Baby mussels in Hauraki Gulf no longer growing together in large numbers, but researchers may have solution

Efforts to restore mussel beds in the Hauraki Gulf have hit a snag - baby mussels are no longer growing together in large numbers. 

But researchers believe there's a solution and the answer may lie in another humble marine organism - seaweed. 

Searching for seaweed in the shallows off Mathesons Bay north of Auckland. It's a watery quest with a purpose - to find out what types of seaweed baby mussels, or spat, are attracted to.

"It's one of the issues. We can get adults to survive on the seafloor. But if we can't get the baby mussels coming in, recruiting and replacing the adults, that becomes an issue for restoration," said Dr Mallory Sea from Kelly Tarlton’s Marine Wildlife Trust.

A red spongy seaweed could unlock secrets about what baby mussels prefer when they settle and start growing

The theory is the compact, grainy arms of certain seaweed offer baby mussels protection. 

"We don't know which ones they like the best or which ones they're going to survive the best on. So that's kind of the point - if we can find out what they like then we can target some of those seaweed species for restoration," Dr Sea said.

Mussels are a bit like ecosystem engineers. They provide food for species like snapper, remove nitrogen and improve water clarity. 

"Really good filter feeders and we say they can filter about a bathtub full of water every single day," Dr Sea said.

In the 1960s, mussel beds, which carpeted the Hauraki Gulf, were dredged to oblivion. 

In 2021 to rebuild the population, iwi, scientists and conservationists dropped 60 tonnes of adult mussels off Okahu Bay.

A year on, they're healthy. 

"By putting mussels in, we actually create this whole new world that's not just mussels by themselves. It then creates habitat, and structure and complexity on the seafloor and food as well from the biodeposits that they produce for many, many other animals," Auckland University marine science lecturer Dr Jenny Hillman said.

But it's the babies that need more attention, which brings us to a lab at Kelly Tarltons, where microscopic spat is introduced to different seaweed-filled tanks. 

"In a few days to a week, we can then pull the seaweeds out. What we then do is wash the seaweeds off and everything we wash off we can then look at under a microscope and then we can count them," Dr Hillman said.

The preferred seaweed could then be transplanted to sites like Okahu Bay.

With sediment choking the seafloor, this type of science is seen as critical.

"I think what's really clear is that time alone is not going to solve the problems of the Hauraki Gulf. We've passed that tipping point," Kelly Tarlton's Marine Wildlife Trust trustee Craig Thorburn said.

"Projects like this help create tools and techniques that can headstart a programme."

And ultimately improve the health of a treasured moana. 

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