One of the Sanctuary's key marine research projects centres around the movement ecology of the critically endangered Bottlenose Wedgefish (Rhynchobatus australiae). The reefs and estuary mouth of the Sanctuary have been identified by marine biologists from partners Marine Megafauna Foundation (MMF) as critical aggregation sites, based on the sheer numbers of wedgefish located here and the behaviours that have been observed - some of which have never been documented before.
The Sanctuary's Taryn Gilroy and colleagues from MMF, Dr Steph Venables and Nilza de Catarina, attended the 15th Oppenheimer Research Conference in Johannesburg from 9-11 October, where Steph presented interim results of this research and what this means for the conservation and management of this species and its habitats.
The ongoing research project entails tagging of these unusual shark-like rays with either an external satellite tracking device, or an internal acoustic telemetry transmitter. An array of acoustic receivers stretching from Bazaruto southward all the way to the southern cape of South Africa detect and log any transmissions within about 400m of the receivers, and through the cooperation of marine researchers who participate in this array network, data is recovered and shared so that scientists can track the movements of their tagged animals.
In the presentation, delivered by Dr Venables, she unpacked the results of the analysis of acoustic tagging data from 18 wedgefish tagged within the Sanctuary and shared some very interesting results. Fortunately the presentation was recorded so that we can share it with the Sanctuary community:
Interesting highlights from the research included:
- All 18 wedgefish tagged in the Sanctuary (randomly caught by line fishing or diving) were female. Scientists don't yet know why.
- Wedgefish have been observed using certain Sanctuary reefs as cleaning stations, having parasites and dead skin removed by fish like cleaner wrasses. This has not yet been documented.
- Aggregations of up to 30+ individuals have been recorded din the Sanctuary - also not recorded elsewhere to scientists' knowledge.
- Tagged females showed very high residency rates, clear seasonality preferring the cooler winter months, and clear patterns in how they use different reefs: some are used only during the day, others only at night, and some equally day or night.
What do these results mean? It's a little early to say for sure, but they certainly provide us with some clear conservation guidelines. For starters, if we use wedgefish as an umbrella species for other sharks and rays - apex predators essential for healthy, functional ecosystems - increasing protection measures in the Sanctuary will bring enormous value. Measures under consideration are declaring the Sanctuary a Shark & Ray Protection Zone, prohibiting all sharks and rays from being caught; extending the sanctuary's boundaries eastward so that critical reef systems are brought under protection; and introducing targeted education to local artisanal and sports fishers of the ecological significance of sharks and rays and more importantly how they contribute to healthy and thriving fisheries.
