of kilometres as they work their way up rivers and into tributaries � finally arriving to within three metres of where they were hatched three years earlier.

How they achieve this is one of the great mysteries of nature, and scientists can not decide whether it is driven by the influence of the moon, the earth's electromagnetic fields or water chemistry or smell.

The fish pair up with males fighting for a spot beside a female and the larger males prevail, with only the strongest individuals breeding.

But, unlike streams on our lakes, the water in arctic streams is so cold it is virtually sterile in terms of food generation, and this is where the adult salmon make the ultimate sacrifice.

After the females deposit their eggs in a shallow depression in the gravel which are fertilised by the males, the adults weaken and die. And it is their carcasses breaking down which provide the nutrients as the basis of the food chain for the hatchlings, so the whole process can start again.

In this country, trout spawn between late April and August, peaking in June and July. Most spawning streams and the headwaters of larger rivers are closed to fishing to protect the fish and allow them to reproduce, and so sustain the population in the lakes.

Salmon are found mainly in the large Canterbury rivers, and they spawn in the autumn but the runs start in November and carry on through the summer.

This is a similar pattern to the different species of salmon in the Northern Hemisphere, which start running up the rivers in mid-summer. This allows the predators which rely on the annual arrival of their main food supply to feed actively before the cold temperatures of autumn arrive.