Okiwi mussels

This is what Okiwi Estuary (Great Barrier Island) used to contain within the main channel close to the estuary entrance. It was 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.

Weeds on Motukorea

After the fire on Browns Island, lots of noxious weeds took hold. Although it looked like the crater was going to be dominated by bracken the weeds are winning. Black nightshade is dominant and there are thousands of woolly nightshades coming up, some as large as 1 meter wide already. Inkweed, apple of sodom and boneseed are also sitting up above the kikuyu.

Click for high res 360° image of the crater

Wooly Nightshade

I casually pulled up about 100 mullien but did not make a noticeable impact. Most disturbing was this animal dropping I found on a large mullien leaf.

Pray mantis identification

New Zealand has two praying mantises, the invasive South African praying mantis and the native New Zealand praying mantis. The easiest way to tell the two species apart is the shape of the head. The thorax (bit that connects their heads to their abdomens) is much wider on a New Zealand mantis so by comparison the South Africans look like hammerheads.

Natural mussel recruitment

After more than half a century Green Lipped mussels have not returned to the soft seafloor of the Hauraki Gulf. However only 100km north at the mouth of the Whangarei Harbour something magic happened. Out of nowhere a huge bed of mussels appeared. I was slow to document it but here are some photos by Dr. Mary A. Sewell in April 2016.

The bed was quickly consumed. Locals could park their cars a few meters from the beds. There were reports of people turning up with wheel barrows and of cars and boats being confiscated for those who exceeded their limits. The beds have now been decimated, (An MPI officer thinks the beds did not disappear but were greatly reduced in their first season, I did not find the original bed on my first trip). I turned up eight months later at low tide and followed some locals to the remaining beds. They looked like this:

Green lipped mussel bed with eleven armed starfish triple fins and fishing line. DEC 2016
Green lipped mussel bed with triple fins and red filamentous algae. DEC 2016
Green lipped mussel bed with alga and octopus. DEC 2016
Green lipped mussel bed showing algae diversity. DEC 2016
Destroyed green lipped mussel bed. DEC 2016

The remaining bed ran along the edge of the drop off about 4m at low tide and supported a wide range of marine life. On my way back to the beach I wondered what conditions bought about this beautiful natural phenomena? There may well have been sufficient spat from mussels growing on the refinery wharf, but did a closure of the cockle beds promote natural the recovery? Could the neighbouring marine reserve have played a part? Was this a once off or part of the recovery of the Whangarei harbour since the Firth cement works has cleaned up its act? Then I spotted these:

Juvenile green lipped mussels. DEC 2016
Juvenile green lipped mussel. DEC 2016
Juvenile green lipped mussels. DEC 2016

It seems the cockle and pipi shells have created a firm substrate for the juvenile mussels to attach to. Is this how the bed started of? No hydroids or red filamentous algae to attach to, just green algae and shell? If so this is a recipe worth exploring for aided restoration elsewhere in New Zealand. Local iwi (Patuharakeke Te Iwi Trust Board) have proposed a 2-year temporary closure to the take of all shellfish at Mair Bank and Marsden Bank, Whangarei.

I dived around the other side of the channel where there was tonnes of algae of every sort but no mussels. Not even on the rocks. It is possible that the mussels did their 1st settlement on this algae then drifted to the bank. It would be interesting to dive a similar distance in the opposite tidal direction to look for recruitment.

Note there was a large pipi & cockle die off on the shell bank where mussels appeared.

UPDATE: Nov 2017
There are isolated individuals ranging for a hundred meters or so west of the main bed. Most mussels were around 6-8cm long. I could not find blue mussels in the bed, I did however see fish eggs, octopus, crabs, evidence of fish feeding on the mussels, triplefins and many shorebirds enjoying the exposed reef. The mussels were also growing in smaller patches off the edge of the sand bank into the channel where I found the remnants in Dec 16.

Isolated mussels. NOV 2017.
Isolated mussels. NOV 2017.
Isolated mussels. NOV 2017.
Isolated mussels. NOV 2017.
Soft sediment intertidal green-lipped mussel bed. NOV 2017.
Soft sediment intertidal green-lipped mussel bed. NOV 2017.
 I saw five octopus in half an hour. NOV 2017.
I saw five octopus in half an hour. NOV 2017.
Green-lipped mussels Marsden Point. NOV 2017.
Green-lipped mussels Marsden Point. NOV 2017.
Humans and variable oysetercatcher consuming mussels. NOV 2017.
Humans and variable oysetercatcher consuming mussels. NOV 2017.

Hopefully this bed continues to regenerate despite the massive human harvesting effort. Identifying and protecting the seed stock would go a long way towards this.

The current here is very strong, here is a short video of 1,000s of young pipi swept in current. They seem to be regenerating.

UPDATE: Jan 2018
Two insights after talking to locals.

  1. Mair Bank was once covered in a thick carpet of large mussels with live pipi underneath.
  2. In the late 1950s / early 1960s (1963?) there was a commercial operation that dredged the whole bank, taking out all the mussels.
  3. The pipi die off could have helped the mussel bed by providing a settlement substrate. The pipi could have consumed mussel spat that may have perviously tried to settle the bed.

Also the beds look choice at high tide, I saw sea hares and piper fish. Look how many species in this photo:

Triplefin, spotty, goby, green-lipped mussel, whelks, common Sydney octopus and algae.
The water clarity is excellent, its just hard to shoot because of the current.

UPDATE: Feb 2018

I managed to track down a report which mapped the bed in Feb 2016. They worked out that the mussel bed then covered an area of approximately 12,800m2 and had an average coverage 38.3%.

Evasive weeds

One of the hardest aspects of restoring and maintaining native habitat is weed control. Cliff faces are the most expensive often requiring carrying heavy loads to remote places and abseiling or a helicopter. Today I watched as DOC, Motuihe Trust and Yamaha trailed spraying pampas from an unmanned helicopter. The RMAX helicopters are piloted by remote control and used in a wide range of industrial and research applications overseas. The trial was a success and the team have plans to further improve the precision of the technology.