Commercial long-line fishers appear to be breaking the law and failing to report bycatch, according to a Fisheries NZ report.
Vessels catching migratory species like tuna were nine times more likely to report non-fish bycatch when there were independent observers onboard, suggesting "a level of underreporting for non-observed trips".
Over the 2017/2018 period, 114 seabirds were reported caught up in the fishery for highly migratory species, including 10 that survived.
This was more than double the previous year, when 51 seabirds were caught, but less than the 131 in 2015/2016.
It is not an offence to kill seabirds while fishing but it is an offence to fail to report catching them.
Similar differences in reporting have been seen over the last four fishing years, and Forest & Bird fisheries adviser Geoff Keey said it showed vast numbers of seabirds and other animals were dying without ever being reported.
"It's simply unbelievable that a commercial fishing trip is nine times more likely to catch seabirds when there's an observer on board," Keey said.
"The only reasonable conclusion is that on trips without observers fishers are breaching their legal obligation to honestly and accurately report bycatch."
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Long-line fishing vessels catch fish by rolling out lines of the back of the boat that can be kilometres long with thousands of hooks. They are used to catch highly migratory fish, like tuna and swordfish.
A range of seabirds including critically-endangered seabirds have been caught, including Antipodean and Gibson albatrosses, and occasionally Northern Royal albatrosses, the albatrosses people see at the Dunedin albatross colony.
The report follows Department of Conservation research last year that found the Antipodean albatross population was decreasing at an alarming rate, with evidence some were being killed by long-line fishing vessels.
The albatross population had been declining rapidly since 2004 and the number of females had halved.
The DoC research showed that of 16 female Antipodean albatrosses being monitored with tracking devices in 2019, two appeared to have been caught and killed by longline fishing vessels.
Five Antipodean albatross killed by a New Zealand long-line fishing boat in the Bay of Plenty region were just one of 30 species of seabird killed in New Zealand fisheries last year.
Keey said there had been similar past misreporting in fisheries that caught penguins, and industry logbooks were so unreliable the Government had to independently estimate the numbers of seabirds, marine mammals and turtles that were caught by the fishing industry every year.
"The fishers who misreport bycatch are destroying the credibility of the entire industry. It appears widespread, it's been going on for years and this is the year it has to stop."
The organisation has declared 2020 the year of the seabird, and is calling for better fishing rules in a National Plan of Action for Seabirds which is currently in its draft stage, with submissions closing on January 27.
"Forest & Bird are asking for a zero bycatch goal, cameras on commercial boats, binding actions and rules and an end to set netting in the habitat of threatened species," Keey said.
Seafood New Zealand chief executive Tim Pankhurst said any seabird caught was "regrettable".
Overall, seabird bycatch was falling and the industry was looking at new ways to deter birds from vessels, he said.
They supported seabird risk management plans for vessels, and many vessels already voluntarily had onboard cameras, particularly in high-risk areas such as the black petrel habitat around the Hauraki Gulf.
He did not answer specific questions about bycatch in the long-line industry, nor about suggestions for cameras and observers to be on all vessels.
A spokeswoman for Fisheries Minister Stuart Nash said the Government was rolling out various digital technologies for the tracking, reporting, and monitoring of commercial fishing, which included onboard cameras.
From November fishing vessels at the greatest risk of encountering critically endangered Māui dolphins were required to operate with onboard camers.
No decisions have been made about any wider rollout of onboard cameras.