Toheroa thieves caught on video?

Oystercatchers on Ripiro Beach that some claim shows them feeding on adult toheroa.

Scientists scoffed when a trio of Ripiro Beach veterans blamed oystercatchers for the disappearance of millions of toheroa.

And the doubters are still dismissive despite seeing a video purporting to show the birds making a meal of the protected shellfish on the Kaipara west coast beach.

Des Subritzky, Joe Yakas and Robbie Sarich claimed three months ago that the birds got their beaks into the tube that Buried toheroa extend to the surface and sucked the mollusc from its shell.

Watch the footage below:

Mr Subritzky felt oystercatchers were such a threat to the unique shellfish that a bird cull could be needed. He got Northland MP Mike Sabin to talk to Conservation Minister Nick Smith, but the notion that oystercatchers were feasting on big toheroa made no traction with the minister.

In a letter to Mr Sabin on May 22, Dr Smith said DoC considered oystercatchers too small to devour mature toheroa.

"The real reason for the decline in toheroa is more likely to be a disease within the species' local environment and/or illegal harvesting by humans," the minister said.

The Northern Advocate sent a copy of the video showing oystercatchers feeding to DoC scientist Dr Hugh Robertson, who had been involved in advising Dr Smith about oystercatchers.

Dr Robertson said the birds in the video were South Island pied oystercatchers weighing around 550g, which tended to visit the North Island in autumn and winter.

"The oystercatchers in the video are not extracting adult toheroa," he said. "It is impossible to determine the species of shellfish being taken in the video. In addition to toheroa, there are probably tuatua and other bivalve molluscs of about that size found at Ripiro Beach."

If the prey were toheroa, they were juveniles less than a year old. When toheroa were legally harvested the minimum size limit - presumably "adult" toheroa - was 76 mm. The largest specimens were up to 150mm long, Dr Robertson said.

Toheroa were found about 300mm beneath the sand. The average length of the bill of a male oystercatcher was 81mm and female bills averaged 91mm.

Dr Robertson said oystercatchers did not have the musculature to "suck" up shellfish.

The video showed them opening bivalves by hammering through the shell or prising them open.

A 1999 survey found there were around 113.5 million toheroa on Ripiro Beach, 3.3 million of them longer than 75mm.

"If 1000 oystercatchers ate 50 toheroa a day for 200 days of the year they would consume 10 million toheroa annually.

"This would leave 100 million for other predators and there would still be a massive number of toheroa left to become the next lot of adults," Dr Robertson said.

He thought oystercatchers had been singled out as they were more visible than other species that ate toheroa, such as paddle crabs.