Whitebait live a perilous life well before meeting the nets.

It was just a shadow passing across the blurred white line under the water. Then another thin shadow appeared, and another, then a mass of tiny, wriggling fish.

The man grasped the long handle of his net and steadily scooped up the small school of fish before swinging the net on to the mud and grass, shaking the mesh to ensure none of the tiny fish stuck to the steep sides. There, at the bottom of the long net, was a dense greenish mass of shimmering whitebait.

He always felt a thrill when he saw the catch secure in the net.

He knew how a prospector must feel when he struck gold after years of searching - to him, the whitebait were like liquid gold.

This weekend, lovers of the delicate fish will sit on riverbanks throughout the country, eyes glued to the water, hoping to spot the moving shadows.

The whitebait season opened two weeks ago and runs until the end of November.

The exception is on the West Coast of the South Island, where the season starts on Wednesday and closes on November 14. This is to allow enough of the migrating fish to pass upstream to replenish the population, for the West Coast is the remaining bastion of large runs of whitebait.

There are rules covering the length of nets and how much of a river's flow may be fished from 5am and 8pm (6am to 9pm under daylight saving). Theories abound as to the best tides and moon phases for sparking runs of bait, but all agree that the days when catches filled kerosene tins and unwanted fish were buried in gardens as fertiliser are long gone.

The precious little fish now fetch close to $200 a kilo in Auckland and, like Bluff oysters, the first fresh bait of the season are eagerly awaited.

Clues as to their origins can be seen on rural road signs marking streams, and the name Kaikokopu Stream can be found in the Bay of Plenty. Kai suggests food and kokopu is the name for the adult whitebait, which belong to the galaxaiid family of freshwater fishes found throughout New Zealand and Australia.

The adults are secretive creatures, living in dark, bush-clad pools. Four members of the family make up the most highly sought whitebait - banded kokopu, giant kokopu, dwarf kokopu and short-jawed kokopu.

A cousin, the koaro, is also involved, as is the inanga. Kokopu require clean mountain streams and do not adapt readily to habitat changes, so have disappeared from much of the country.

The banded kokopu can still be found in many streams and lakes in the North Island. The hardy inanga can tolerate water which carries sediment and effluent, and 80 per cent of the whitebait caught in rivers which run through farming country, such as the Waikato River, are inanga. In autumn, the adults migrate downstream to rivermouths and lay their eggs on reeds and grasses covered by the highest tides.

They remain exposed until a month later when the next highest tide covers the eggs, which then hatch and the millions of tiny larval fish are swept out to sea.

It is a hazardous existence and many fall prey to predators. The survivors grow in the plankton-rich environment and in the spring migrate back up the rivers and streams as whitebait.

The strong currents in the middle of the rivers force them close to the banks, where anglers await with mesh traps and long-handled nets.

But it is not just predators and fishing causing whitebait runs to diminish. Grazing cows destroy the eggs, floods can sweep the fish out to sea, and as trees are stripped from the hills the once-strong banks of the dark pools scour and muddy.

Whitebaiters still boast of record catches occasionally, but most are content to turn their catch into fritters patterned with individual white fish and dark eyes.