Canopy algae are a major component of Blue Carbon fixation. Whilst diving at The Bluff (Rosetta Head) on 27th October 2022, I found this Western Pacific Purple Sea Urchin, Heliocidaris erythrogramma: –

I have posted it on iNaturalist, along with the following details, at .

Overall, I saw relatively few of these, happily so with regard to their known impact on canopy browns e.g. common kelp.

But of concern, I saw only one Western Blue Groper, a juvenile about 30 cm total length, that species being an important predator of this urchin species.

I also saw, disappointingly, few mature Bluethroat and Brown-spotted wrasses, with males very scarce despite ideal habitat.

So it appears that recreational fishing has impacted significantly on the main local wrasse taxa that historically have helped limit such urchin densities. I expect we’ll see increased evidence of urchin barrens in this area within a few years, and barrens are already documented in some other parts of Encounter Bay.

At the risk of being boringly repetitive, one simple measure to minimise the likelihood of canopy loss here and everywhere in SA would be full state-wide protection for Western Blue Groper, and greater focus on reducing recreational taking of Bluethroat Wrasse (which increasingly occurs intentionally but, probably still more often, occurs as non-target bycatch).

Not forgetting that canopy algae are a major component of Blue Carbon fixation.

(Header photo taken by Steve Reynolds)

By David Muirhead

Life member David is a long-serving Secretary of the Marine Life Society of South Australia. He has dived and snorkelled in South Australian waters for around five decades and has a particular interest in bony fishes. He is a diver photographer who loves posting photos from his dives to iNaturalist

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