Recycling project to help transform NZ coasts

University of Auckland senior lecturer in marine science Dr Jenny Hillman is investigating whether artificial horse mussels made with shell waste can help bring back marine life to coastal waters.

Her team received almost $1 million in Smart Ideas funding from the Ministry of Business, Innovation and Employment for the research.

“Dredging by commercial fishing companies has damaged the sea floor so severely that little life is left on the seabed around much of New Zealand.

“We want to find the most environmentally sustainable way to restore New Zealand’s coasts, so we’re trying to upcycle shells from the mussel aquaculture industry that would otherwise go to landfill,” says Hillman, who works with the University’s Centre for Climate, Biodiversity and Society – Ngā Ara Whetū.

Dense horse mussel beds once carpeted the sea floor around New Zealand, but have been wiped out by commercial bottom trawling or smothered with sediment that has washed off the land, she says.

New Zealand horse mussels are now functionally extinct – meaning they no longer fulfil their role in the natural world.

Similar species around the world are also severely depleted.

The research will therefore be relevant nationally and internationally, she says.

“We will use the shell waste to make horse mussel mimics that will be placed on the sea floor to recreate the critical habitat for shellfish and other marine life that horse mussels once provided.

“Research has shown shellfish restoration can have massive benefits, bringing back higher numbers of fish, so that improves fisheries and offers benefits for tourism too,” says Hillman.

The research team includes University of Auckland environmental science Professor Nick Lewis, head of the Institute of Marine Science Professor Conrad Pilditch and senior lecturer in engineering Dr Enrique del Rey Castillo.

They expect the numbers of young fish, such as snapper, and shellfish, such as scallops, to increase, as these and other species find habitat and shelter from predators among the artificial horse mussels.

“The shell mimics will include natural shell and be mixed with a concrete that’s 100 percent non-toxic, so that should increase the chances of other species growing on and around the artificial shells,” says Hillman.

Horse mussels can grow up to 47 centimetres long and often stick out about 20 centimetres from the seabed.

They once stabilised the sea floor and enhanced nutrient processing and carbon sequestration, helping to attract other species and reducing the impacts of climate change.

Placing artificial horse mussels on the sea floor could later help scientists successfully transplant juvenile horse mussels raised in hatcheries into the coastal environment. Natural horse mussel beds might then have a chance to return.

Most of the research in New Zealand up until now has focussed on restoring coastal areas by transplanting green-lipped mussels grown in aquaculture farms into the natural environment.

“Growing horse mussels in hatcheries is a slow process, so we’re hoping the shell mimics might speed things up,” says Hillman.

The scientists will work closely with iwi on the project.

Mātauranga Māori will be used in selecting sites across the Hauraki Gulf, where artificial horse mussel beds will be created that mimic the scale and characteristics of the lost beds.

Remote underwater cameras, videos shot by divers, and samples will be used to gauge whether the artificial shellfish beds increase biodiversity, compared with areas of degraded seafloor.

“We’re interested in predator-prey dynamics, particularly whether juvenile scallops fare better around the shell mimics.

“We will also run experiments to see what influence the artificial beds have on nutrient processing, sediment movement and water flows,” says Hillman.

In New Zealand, about 35,000 tonnes of mussel shells are disposed of each year, at a cost of more than $5 million.

“Recycling some of that shell waste makes sense on so many levels,” says Hillman.