The whitebait fishery is under threat and more data is needed to help protect it, say three freshwater scientists.

LISTEN ABOVE: Premium Whitebait chief executive Jeremy Gardiner spoke with Kerre McIvor and Mark Dye

Dr Stéphane Boyer, senior lecturer of environmental and animal sciences at Unitec, said whitebait fishing practices in New Zealand were unsustainable.

"One of the big issues is that we don't know what species we catch."

Dr Boyer said whitebait comprised five different species of fish which looked similar when they were young. Three of the species were declining and one was threatened.

"Yet we fish, sell and eat them all without distinction.

"To ensure that fish stocks are renewed, first we need to know what species are being fished and where they occur. This way we could protect rivers and streams where endangered or declining species occur.

"We need reliable data about catch to monitor population size, but this will only be useful if the data is collected for each of the five species comprising whitebait."

One way to tell the species apart was by analysing their DNA, she said.

Dr Mike Hickford, research associate in marine ecology at the University of Canterbury, said the whitebait fishery had always been regionally patchy and had varied enormously from year to year.

"There are many people around the country that would tell you that 2016 was, in fact, their best season ever. It's also the case that whitebaiters have notoriously short and fluid memories.

"Many of the people who describe this year as yet another lacklustre year conveniently forget that last year (or the year before) they had a great whitebaiting season."

Dr Hickford said the main problem was the lack of comprehensive catch data to determine longer-term trends.

"We have no idea if fluctuations in the whitebait catch are even related to fish stocks, let alone whether the whitebait catch is in decline... I despair at the lack of data on the whitebait fishery."

He believed there would in future be a whitebaiting licence much like a Fish & Game Sports Fishing Licence. Whitebaiters would buy the licence for the number of days they wanted to fish and would have to nominate rivers and regions.

The nominal licence fee would support research and compliance efforts, but the major benefit would be data on how many people were fishing, when and where.

Dr Hickford said numerous environmental factors could affect the species. Introduced predators, including trout, had also had a huge impact on whitebait and adult populations.

Dr Mike Joy, senior lecturer in environmental science/ecology, Massey University said New Zealand would lose whitebaiting if it did nothing about habitat loss, pollution and commercial harvesting.

Lack of catch data meant no one knew whether current whitebaiting practices were sustainable, he said. However, the number of adult fish was declining and their distribution was reducing, according to 30,000 site records in the NZ freshwater fish database.

New Zealand probably had the highest proportion of threatened and at-risk freshwater fish species in the world, Dr Joy said. New Zealand's water and habitat was bad and still declining.

He blamed increases in nutrients and sediment, and physical impacts like dams, engineering (stop banking, flood control) and wetland drainage.

He said sea temperatures were rising and there was more acidification.

"It is a complex set of factors that determine the recruitment success of whitebait in any given year. But without detailed data [on] how these pressures interact with whitebait populations, we just can't be sure of long-term trends.

"What is obvious is the need to reduce pressure on the juvenile galaxiids (whitebait). We should give native fish the same protection as trout and, in the short term, ban commercial harvesting in order to achieve this."

- Westport News