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.
  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.

Indigenous biodiversity

I have 1,612 verified observations on iNaturalist between Auckland and Whangarei documenting 552 species (mostly invertebrates) covering forest, freshwater, intertidal and marine habitats. I don’t take many photographs of plants. Of these observation 96 or 17% of species were introduced. Here is a break down showing areas where I have found more or less introduced species:

Hauturu Aotea Tawharanui East Auckland Waitakeres Hunua Ranges Rangitoto / Motutapu / Motuihe Motukorea Mungatapere
Observations 206 149 141 197 77 94 71 33 159
Species 86 84 86 120 57 64 52 25 83
Introduced 6 12 8 35 1 4 11 4 9
Percent introduced 7 14 9 29 2 6 21 16 11

I expected the restored and protected islands in the Hauraki Gulf to have a smaller percent of introduced species. I think the high number of introduced species (compared to the Waitakeres and the Hunua Ranges) reflects the islands farmed history with islands like Motukorea and Motutapu still dominated by kikuyu. The larger and older the forest the more indigenous biodiversity.

Tracking footprints

I teamed up with computer and environmental scientist Jordi Tablada to build a website for identifying New Zealand animal sign. I met Jordi through the New Zealand Dotterel Forum as he looks after dotterel at Piha. We had overlapping skills and were looking for a project to collaborate on. He came to me with the idea inspired by some materials produced by another dotterel minder Emily Roberts.

Now when I spot tracks in the sand and wonder what made them I load up the website and check them against the examples. It’s working really well and I hope to expand it to include other animal sign and more species. Others are using it too, mostly due to some great press. It was inspiring to see another citizen science identification guide go live this morning which will also help on beaches. This one is for shells.

These guides join others produced by organisations like nzbirdsonline.org.nz and help users of tools like eBird and iNaturalist.nz map the diversity and abundance of New Zealand flora and fauna.

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.

Hauturu kiwi monitoring trip

Cave weta, Pachyrhamma

I was recently lucky enough to monitor kiwi on Hauturu with the Little Barrier Island Supporters Trust. I have done this many times before at Tāwharanui with TOSSI. The exercise involves a good hike to the destination at night then sitting quietly in the dark listening to the sounds of the forest for a couple of hours. I have met some great people doing this and this trip was no exception. They patiently waited for me as I inched along the tracks on the way back to the hut, inspecting every tree for hidden treasures and some even came out with me again on their free nights. I have posted all my invertebrate observations here on iNaturalist where the community is helping me identify them all. I have also posted some fungi which were a hot topic among the volunteers.

It’s an incredible island where one can get a sense for pre-human New Zealand. Here is a list of some observations I made over the 12 days I was there.

  • I saw less invertebrates, birds and geckos at elevation.
  • Piwakawaka following Tieke on two occasions.
  • By weight there was much more industrial plastic pollution (mussel floats) than domestic plastic pollution on the beaches.
  • Similar gecko numbers in winter compared to summer. One seen every 20 minutes on night walks.
  • Cave Weta were the dominant weta by a factor of 10 or more. Tree Weta, Ground weta and Wētāpunga active.
  • Winter invertebrate abundance similar to mainland sites with predator control (and less birds). At night, in two out of three tress invertebrates were easily found.
  • Invertebrates not much bigger than raindrops stay hidden during the rain.
  • NZ Giant Centipede found 3m above ground inside a Kanuka tree that fell in the middle of the day. The species is very arboreal.
  • Korimoko chasing Ruru during the day.
  • Flock of 18 Kereru feeding on Muehlenbeckia.
  • A high of 12 Shore Skinks seen from one observation point. No other skinks observed.
  • Invertebrate diversity high with many species I had not seen before. Some had not been previously photographed in the wild.
  • Strange absence of Katydid calls.
  • Very empty streams with only Shortfinned Eel and Banded Kokopu observed. Huge flushing events the likely cause of low freshwater diversity and abundance.

Training iNaturalist

The artificial intelligence that reads photos on iNaturalist and identifies species is incredible. It often makes me double check an ID and has helped on many occasions. However I was pleased to see it bested by this stick insect I photographed who completely out-witted it 😀

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.

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