Posted on by Shaun

Okiwi mussels

The Okiwi Estuary (Whangapoua, Aotea / Great Barrier Island) used to contain the last naturally occurring soft-sediment mussel reef in the Hauraki Gulf.

Green-lipped mussels – Okiwi estuary. Photos by Dr Darren Parsons

“Aggregations ranged from a few individuals to meters in diameter. Mussels were frequently attached to pipis, partially buried in the sandy substrate. There were an estimated 3.2 million adults. Compared to other sites Okiwi mussels had the poorest condition but highest densities of invertebrates” – Ian Mcleod’s 2009 thesis on soft-sediment mussel systems in northeastern New Zealand.

Unfortunately they were nearly completely wiped out by a major storm.

Okiwi June 2015. Photo by Richie Robinson

I went for a swim on the outgoing tide (excuse the murky tannin stained water) to investigate 2 years later (April 2017).

Some areas were thick with shell but most of the substrate was sand.

I found one spot that was thick with old mussel shell (and just the one live adult).

Only 50 meters or so upstream I found some small regenerating clumps.

I was really pleased to see juveniles. Note the abundant red (not green) algae. Short video below.

If we left the bed alone, I wonder how many years it might take for it to completely regenerate?

UPDATE January 2018

I was able to visit the site again and was so impressed with it’s growth. The bed is awesome:

  • There are still areas with lots of shell including old mussel shell that have not been colonised by green-lipped mussels.
  • Most mussels were 4-5cm long but there were much smaller juveniles too.
  • The estuary has a lot of cushion stars and sea hares but no eleven armed starfish or octopus which predate mussels. It was quite strange to not see eleven armed starfish there.
  • The network pattern was similar to that of both restored and reefs and the one at Marsden Point with a wide range of density. It is most compact in the center where the mussels will run out of space if they grow.
  • I was told by a local that there were adults at the mouth of the estuary, they maybe crucial seed stock for the bed.
  • Although I saw eagle rays in the estuary I did not see them in the bed. There was no evidence of rays or snapper feeding in the bed but it looked like rays had been digging elsewhere.

These clumps feather the edge of the bed.

It was low tide and the tide was coming in.

There were gaps in most of the bed.

The network pattern.

The clumps came up very easily, you can see they are attached to old shell including mussel shell left over from the old bed.

Numerous sea hares (Aplysia keraudreni).

There were not many fish (yet), but the triple fin were lovely.

These two Bigbelly seahorses (Hippocampus abdominalis) were a real bonus!. It’s been so long since I saw a seahorse alive.

Update January 2019

The bed is doing well, there has been some harvesting at the eastern end but it may have been for pipi. I located the parent stock 1km away at the mouth of the estuary. The adults were only 6-7cm long so they are growing much slower than farmed mussels. Still no eleven-armed starfish. I hope the bed continues to grow. It would be interesting to survey the bed to see of recruitment was happening more at the edges (or not). The bed, size density and associated fauna and flora will help inform restoration efforts elsewhere in the gulf. Video here. Observations below:

  • Disturbed areas, eastern end of bed
  • The dominant algae (crucial to lifecycle) has changed, it’s less filamentous.
  • Juveniles growing adult mussels
  • Network pattern very similar to beds restored in 2014

Update January 2020

No major changes to density, heavy with algae, no juveniles seen. Locals driving vehicles on cockle beds to get to the pipi which surrounds the mussel bed and set a net right on its eastern side.

  • Abundant seahares and cushion stars
  • The dense algae makes density hard to estimate
  • I really hope this is not an invasive asian paddle crab, it sure acted like one.
  • I was swimming just after low tide so these photos have been colour corrected to remove the tannin colours.

Update February 2021

Unfortunately the crab above was the invasive species and my find triggered a $10,000 biosecurity response.

A local has anecdotal evidence that the mussel bed may not be 100% natural. A mussel farmer from the other side of the island who has since passed (Dick Anderson) claimed to have seeded the bed with mussel spat around 1990. The same local who talked to the farmer reports having seen mussels in the channel at the time. The story questions the genetic makeup of the bed, however in 2015 (when the bed was nearly wiped out) most of the bed was replaced then by local spat (there are beds on nearby rocks). Locals recall mussels on these rocks from the late 1980’s to early 1990’s. I am now checking with Auckland University to see if surveys for a marine reserve in the 1970’s recorded the bed.