Starfish and pa¯ua - maybe not
It seems like such a simple, useful idea. Put a starfish in your catch bag when you’ve got pa¯ua in it and it’s still underwater. The shellfish will then spawn, starting the cycle for the next generation of pa¯ua.
That’s the theory pa¯ua diver Phil Walker, who works around the top of the South Island, has been putting into practice for the past three years and he says its showing promising results. But others in the industry say it doesn’t work and may even be harmful to the pa¯ua.
Mike Radon has had a pa¯ua hatchery at the entrance to Tory Channel in Marlborough for more than 15 years and says he tried the idea six or seven years ago.
It could be difficult to get pa¯ua to spawn, so he tried introducing starfish, and the pa¯ua did give off a fluid that looked a bit like sperm, although nothing like eggs, he said.
The pa¯ua produced the same fluid when starfish were put into tanks with only male or only female shellfish.
‘‘When I looked under the microscope, there’s no sperm, there’s no eggs,’’ Radon said. ‘‘I think it’s a reaction pa¯ua have to starfish. It’s to confuse the starfish, I think.’’
The fluid looked like pa¯ua blood, and might have a similar role to octopus ink.
Starfish were one of the main predators of pa¯ua and could devastate populations of the shellfish. When a starfish touched a pa¯ua it could burn the shellfish somehow, Radon said. If there was enough contact the pa¯ua could die, although it could take a couple of months for that to happen.
There seemed to be quite a lot of enthusiasm on the internet for the idea of taking starfish to pa¯ua underwater following a story in the Express earlier this week, but he was concerned that could be detrimental to the shellfish.
The pa¯ua fishery around the top of the South Island - known as Pa¯ua 7 - is under major stress. In 2016 thenMinister for Primary Industries Nathan Guy slashed the total allowable commercial catch in the area in half to 93.62 tonnes for the 2016-17 fishing year. That followed four years during which the industry had already voluntarily cut the amount of pa¯ua being harvested.
The amount of pa¯ua in the area was well below the target level, and rebuilding the fishery was likely to take some time, Guy said. He acknowledged recreational take could be a drag on the rebuild of the fishery in the longer term, but did not expect it to pose a risk over the first three years of the reduced catch.
An ecosystem service review of pa¯ua in New Zealand commissioned by Aotearoa Fisheries said the shellfish had been over-exploited in many fisheries around the world, with the population declining massively.
‘‘Abalone’s susceptibility to overfishing can be attributed to its sedentary nature and tendency to be found in groups. The depletion of mature adults can result in low natural recruitment, which limits population recovery,’’ the review said.
‘‘While commercial fishing is the primary reason for the worldwide decline in abalone populations, the high-market value and relative accessibility to all fishers exposes them to significant illegal, unreported and unregulated fishing.’’
Having peaked at 2000 tonnes in 1981, the allowable commercial pa¯ua catch in New Zealand is now below 1000 tonnes, because of overfishing concerns. The Aotearoa Fisheries review said that while levels of commercial catch could be monitored there was uncertainty about levels of recreational and illegal catch.
Other problems that could affect pa¯ua population health were ocean warming and acidification as a result of climate change, sedimentation, and a loss of kelp beds, the review said. Juvenile abalone appeared to be particularly susceptible to changes in temperature and acidification. High rates of sedimentation in the wild have been linked to the death of adult pa¯ua.
Pa¯ua Industry Council scientist Tom McCowan said reseeding had been trialled in isolated areas in the Pa¯ua 7 area. The amount of pa¯ua in the trial areas had increased, and he was confident the reseeding was a factor in the improvement.
But he was not confident it was an ‘‘economically viable great idea’’.
It was more likely to be a good option in areas that were seriously struggling or, like Kaiko¯ura, where there had been mass mortality of juveniles, McCowan said. A process was now under way to set up a hatchery in Kaiko¯ura.
Reseeding was becoming more important, council chief executive Jeremy Cooper said.
‘‘If we had to we could produce millions of seeds the size of your thumbnail,’’ he said.
The question was who would pay for it. A funding application had been made for the Kaiko¯ura project, which could potentially results in millions of seeds being introduced to the earthquake-damaged area.