8 Feb 2018

Safe houses for Kaikoura's baby paua

From Our Changing World, 9:07 pm on 8 February 2018

Pāua scientist Reyn Naylor, from NIWA, is testing small ‘safe houses’ for baby pāua, with the idea of boosting pāua stocks on the earthquake-damaged Kaikōura coast.

Reyn Naylor from NIWA and Tom McCowan from the Paua Industry Council with a pāua 'house' being trialled in Wellington and Kaikōura.

Reyn Naylor from NIWA and Tom McCowan from the Paua Industry Council with a pāua 'house' being trialled in Wellington and Kaikōura. Photo: Dave Allen / NIWA

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Two hatchery-reared baby pāua, identifiable by their distinctive blue colour, alongside a large wild-raised pāua.

Two hatchery-reared baby pāua, identifiable by their distinctive blue colour, alongside a large wild-raised pāua. Photo: RNZ / Alison Ballance

The pāua houses – or habitats as Reyn calls them - are essentially concrete blocks, with a concave underside, and a series of small entrances around the side. The structure sits on the sea floor, and the entrances are large enough for small pāua to come and go, but small enough to prevent starfish and other hungry predators from getting inside.

“They’re designed to mimic under boulder habitat, which is the habitat that juvenile pāua like,” says Reyn, “because when they’re young they don’t like the light and they live under rocks.”

Reyn originally designed the pāua habitats as part of a long-running research programme with the pāua fishing industry. He wanted to measure how many small pāua were around, and he needed structures that were all the same size, heavy enough to withstand waves yet light enough to be turned over for ease of counting, and that would attract pāua between 40-80mm long.

Reyn says he counts small pāua because in two to three years time they will be large enough to be recruited into the fishery, which means they will have reached the harvestable size of 125mm. A big cohort of juveniles means lots of adults to harvest, while a small cohort indicates a lean year coming up.

Upturned pāua 'houses', showing small entrances and plastic mesh to exclude hungry predators. The small blue-shelled pāua in the foreground have been reared in a hatchery, and the large pāua shells are to provide more shelter.

Upturned pāua 'houses', showing small entrances and plastic mesh to exclude hungry predators. The small blue-shelled pāua in the foreground have been reared in a hatchery, and the large pāua shells are to provide more shelter. Photo: RNZ / Alison Ballance

Kick starting Kaikōura’s pāua populations

It was only after the first habitats had been built and Reyn had seen that “the little guys like them”, that the idea of using them as safe houses for pāua on the Kaikōura coast came up.

The Kaikōura coast line was strongly affected by the destructive November 2016 earthquake, which saw a large part of the Kaikōura coastline uplifted by one to six metres. As a result, there was a significant loss of intertidal and shallow subtidal habitat which had previously been home to sizeable pāua populations.

Reyn and his colleagues in the pāua fishing industry thought that they could help kickstart the pāua population’s recovery by seeding areas with small pāua grown in a hatchery.

The most dangerous time for reseeded pāua is the first two to three days in the wild, so the thought was the ‘research houses’ could provide ‘emergency shelter’ for that vulnerable period.

Reyn is running two pilot schemes to test the idea. The first took place for two months on Wellington’s south coast in late 2017, and the second is taking place in Kaikōura.

Initial results from the Wellington trial suggest that most of the small pāua only stayed in the houses for a few days before moving out to hide amongst nearby boulders, and Reyn suspects that the houses don’t offer enough complex structure for the pāua to hide in.

But he says it “doesn’t matter if the pāua come out of these things – the main thing is that they survive.”

Reyn will also use the concrete shelters to monitor natural recruitment in the wild pāua populations around Kaikōura.

Reyn Naylor (left) and divers Peter Notman and Tom McCowan checking pāua 'houses' during a king low tide on Wellington's south coast.

Reyn Naylor (left) and divers Peter Notman and Tom McCowan checking pāua 'houses' during a king low tide on Wellington's south coast. Photo: RNZ / Alison Ballance

Interesting pāua facts

In most places in New Zealand, the legal minimum size for harvesting pāua is 125 mm.

However, the legal minimum size for harvesting pāua in Taranaki is just 85mm.

This is because pāua in Taranaki are much smaller than elsewhere, and reach sexual maturity when they are just 60mm long.

Around most of New Zealand about half of the pāua aren’t sexually mature until they reach 90mm long.

In Wellington, the Marlbrough Sounds and Stewart Island it takes pāua about 5 years to grow to 125mm, whereas around Banks Peninsula it takes about 8 years.

Small pāua grown in hatcheries have bright blue shells, as a result of the food they eat.

Diver Tom McCowan searching for young pāua under boulders on Wellington's south coast.

Diver Tom McCowan searching for young pāua under boulders on Wellington's south coast. Photo: RNZ / Alison Ballance

To find out more, check out this Our Changing World story about pāua research at Karitane, near Dunedin, this story about a pāua hatchery near Wellington, and how ocean acidification might affect pāua stocks.

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