A small kūtai reference bed

One of the interesting challenges associated with kūtai / mussel reef restoration in the Hauraki Gulf Marine Park (HGMP) is the lack of a reference bed.

The last natural soft sediment kūtai bed left in the HGMP is in an estuary and recovering after being wiped out by a storm in 2015. This means our best reference beds are those under or adjacent to kūtai farms or previously restored. Myself and other divers have helped look for a natural bed in The Noises group of islands which still has old kūtai growing on rocks. On the first of April (to celebrate the tipa / scallop closure which will temporarily protect the seafloor around the Noises from dredging for the first time ever!) I went for a dive / snorkel with Sue Neureuter who wanted to show me kūtai recruiting on shell near the base of some rocks. It was disappointing to see large losses the kūtai growing on the rocks due to a devastating combination of recreational harvest and this summers cyclone impact. The kina barrens were shocking and the area dearly needs protection. However we did find some recruits. The juveniles look very cosey in the old shell showing how dead shell still provides many ecosystem benefits, possibly including a suitable recruitment substrate. There is not much primary recruitment substrate around (kelp) but I did see some beautiful christmas tree hydroids.

Juvenile kūtai growing in adult shell

I was most excited to find a small c4mx4m bed of kūtai growing on soft sediment. I lifted kūtai up at the edge of the bed and they were only attached to each other and some dead shell. I don’t know if the kūtai recruited on to the shell hash here or if they came off the rocks and built up over several years. Some of the kūtai were old and large but most were closer to 9cm long. I did not measure any. I took some photos and spent two minutes doing some photogrammetry on part of the bed using this process I also took a sample of the shell hash.

The bed was remarkable because:

  • the kūtai sat at a very similar depth in the shell hash compared to softer sediments
  • the bed was lacking any signs of bio deposits from the kūtai (I have seen this once before at Marsden Point but thought it was just due to really high current which there is not here)
  • the bed had less colonial epifauna than restored beds
  • the bed showed no signs of clumping which is a very important feature of established beds
  • the bed had some young ecklonia radiata growing in it (I should have lifted one up to see what it was attached to)
  • the portion of the bed I mapped contains some >10 cooks turban suggesting shell is or was swept into the area

Let me know if you notice anything else unusual. If the Noises kūtai were protected and able to regenerate it would help us learn a lot about kūtai restoration.