Using mussel to save the gulf

By Geoff Cumming

The declining health of the Hauraki Gulf is linked to the disappearance of mussel beds, which provide habitat for marine organisms and fish. Fifty years after dredging destroyed the reefs, a group plans to restore mussels to the seabed. Geoff Cumming reports

John Laurence (left) and Darren Parsons, who are leading the idea of restoring mussel beds, at Achilles Point, overlooking the Hauraki Gulf. Photo / Sarah Ivey
John Laurence (left) and Darren Parsons, who are leading the idea of restoring mussel beds, at Achilles Point, overlooking the Hauraki Gulf. Photo / Sarah Ivey

As most swimmers and any snorkeller could tell you, the shallow waters of the inner-Hauraki Gulf are not renowned for their clarity. Nor are there signs of abundant marine life. Nothing much grows on the murky, sediment-laden seafloor.

"Often when you dive, even quite well out into the Firth of Thames, you can't see more than a few centimetres in front of your mask," says Niwa scientist Darren Parsons.

It was not always so. The sheltered waters were once clearer and supported much more marine life thanks to mussel beds which carpeted swathes of the seafloor in the Rangitoto Channel, Tamaki Strait, Firth of Thames and along the Coromandel coast.

The self-replenishing mussel reefs were the engine room of one of the country's most productive shallow-water ecosystems. The mussels themselves supported a commercial dredging industry from the early 1900s. In 1961, 2800 tonnes were taken. But within eight years the industry had collapsed.

The reefs have never recovered, and the barren seafloor offers stark illustration of how quickly human greed and ignorance can destroy a resource. The greenlipped mussels we get from the supermarket grow on ropes beneath pontoons which effectively privatise surrounding waters and bays and raise visual pollution complaints.

Fifty years on, Parsons is one of a band of marine scientists and environmentalists hatching plans to improve water clarity and the abundance of marine life in the inner-gulf by restoring mussel reefs to the seafloor.

Their plan is not to compete with, or discourage, mussel farms but to improve the environmental health of the Gulf, an ecosystem reduced to perhaps 20 per cent of its natural abundance, the 2011 State of the Gulf report revealed.

Just how they'll manage to "re-seed" the seafloor with mussels is as unclear as the habitat has become thick with suspended sediments. But the vision is compelling.

The hard shell reefs of living and dead mussels provide attachment for sponges,seaweeds and tiny creatures which provide food and shelter for juvenile snapper and other finfish. Crayfish cluster around them during seasonal migrations. A study of remnant beds in the Firth of Thames found they had four times the abundance, seven times the biomass and 10 times more fish than the surrounding sediments. Productivity for invertebrates (including sponges, worms and shellfish) was the highest of any marine habitat recorded in New Zealand.

Extensive mussel beds would also filter vast amounts of water and improve overall water clarity in the Firth of Thames, which receives tonnes of sediments and chemical nutrients annually from the Hauraki Plains, the restoration group says in a submission to the Hauraki Gulf Forum. Mussels obtain food by filtering microscopic algae out of the water column, filtering out sediment and other particles at the same time.

A paper on the fishery's collapse by Niwa scientist Larry Paul says the loss of the mussel beds through dredging removed hard surfaces which many invertebrates settled on, impoverishing communities right up the food chain, including popular fish species.

Marine scientist Shane Kelly, lead author of the State of the Gulf report, says mussels were one of the major biomass elements in the gulf. "It's one of those things that gets lost in our memories - people don't even realise what's changed in terms of the marine environment."

Finding a successful way to re-seed the seafloor with mussels will be a trial and error process, says Parsons, who admits scientists are still grappling with why the beds have never recovered. In their early stages, planktonic larvae expelled from mussels drift with tides and currents, attaching to algae, seaweed and other marine organisms. As they grow into spat (juvenile mussels) they move on to firm surfaces - gravel or rock or the shell of adult mussels - to settle. It is thought they may be attracted back into established beds using chemical cues from adult mussels.

The dredging, which peaked in the 1950s and 60s, destroyed almost all of the substrate which juveniles settle on. Scientists also suspect increased sedimentation and nutrients have played a role in the failure of beds to recover, by making it more difficult for mussels to spawn and for larvae and juveniles to settle and survive.

"Mussels have a certain tolerance for sediment in the water," Parsons says. "But if they are using up too much energy filtering out sediment for food, this may affect spawning. And we don't know too much about the tolerance of larvae and spat for sediment and nutrients."

But research by Auckland University masters student Ian McLeod has raised hopes that re-establishment may be possible if juveniles can make it to the adult stage. McLeod's paper, published in Marine and Freshwater Research, found adult mussels can survive on the muddy seafloor, even if their growth rates are affected. The scientists now think factors affecting the early life stages of mussels may be more important in the reefs' failure to re-establish.

McLeod's paper notes that active restoration -such as replacement of shell or juveniles - has been successful overseas, for instance with oysters off the United States' east coast. It suggests the local mussel farming industry could help by offering shells and undersized mussels for trials to establish new settlement surfaces and seeding on the seafloor.

Parsons says spat could be obtained on seaweed from the same sources that supply mussel farms.

The project has the support of the Hauraki Gulf Forum and the group will seek to have restoration areas formally designated in the marine spatial plan being prepared for the gulf.

Co-ordinator John Laurence says the project will be long-term and will rely on local groups, iwi and corporate support. Potential trial sites have been identified around the Tamaki estuary, northeast Waiheke and the bottom end of Ponui Island, where sedimentation is not too bad. The group aims to establish three demonstration beds of about 100 sq m within five years and have 1 sq km of seabed restored within 15 years.

The ultimate goal is to have self-generating mussel beds - a critical mass of adult mussels producing enough offspring for the beds to expand.

The group's strategy is not dissimilar to how the reefs themselves grow - gradually replicating what is there.

"It will be bay by bay," says Kelly. "We are talking decades to get back to anywhere near what was in Tamaki Strait and Firth of Thames."

There is clear enthusiasm among conservationists - a call for expressions of interest in a restoration group at this year's gulf forum seminar attracted around 25 volunteers.

This summer, they are asking the public to help by reporting the location of remnant beds before trials to re-establish a small colony begin next winter.

"It's surprising what locals know about and keep to themselves," says Kelly. "If they are willing to share information it will be kept within the science people rather than widely shared."

Laurence says identifying existing beds will help scientists to build knowledge.

"We need people to be our eyes and ears and tell us where mussels are growing on the seafloor - not attached to rocks in the inter-tidal zone," says Laurence, a driving force in the volunteer-led restoration of Motuihe Island. "We would then hope to get others to set up groups around the gulf and do their own thing, using our technical support and assistance.

"If this project is successful there will be a huge amount of biomass in the water. Rather than being just a bare, silty seafloor it's going to be covered in mussels attracting a huge number of invertebrates and small fish, supporting more commercial and recreational species such as snapper."

Laurence says he is motivated not by fishing but the potential to enhance the gulf's international reputation as an ecological tourism gem.


To volunteer or report the location of seafloor mussel beds: Email musselreef@gmail.com

- NZ Herald

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