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ISRA FACTSHEETS

AUSTRALIA AND SOUTHEAST INDIAN OCEAN

ISRA FACTSHEETS

AUSTRALIA AND SOUTHEAST INDIAN OCEAN

Christies Beach ISRA

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Christies Beach ISRA

Christies Beach

Summary

Christies Beach is located on the eastern coast of the Gulf St Vincent, South Australia, Australia. The habitat is characterised by sand substrates, seagrass, and rocky reefs with high macroalgae coverage. The area is bordered by Port Noarlunga reef to the south and contains a circular rocky reef (Horseshoe Reef) ~500 m from shore. It is influenced by freshwater input from Christies Creek and the Onkaparinga River. The area overlaps with Encounter Marine Park. Within this area there are: reproductive areas (Port Jackson Shark Heterodontus portusjacksoni) and undefined aggregations (Southern Eagle Ray Myliobatis tenuicaudatus).

Christies Beach

Christies Beach

DESCRIPTION OF HABITAT

Christies Beach is located in South Australia, Australia. It is situated on the eastern coast of the Gulf St Vincent, ~30 km south of Adelaide. The area is a heterogeneous environment characterised by sand substrates, seagrass (e.g., Posidonia, Zosteraceae, Amphibolis spp.), and rocky reefs with high macroalgae coverage. Nearby is a small river, Christies Creek, and to the south is the Onkaparinga River, which delivers terrestrial nutrients to the surrounding environment. Horseshoe Reef is a ~0.12 km2 shallow reef, ~500 m from shore, which becomes exposed at low tide. At the southern end of Christies Beach is a shallow rocky reef extending from Witton Bluff, which loosely connects with Port Noarlunga reef.

The area is influenced by the dynamics of the Gulf St Vincent, which is a shallow gulf environment (maximum depth ~40 m) that supports an abundance of tidal wetlands, seagrass meadows, mangroves, rocky reefs, and saltmarshes (Edyvane 1999). This is an inverse estuary system, with salinity increasing with distance from the open ocean because of the evaporative processes occurring at the top of the gulf (Bye 1976). Water is transported into the gulf via Investigator Strait and Backstairs Passage, but Kangaroo Island blocks most of the gulf mouth from the open ocean, limiting water exchange and sheltering the gulf from high wave energy (Tanner 2002). At Christies Beach, the sea surface temperature ranges ~11.7–21.3°C from August to March (Rodda 2000).

This area overlaps with Encounter Marine Park (NPWS SA 2025).

This Important Shark and Ray Area is benthic and pelagic and is delineated from inshore and surface waters (0 m) to 15 m based on the bathymetry of the area.

CRITERION C

SUB-CRITERION C1 – REPRODUCTIVE AREAS

Christies Beach is an important reproductive area for one shark species.

Underwater visual census (UVC) surveys and observations from divers and snorkelers have revealed the regular and predictable observations of egg cases and neonate/young-of-the-year (YOY) Port Jackson Sharks and mating events in the area (Rodda 2000; Rodda & Seymour 2008; Adelaide Freedive Tribe 2021, 2022; Gervais et al. 2021; iNaturalist 2025; RLS 2025; C Huveneers unpubl. data 2025; CN Roberts unpubl. data 2025).

Historically, this area has been reported as an area with high abundance of egg cases of Port Jackson Sharks (Rodda 2000; Rodda & Seymour 2008; RLS 2025). Between 1988–1991, egg cases were collected by diving at Christies Beach for physiologic studies. This area was chosen due to the highest abundance of egg case across the whole Gulf St Vincent (Rodda 2000; Rodda & Seymour 2008). Egg cases were collected at a specific site in this area (7 m2 of reef), which was surveyed every 7–14 days for nine months each year. A total of 305 fresh egg cases were collected, with an average of four per dive. In addition, older egg cases were observed but not quantified nor collected. Egg cases were collected mostly between August–November (Rodda 2000). In addition, 29 egg cases were collected in the area in November 2016 (Gervais et al. 2021). Egg cases of Port Jackson Shark are regularly observed by divers and snorkellers in this area, mostly around nearshore rocky reefs (e.g., Horseshoe Reef), with >100 egg cases sighted during a 1-hr snorkel (iNaturalist; C Huveneers pers. obs. 2025). In addition, egg cases are regularly observed washed on the beach in this area by citizen scientists (iNaturalist 2025). From observations of egg cases reported between 2012–2024 across the whole Gulf St Vincent (n = 113), Christies Beach was the location with the largest number of egg cases recorded (n = 35, 30.9%; iNaturalist 2025). In addition, a large number of egg cases were recorded in 2025 (n = 110) after the impacts of an algal bloom in the Adelaide region, with a large proportion (n = 45, 40.9%) being recorded in this area. Records comprise mostly single or pairs of egg cases with observation of more than five egg cases being rare. While stranded egg cases are observed across all the Gulf St Vincent, Christies Beach is the only location where these are observed in large numbers and across multiple years (iNaturalist 2025).

