23 January 2023
High tide 9:20am
Calm, overcast
With Ed Chignell (Skipper / Freediver)
I had given up on finding these reef balls until @jordi_nz figured out why the GPS coordinates I had been using needed adjusting. I made a lot of noise underwater when I first saw them :D
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Octagon Reef 174°45'20"E 36°40'45"S
9.8m right on slack tide
40 mins
We did not see anything on sidescan at the GPS point, but the fish finder was showing lots of activity. We dropped a float and sinker on the GPS so that our anchor did not hit the reef and anchored 20 meters away. I swam to the float and started descending the line. I saw the reef balls right away, and cheered into my regulator! The visibility on the seafloor was great (for Long Bay) at 4m or so. Two things stood out the fanworm and the fish. I surfaced to let Ed know and grab my camera.
The reef balls were covered in tube worms (probably parchment worms) and a fine growth. Mediterranean fanworm, sea quirts and various sponges dotted the surface. The silver sweep were the dominant fish and were on every ball, they also use the holes a lot, it was fun watching them pop in and out of the hollow balls. c1kg tāmure / snapper moved away from the reef balls while I was scuba diving but were regularly observed by Ed who was free diving. I was most impressed with the numbers of juvenile fish on the reef, mostly tāmure and jack mackerel. This was the first time I've seen juvenile tāmure shelter in fanworm. We saw the occasional spotty, a parore, a few juvenile goatfish and the odd triplefin. I didn't do a good job of photographing the shells but I dont remember seeing any bivalves and most of the molluscs were home to hermit crabs. The floor of each reef ball was littered with shell. Drifting ecklonia and some other algae had been trapped the structures adding complexity. I suspect some of the shell around the bases of the balls had also been swept in.
Two minute video here https://youtu.be/H3g6C2gaoCI
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Zig Zag Reef 174°45'16"E 36°40'45"S
8m one hour after high tide
36 mins
We used the same technique but noted that the fish were not on the GPS location of the reef. We forgot to accurately re-log the waypoint but think it may be 20m west of the GPS. However we had no trouble finding the reef by free diving. The visibility had noticeably dropped. After the dive we adjusted the sidescan and were able to clearly see the reef balls (20m spread, 800kHz).
There was more sediment in the water here, the tide was now going out but there was little current. Although there was sediment in the sand at Octagon Reef, here it seemed softer and in one spot it was thick and sticky. I had to be more careful not to touch the bottom. I also thought the seafloor sloped down to each reef ball more at this reef (just like a boulder on a sand beach). This reef had caught a large log which added complexity and biodiversity to the reef.
The biodiversity and abundance was were similar to Octagon Reef (including numbers of juvenile fish) with a few exceptions. There were significant numbers of white striped anemone here, I dont remember seeing them on Octagon Reef. A patch of feather hydroids and a different sponge (Dysidea).
I saw more fish in five minutes at these reefs than I did searching hundreds of meters of seafloor in Long Bay looking for the reefs. I have also spent hours on the rocky reefs in this bay (which have abundant fish life) and thought the biodiversity created by the reef ball habitat was unique.
