As whitebaiters dust off their nets for the start of the season, research is under way to map out "love zones" to see where the species are spawning.
The Bay of Plenty Regional Council's freshwater ecologist, Alastair Suren, had been working on the research to gather accurate information to identify what and where other tactics could make the most difference for whitebait breeding success in the future.
"We've spent more than 100 hours in a small tin boat since February, mapping and assessing the current state of whitebait spawning areas on 23 Bay of Plenty rivers," Suren said.
He said changes in water quality and the loss of suitable spawning habitats played a large part in the decline of whitebait species over time.
Suren said riparian restoration and protection of spawning areas could help turn the tide on that decline.
"Regional council staff already work closely with landowners to encourage protection of whitebait habitat, as part of our Riparian and Biodiversity Management Programmes," he said.
The Department of Conservation administered the rules for the whitebait fishing season which opened from August 15 to November 30 in the Bay of Plenty.
Suren said a lot of people did not realise the whitebait they catch were generally a mix of five different species of native fish that spent most of their lives in freshwater rivers and streams.
"One, the shortjaw kōkopu, is classified as a threatened species, while the giant kōkopu, inanga and koaro are classified as at risk and in decline.
"Inanga are the most common whitebait species, and they don't climb as well as the others," Suren said.
He encouraged whitebaiters to let the fish that try to climb out of their buckets go.
"That way they're helping the most threatened species to survive."
Suren said one mature female inanga could lay thousands of eggs at a time.
"They lay on the highest tides in autumn, placing their eggs in well-vegetated areas along the edges of estuaries and stream mouths in the 'love zone' – the area where seawater and freshwater meet," Suren said.
The larvae then hatch and wash out to sea a month later, on the next spring tide event.
"Those that survive at sea will return as juvenile fish to run the gauntlet of whitebait nets and natural predators in spring. Most whitebait species then work their way upstream and grow to become breeding adults, except for inanga which stay near the coast."
Suren said whitebait breeding potential was "huge", but the species was incredibly picky about where they spawned.
"The bank angle, salinity levels and vegetation cover needs to be just right, and the spawning area needs to be protected from trampling or other disturbance for the eggs to be able to survive," he said.
"The exact location of spawning sites varies with different tide and river levels."
Suren had surveyed each rivermouth on multiple high tides, including early mornings and weekends, especially over Easter, to be able to fully map the extent of spawning areas.
The survey included collection of salinity measurements and observations of streambank angle, vegetation type and other factors that affect spawning suitability.
"We have to work around the tides, with just a few hours to gather as much data as we can during each site visit," Suren said.
Suren would be writing up his findings in the coming months and expected a final report to be available by the end of the year.
For more information visit- www.boprc.govt.nz/freshwaterfutures.