Dive report Horuhoru Rock

13 February 2024
10:40am High tide
Vis 5-7m
Wave height 0.5m
Wind 5 knots
Current minimal
45 minutes
Max depth 22m

[ I arrived around 1pm to see two dead juvenile gannets on the rocks. I checked to see they were not alive after the one I saved last week made an amazing recovery. I'd love to know if these are juveniles falling from the colony above or crash landing, the weather has been very mild. ]

Efforts to restore soft-sedimen kūtai / green-lipped mussel beds in the Hauraki Gulf are hampered by a lack of a reference bed. I have been visually documenting soft-sediment beds for 10 years now and was quite interested in kūtai I found growing in mussel shell near Ahaaha Rocks. A beautiful and better reference bed with similar shell dynamics was found off Waiheke by Craig Thorburn. It's small and may be fed by kūtai shell from the rocks above. I theorised there would be similar habitat below the kūtai on this wall at Horuhoru Rock.

I anchored 10-15m away from the rock and swam over to the rock face so I could descend the wall. It was nice to see juvenile kūtai recruiting below the low tide line. Swimming north I found a shelf at around 7m covered in kūtai shell with small live kūtai growing on the shell (not attached to rock). There were fine branching red algae (coralline) nearby which could have acted and settlement substrate. Last week I documented large beds of juvenile mussels growing just above and below the low tide line here. These mussels have been protected by rāhui for the last two years.

Dropping to about 18m I found what I was looking for at the bottom of the wall, live kūtai growing in a kūtai shell drift. The drift changed elevation dramatically as I headed south (both up and down) I recorded a max depth of 22m. Unlike the shallower Waiheke bed there was no Ecklonia in the bed and no association of anemones with the live kūtai. I have not seen predatory eleven-armed starfish at either bed. I did see two starfish eating triton here. In places the drift was more than 30cm deep with large kūtai shell (much thicker than anywhere else).

The kūtai shell bed extended from the rock for several meters but the live mussels did not continue and soon the habitat was better described as dog cockle shell drift. Heading back I found my anchor on the kūtai shell which may be one of the reasons the kūtai bed is not bigger. However I did not observe any kūtai recruitment or settlement structures at this depth.

I explored the edge of the rock further south which had ledges and a pretty cave before heading back and up my anchor chain.

Posted on February 14, 2024 04:31 AM by shaun-lee