The mussel line

Marine restoration is a lazy business. All you have to do is stop fishing an area and marine ecosystems heal themselves. However this is not the case with green-lipped mussels in New Zealand.

100’s of square kilometres of sub-tidal mussel beds were fished to extinction in each harbour around New Zealand.

The industry collapsed and more than half a century later they have not returned. In the Hauraki Gulf there are a few places you can still find Green-lipped mussels. You would think that these places would be deep under the ocean (Green-lipped mussels have been found at 50m deep), but they are not.

Most are in the intertidal zone on rocky shores. Here there is usually a gradient with mussels thin higher up and getting thicker towards the low tide mark where the abruptly stop. I have asked several local experts and no one has a solid answer why they stop so abruptly.

As we spend 100’s of thousands of dollars restoring sub-tidal beds maybe the key to unlocking a lazier (and cheaper) solution is staring us in the face. Here are some thoughts on why the line exists:

Mussel predation
  1. Avian predation. Every exposed mussel bed has at least one pair of Variable oystercatcher eating the smaller mussels every low tide. So living in the subtidal zone would be of some advantage but we are more likely to see juveniles higher up. This suggests there is even more predation from below.
  2. Starfish predation. Eleven-armed star fish were a big problem in the first beds put down by Revive our Gulf. However starfish do okay in the intertidal zone and are not particularly abundant in intertidal beds. We don’t see lots of them waiting below the low tide mark next to intertidal mussel reefs.
  3. Fish predation. This seems like the most obvious cause but surely it can’t be Snapper as they have been fished down to 20% of their natural biomass. Rays are a possibility but I thought I would see more of them in the shallows if this was the case. This southern study looked at predation and found it to be largely subtidal and nocturnal, by fish and large crabs.
  4. Octopus predation. They are nearly invisible and love eating mussels so at first this is a good fit. But octopus leave the shells, I will look for evidence on the next intertidal bed I explore. None of the predation theories show why the line is so strong.
  5. Food. Mussels eat phytoplankton and algae. I am sure there will be more of this close to the surface. I am pretty sure this is why mussel farmers grow their mussels high in the water column. Wild mussels might also benefit from wave action on the rocks as it would increase the oxygen in the water. However I would have thought that these benefits would be offset by the fact they can’t feed while they are exposed to the air.
  6. Sediment. If there is some other benefit to being exposed to the air maybe it’s that the water in the Gulf just has too much stuff in it. Mussels have to work hard sorting out the food from the dirt, maybe mussels are not good at taking a break and being forced to is good for them. Comparing the condition of intertidal and sub-tidal mussels would help dismiss this idea.

Or maybe like a lot of things in biology it’s a mixture of the above factors. As we are slowly losing our intertidal mussel beds it might be wise to set up a long-term monitoring project that might solve this mystery and inspire lazier restoration methods.

Probe holes

With a lot more mowing at Point England this Winter the flocks of South Island pied oystercatcher are leaving a visible sign in the paddock. The probe holes are very dense right to the edges of the paddocks which means that I am also seeing holes from other species like White-faced heron and Pukeko. Casual counts put the number of little holes at around 100 per square meter.

Roost heights at Tahuna Torea

Today I took photos of the shorebird roosts at Tahuna Torea when the tide had just covered the main spit roost. The tide height was predicted to be very high today (3 August 2019) at 3.4M Westhaven 8:53am. Conditions were relatively calm and the tide had not (yet) hit the high tide line on the Little spit.

Names and locations of roosts
The tide was about 20cm (in height) from the high tide line.
Pathway
And the pathway which is sometimes flooded was about 20cm above the water.
The spit had just been compromised with just a few Black-back gulls and one Caspian tern remaining
Little spit was about 50cm above the tide
Little Godwit Island was about 60cm above the tide
South Godwit Island was about 75cm above the tide
North Godwit Island had about 40cm above the tide
The Southern end of Lockley Island had about 40cm above the tide
The Northern end of Lockley Island had about 40cm above the tide

I have seen tidal debris in the mangroves around Lockley Island that were above the height of the island (indicating that it sometimes gets swamped) but I think it would have made an ok roost today.

Reversing the decline of the Shorebirds of the Tāmaki Estuary

I presented this report to the Maungakiekie-Tāmaki Local Board today as they are yet to seek advice from a shorebird expert at Auckland Council in developing their Open Space Network Plan (OSNP).