Anecdotal observations by snorkellers and citizen scientists have confirmed the presence of neonate/YOY Port Jackson Sharks in 2015, 2021, and 2022 (Adelaide Freedive Tribe 2021, 2022; iNaturalist 2025). Christies Beach is the only confirmed location where neonate/YOY have been recorded alive across the Gulf St Vincent (Adelaide Freedive Tribe 2021, 2022; iNaturalist 2025). While individuals were not directly measured, estimated sizes from photographs clearly indicate that they measured <30 cm total length (TL) which is close to the reported size-at-birth (23–24 cm TL; Ebert et al. 2021) and to the YOY size in Australia (30 cm TL) according to growth rates (Izzo & Rodda 2012). In addition, after the impacts of an algal bloom in Adelaide in 2025, a large number of neonate/YOY Port Jackson Sharks (n = 61) were observed stranded on the beaches across the whole gulf, with most of them (n = 39; 63.9%) recorded at Christies Beach (iNaturalist 2025).

Aggregations of ~6–20 Port Jackson Sharks (juvenile and adults) are regularly observed in the area by divers and snorkellers with Christies Beach being the main location to observe the species in the whole Gulf St Vincent (iNaturalist 2025; Snorkelspots 2025; C Huveneers unpubl. data 2025). Between 2019–2025, 89 Port Jackson Sharks (70 females, 19 males) were caught in the area for tagging purposes. During these activities, up to 30 individuals were sighted at one time under rock ledges. Forty-two of the Port Jackson Sharks caught were tagged with acoustic transmitters (CN Roberts unpubl. data 2025). Tagged Port Jackson Sharks displayed both resident and transient movement patterns, with the majority (n = 25; 60%) being detected between November–March (when waters are warmer). Six individuals were recorded year-round, and the rest were sporadically detected throughout the monitoring period (CN Roberts unpubl. data 2025). Of the 70 females sampled at Christies Beach, 66 measured >65 cm total length (TL) and 54 measured >75 cm TL. In South Australia, female Port Jackson Sharks reach maturity between 65–75 cm TL (Izzo & Rodda 2012; Rodda & Svane 2007), indicating that aggregations at Christies Beach are largely composed of mature females for reproductive purposes. Residency of females predictably peaks between November–March each year, which coincides with the increased presence of males, and with the anecdotal observation of mating and courtship events recorded in 2019 and 2021 by scientists, divers, and snorkellers in the south corner of Christies Beach (C Huveneers unpubl. data 2025; CN Roberts unpubl. data 2025). Acoustic receivers deployed at Christies Beach detected the most Port Jackson Sharks, and most often, compared to an array of 47 receivers deployed along a 60 km stretch of the Adelaide metropolitan coastline, highlighting the importance of this area (CN Roberts unpubl. data 2025). Christies Beach is the only location in South Australia where mating events are regularly recorded (C Huveneers unpubl. data 2025).

The high abundance of egg cases, the regular presence of neonate/YOY, and observations of mating and courtship events highlights the reproductive importance of the area. It has been hypothesised that the rocky reefs that characterise Christies Beach provide suitable areas for egg deposition (e.g., in rocks and crevices) and retention during the 10–11 months of incubation (Rodda & Seymour 2008).

CRITERION C

SUB-CRITERION C5 – UNDEFINED AGGREGATIONS

Christies Beach is an important area for undefined aggregations of one ray species.

Southern Eagle Rays regularly aggregate at Christies Beach. Between 2019–2021, 10 Southern Eagle Rays were tagged with acoustic transmitters and detected in the area (Davey et al. 2023). In 2020–2021, Southern Eagle Rays had high site fidelity to the area, with residency indices of ~85%, and 30% of individuals occupying the area year-round (Davey et al. 2023). In addition, Seven Southern Eagle rays were tagged and monitored in the area between 2022–2024 (CN Roberts unpubl. data 2025). Individuals had a mix of high and low residency, with 25% occupying the area year-round, but others having sparse detections (CN Roberts unpubl. data 2025). However, Southern Eagle Rays were tagged externally, and the reduced number of detections for some individuals could be related to tag shedding. Both sets of data indicate that this species use the area more frequently than adjacent locations.

Southern Eagle Rays aggregate in the area year-round in groups of 3–5 individuals. This many individuals were detected on the same receiver within the same hour on 128 days (in a 457-day period between 2020–2021) (CN Roberts unpubl. data 2025). Individuals tagged at Christies Beach measured >140 cm disc width (DW) indicating that these were mature females, as the size-at-maturity is 80–100 cm DW (Last et al. 2016). These aggregations potentially use the warm, productive areas of the nearshore rocky reefs to rest or aid in embryonic development. There are additional opportunistic observations by snorkelers and divers of multiple rays aggregating at the shoreline of Christies Beach (C Huveneers unpubl. data 2025). Sporadic snorkel surveys conducted in the area since 2019 have recorded a group of 3–4 Southern Eagle Rays regularly with no observations of the species during surveys conducted in adjacent areas (C Huveneers et al. unpubl. data 2025). Additional information is required to understand the nature and function of these aggregations.

Christies Beach

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