My intention was to compare population structure and growth rates at sites with varying degrees of exposure to ocean swells and different subtidal habitat structure. As with any marine diving research, being able to access the habitat on sufficient days per year is a real, limiting consideration. Given my previous research on abalone, I assumed the best growth sites would have significant water movement which is not conducive to fixed-schedule diving, so I knew there would be limited days at some sites.
A map of the Sydney sites, below, is from Kienzle et al (2022). MP=Mahon Pool; LB=Little Bay 1 and 2; C1 C2 C3=Congwong Bay 1,2 and 3; YB=Yarra Bay; K=Kurnell 1 and 2.
I chose some sheltered tagging sites in Congwong Bay and Yarra Bay within Botany Bay (C1, C2 and Y1) and Little Bay LB2 which had a mix of turf algae and kelp beds, and a mix of boulders and crevices. I also included a suburban rock pool (Mahon Pool, North Maroubra) that showed evidence of good recruitment (especially Turbo militaris) and provided a ready source of small juveniles. More frequent visits were possible at low tide there, but I had to dodge the swimmers or dive at night.
For comparison, sampling and tagging were done at more exposed sites at Congwong Bay (C3), Kurnell (K1, K2) and Little Bay (2 kms north of Botany Bay; LB1). Another exposed ocean site was Merry Beach (near Kioloa, 200km south of Sydney between Ulladulla and Batemans Bay; U1). Realistically, only Little Bay and Merry Beach were fully exposed ocean sites with no real estuarine influence.
All these sites were authorised for research collections in a 5-year scientific collection permit from the relevant fisheries authority (NSW Fisheries Research Permit P18/0028).
CONGWONG BAY
Congwong 1 and 2 are relatively sheltered sites about 50m apart in the middle of the photo below. They were selected to identify the degree of local movement between sites, and to enable numerous diving days due to being in the lee of north-east, south-east and north-westerly winds. Both have dense Ecklonia beds outside turf algae meadows and sea urchin (Heliocidaris spp) honeycomb habitat.
Congwong 3 is 500m south, on the western side of Henry Head near the mouth of Botany Bay, and while subject to much more surge than C1 or C2, is still sufficiently sheltered from north-easterly winds and swell to enable frequent, safe diving. Turf algae and crustose corallines grade down to Ecklonia stands and then tent shell/sea urchin barrens. I subdivided this site into C3N and C3 to examine differences between a habitat with small boulders (watermelon to lawnmower sizes) and one with larger rocks (lawnmower to car sizes) and to explore the extent of localised movement.
Fig 1. Congwong Bay. Site C1 is located south (just to the right) of Little Congwong Beach. C2 is halfway along and C3 is just inside Henry Head at the right.
KURNELL
Kurnell is more wave-exposed and allows for fewer diving days. Like Congwong Bay, it grades down to “white rock” barrens populated by tent shells, then to deeper sponge gardens popular with scuba divers (K1 is known by divers as “The Steps”). Tidal currents and swell made anchoring and diving tenuous compared to other sites. Ecklonia beds are sparse and there are significant beds of the cunjevois, Pyura praeputialis. I tagged at two spots at this site (K1, K2) to ascertain the extent of local movements. K2 was closer to the southern headland of Botany Bay and marginally rougher than K1, but both were slightly sheltered from southerly seas.
Fig 2. Kurnell sites. K2 at the left and K1 at the right. The subtidal terrain is undercut shelves, gutters and very large boulders, like what can be seen above water level.
YARRA BAY
Yarra Bay is well inside Botany Bay but still has extensive and dense Ecklonia beds (except after storms) and shallow turf algae meadows. The rock platform reef has crevices rather than boulders and grades down to a sandy bottom populated with seagrass (Halophila sp.) More frequent diving was possible but there was still significant water movement with southerly weather. Large specimens can be found on the nearby seawall (north side of Molineaux Point) which protects container ship wharves in Port Botany. The largest shell in my collection was washed up on Yarra Bay beach after a storm. This site used to be the warm water outlet canal from the old power station.
Fig 3. Yarra Bay reef in rough weather. The sailing club, La Perouse Museum and Bare Island are in the background.
LITTLE BAY
I tagged at two sites in Little Bay. At these locations, beds of Ecklonia were sparser than at other sites.
Fig 4. Little Bay in unusually calm weather
LB1 at the front (east, top left of picture) is the most wave-exposed of my Sydney sites with large turf algae meadows and incised gutters on a large, horizontal rock shelf. This drops off to very large boulders and sea urchin/tent shell barrens. There are dense Pyura colonies to the east. Good diving conditions here (as shown above) are uncommon.
To the west, LB2 is at the shore in the swimming beach area (bottom left of picture) and is much more sheltered, and therefore more exposed to recreational harvesting. The reef is fractured rock shelf and large boulders.
MAHON POOL
This is a man-made rock platform pool, one of many in Sydney apparently constructed as work projects in the 1930’s depression. It has small boulders at the base of the eastern wall, several vertical crevices across the floor of the pool, and turf algal coverage on the walls. Undercut crevices provided refuge for juvenile Turbo militaris whereas Lunella torquata individuals were found in the turf algae, grazing across the bottom, and around the boulders. Night-diving revealed the Turbo snails out grazing across the pool. The pool receives fresh water from a swimmers’ shower and seepage from the grassy headland above but is well-flushed on each tidal cycle and even at low tide with moderate swells. Water quality is poor on a summer day when crowded with swimmers.
Fig 5. Mahon Pool at the north end of Maroubra Bay. In the middle of the picture is a large rock that appeared overnight during a May 2023 storm.
KIOLOA
Merry Beach is south of Ulladulla on the mid-south coast and is more typically cool-temperate reef with dense Phyllospora, Ecklonia and Cystophora kelp beds in the shallows. These are interspersed with coralline turf algae and grade down to sea urchin barrens then sand. It is a mix of crevices and gutters and large granite boulders, as opposed to the sandstone geology of my Sydney sites. It is highly wave-exposed and tagging trips were frequently aborted or cut short by rising swells. Safe diving for me was possible only with northerly swells and light winds, so I did fewer days there and could not go at short notice since it was an overnight trip from Sydney. It was one of my abalone research sites back in the 1970’s.
Fig 6. Tagging site at Merry Bay just north of Merry Beach, over the headland south of Kioloa.
Reference
Kienzle M, M Broadhurst , G Hamer. 2022. Bayesian estimates of turban snail (Lunella torquata) growth off south-eastern Australia.
Fisheries Research 248, 106218. https://doi.org/10.1016/j.fishres.2021.106218