It recommends:

  1. Shorebird expert to review the report.
  2. Local Boards commit to reverse declining shorebird numbers.
  3. OSNP revised with dedicated and enhanced shorebird roosts.
  4. Serious investments in shorebird roost protection and enhancement.

I have also attached my amendments to the OSNP and my submission on the OSNP above.

UPDATE: 1 May 2019.
Great response from the Ōrākei Local Board, nothing from the Maungakiekie-Tāmaki Local Board (yet) who have approved their Open Space Network Plan with no space for shorebirds.

UPDATE: 18 June 2019
Great article on the report by Farrah Hancock here

UPDATE: 12 July 2019
“The report’s recommendations accord closely with the Council’s policies and objectives to protect biodiversity in the Auckland region.”
– Dr Tim Lovegrove

UPDATE: 01 September 2019
Report covered on page 9 of the Panmure Business & Community Newsletter

UPDATE: 12 September 2019

The MTLB have made changes to the OSNP as follows:

Page/title Amended text
Page 10 (Coastlines and Waterways)

The Tāmaki Estuary is home to a varied number of shorebird species. The shorebirds roost in the reserves along the coastal edge throughout the year including Point England Reserve, Wellington War Memorial Reserve and Tahuna Torea Nature Reserve.

Several species are threatened and the populations are in decline.

Threats to the shorebirds include disturbance of their roosts preventing them from resting, particularly at high tide, from lighting of the reserves at night, dogs, humans, unsuitable vegetation and construction.

Page 20 (Environmental Quality) + Shorebird populations are in decline due to their roosts within the coastal reserves being disturbed
Page 36 (Improve biodiversity and water quality)

Protect and enhance shorebird roosting areas:

+ Investigate opportunities to restore roosting areas for shorebird populations

+ Provide opportunities to protect existing roosting areas.

The wording is well chosen and at a high level it addresses my core concerns. A good start to reversing the decline of the shorebirds in the Tāmaki Estuary.

UPDATE: 7 May 2020
The authors have really taken on board the feedback! Shorebirds (which contain the most conservation dependant species in the region) were excluded from the plan. They now feature on pages 10, 21, 37, 51, 52, 55, 93, 94, 109. You can download the finalised copy of the Tāmaki Open Space Network Plan here. [Published here first!]

Pest monitoring Selwyn Bush 2019

We identified 19 tracking locations (S1-S19) in 2014. The first 10 (S1-S10) were used in this survey. The survey was repeated in March 2019. The first 10 locations were searched and three out of 10 tracking tunnels were found. Seven new tracking tunnels were installed on the 10th of March. On the 16th of March another 9 tunnels were installed and all the tunnels were filled with an inked tracking card with peanut butter in the middle. Some locations were adjusted to make access easier. I noticed when trying to find the tunnels again with the Garmin InReach and App that the positions were often off by up to 16M. Thats a lot in the bush!

Locations of the 2014 tracking locations compared to the actual locations of the tunnels placed in 2019.

RESULTS

I find these mashed up prints very hard to identify, but here is my best guess:

S1 Hedgehog
S2 WW, Rat, Hedgehog
S3 Rat, Hedgehog
S4 Hedgehog, WW
S5 –
S6 –
S7 Hedgehog
S8 –
S9 –
S10 –
S11 –
S12 –
S13 –
S14 –
S15 –
S16 –
S17 –
S18 –
S19 –

WW = Winged weta, I was particularly looking for this species. See my blog post on tracking winged weta here.

Suspected winged weta tarsal pad print

I think we should probably ignore the tracking tunnels put out recently (S11-S19) as they may have been avoided due to (at at least some of) the target species being neophobic.

This would give us a result for 2019 of 20% Rat, 0% Mice, 50% Hedgehog. This indicates we have less rats & mice and more hedgehogs than 2019.

I have left the tunnels out there, it might be a good idea to retry in a few weeks. It’s interesting that we see more pests near the top of the bush, this has been observed before.

KMZ file with locations

Freshly planted mussel bed

Mussel bed on hour old

I think this is the worlds first photo of a freshly planted mussel bed. Altho I have been to most of Revive our Gulf’s deployments (including the first one in 2013) we have’t dived on them straight away. After taking this photo I deployed three time-lapse cameras to record suspected predation (I was happy to discover none). I also photographed a nearby bed that was laid a week ago where the mussels are standing up and had knitted together. It would be interesting to know how long it takes them to jostle into position.

Volunteers shoveling mussels
Volunteers shoveling mussels

Mussel bed one hour old
Mussel bed one hour old

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:

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.