RESEARCHERS say that unless action is taken mussels will disappear completely from Ohiwa Harbour.
A triple-whammy of starfish, silt and pollution has reduced mussel numbers from 112 million in 2007 down to just over half a million in 2016.
Research diver Joe Burke and his wife, Kura Paul-Burke, have been involved in surveying Ohiwa Harbour for almost 10 years and say the situation is critical.
“For the first time ever, we’ve had to wipe the silt off the mussel shells to see what they were,” Dr Paul-Burke said.
Mr Burke said a concentrated community effort was required to stop the mussels from disappearing completely.
“This is where the rubber meets the road,” he said.
“It will go badly unless we act.”
With both the green-lipped mussels and the starfish being native to New Zealand, the mussels appear to have survived the threat for a long time, but nutrients coming off the land and flowing into the harbour appears to have upset the balance.
“A total of 15 starfish per hectare is considered sustainable,” Mr Burke said.
“While diving we have seen five starfish per square metre, meaning a total of 50,000 starfish per hectare.”
In 2009, when the researchers assessed there were 16 million mussels and 1.2 million starfish in the western part of the harbour, the starfish were observed marching.
“It was a like a herd moving across, and any starfish that were by themselves would get enlisted as the march over the mussel bed progressed,” Mr Burke said.
Both mussels and starfish were abundant during the 2009 observation.
In 2013, the numbers had dropped to two million mussels and 98,000 starfish.
“It’s like the mussels then re-start, being smaller and fewer, with the starfish also being smaller and fewer.”
At the latest count, in 2016, there were 543,000 mussels in the entire harbour and the starfish count was down to 4700.
Starfish could multiply by fission, splitting off, as well as by “having babies” sexually.
Mr Burke said that in Australia, starfish were found to have only a one-in-a-million chance of survival “at birth,” but that run-off from farms had allowed them to live and grow, generating huge numbers.
“Overpopulation of starfish is happening globally,” he said.
“It could be due to farming.”
Concerns about the situation were raised as far back as 1965, when the Opotiki News published a map of the harbour for residents to indicate where they gathered seafood.
The then-Opotiki County Council was disputing collection areas outlined by the Pollution Advisory Council.
Dr Paul-Burke said the resulting, revised maps from 1965 fitted well with what the marine researchers had learned when they interviewed kaumatua about the locations of the traditional mussel beds.
The eastern side of the harbour was surveyed for the first time in 2016, when Mr Burke swam the entire eastern side.
“There were masses of dead shells where the beds used to be,” Mr Burke said.
“Only one small bank remains. We also found lots of silt.”
The remaining bank was widespread and sparse, and estimated to hold 59,000 mussels.
What remained of the once-large mussel beds were now small areas near the harbour mouth where the water was cleaner.
Mr Burke said the silt was smothering the mussels.
“There’s more sediment on the western side,” Mr Burke said.
“It makes sense, because it gets flushed less.”
WHAT CAN BE DONE
MARINE scientist Kura Paul-Burke said the best option was likely to be working with what was already in existence.
“We find mussels growing on floats in the harbour, because there the starfish can’t get to them,” she said.
“We want to run spat lines on floats and grow mussels that we can use to build up the mussel beds.”
Retaining and expanding existing beds was more likely to be successful than creating new beds.
In other parts of the world, millions of mussels had been deposited in places in an attempt to start a mussel bed.
These attempts had not been very successful, however.
Dr Paul-Burke said moving old shells so the new mussels could attach to them might be required.
“If you just put the mussels on the bottom they will sink into the silt,” she said.
On the eastern side, the couple suggested trying to regrow the mussel reefs.
“We need to establish what are good sites today, what makes a good site today,” Dr Paul-Burke said.
The couple envisaged that a variety of methods needed trying to establish what works.
A report on the current situation will go to the Ohiwa Harbour Implementation Forum in a couple of weeks.
“It’s quite an impressive forum, everyone is doing their part,” Dr Paul-Burke said.