Scientists from the Marine Megafauna Foundation (MMF) have placed acoustic and pop-up archival satellite tags on two species of wedgefish, the bottlenose wedgefish (Rhynchobatus australiae) and the bowmouth guitarfish (Rhina ancylostoma), kickstarting a first of its kind study for these species in Mozambique.
Bottlenose wedgefish and bowmouth guitarfish are listed as Critically Endangered by the IUCN Red List of threatened species. Both are species of ray and members of the family Rhinidae, collectively known as wedgefish, a group recently identified as one of the world’s most endangered marine fish.
Wedgefish are characterised by a life history of slow growth, late maturity, and low reproductive rates, making them particularly susceptible to population decline from overexploitation. They are captured across the globe in commercial and artisanal fisheries using various types of fishing gear including nets, trawls, longlines, and handlines. They are either targeted or retained when caught as bycatch, as their fins are highly valued in the shark fin trade. Very little is known about the biology or ecology of these rays and consequently few management plans exist to protect declining populations worldwide.
The study is taking place in the protected waters of the Bazaruto Archipelago National Park and the Vilanculos Coastal Wildlife Sanctuary. MMF researchers are working alongside park authorities and managers to identify primary aggregation sites for these species, understand their movements and home range, and identify the threats they may face in the region.
The two types of tag provide different, yet complementary, information that can be used to investigate fine and broad-scale movements. “By using this particular combination of tags, we can learn where the animals spend most of their time, whether visits to specific sites are year-round or seasonal, how far they move, how deep they dive, and which temperatures they prefer” explains Dr. Andrea Marshall, MMF co-founder and co-lead of the project. “This will help to identify areas of critical habitat that must be prioritised for protection.”
Acoustic tags are small transmitters that emit a unique acoustic signature, which is detected by an array of underwater acoustic receivers or ‘listening stations’ whenever a tagged animal comes within a few hundred meters. The Mozambican acoustic array spans from the Bazaruto Archipelago to Ponta do Ouro on Mozambique’s southern border and is managed collaboratively by MMF, Oceans Without Borders, and South Africa’s Oceanographic Research Institute. The receivers store the information from tags and researchers collect the receivers every few months to download the data and see which of the tagged animals have been detected. The batteries on acoustic tags last up to 5 years, providing valuable long-term insight into the movement patterns of tagged species.
Pop-up satellite archival tags (or PSAT/miniPAT tags) are attached to the dorsal fin of the wedgefish and record depth, temperature, and light-level data. Light-level data is used to determine the location of the animal through ‘light-based geolocation,’ where sunrise and sunset times are used to predict location. Data are collected and summarised in the internal memory of each tag, ready to be transmitted. These particular tags are programmed to stay attached to the wedgefish for 6 months, before detaching, floating to the surface, and transmitting a summary of the archived data back to scientists via the ARGOS satellite network. These tags also provide valuable information on vertical movements (diving behaviour), depth, and temperature preferences.
With so little known about wedgefish and their relatives globally, researchers are intrigued to see what these tags will show. “We are very excited to see what the tags can tell us about these curious animals,” says Dr. Marshall. “With such little information available, we truly aren’t sure what to expect. Wedgefish could easily be compared to the pangolin of the ocean. They are rare, elusive, critically endangered, and fished intensively for their fins. Just like pangolins, they require urgent protection. No doubt the data from this study will give us a better understanding of how to better protect these species in Mozambican waters — a global stronghold for wedgefish.”
“We are proud to be able to participate in and benefit from this groundbreaking research,” says David Gilroy, General Manager of the Vilanculos Coastal Wildlife Sanctuary. “We rely heavily on the input of MMF scientists to inform the management of our protected area and the local environment at large, and we hope that the data shared with us in time will help us to better protect and safeguard these incredibly important species and their habitats.”
The MMF team in Vilanculos will continue to deploy additional tags in the coming months. The work is supported by the Blue Action Fund, the Mohamed bin Zayed Species Conservation Fund, Ocean Wildlife Project, and a number of private donors.
Header image: Marine Megafauna Foundation.