- Wedgefish, a type of ray, are some of the least-known and most endangered fish in the ocean.
- A new research project in Mozambique is employing two types of tags, acoustic and satellite, to better understand two of these critically endangered species.
- Researchers aim to uncover the species’ range and habitat requirements to preserve them from extinction.
- Wedgefish are heavily targeted by the shark-fin trade, and their populations have declined precipitously throughout much of their range.
An assessment in 2019 by shark experts identified the little-known family of wedgefishes as among the most threatened marine fish families worldwide. This prompted researchers to roll out a first-of-its-kind study to track two critically endangered species in Mozambique and beyond.
Scientists with the Marine Megafauna Foundation (MMF) are employing two different types of tags, acoustic and satellite, to track the bottlenose wedgefish (Rhynchobatus australiae) and bowmouth guitarfish (Rhina ancylostoma) in a bid to better understand their movements and range in order to increase protections.
“With such little information available, we truly aren’t sure what to expect,” said David Gilroy, general manager of the Vilanculos Coastal Wildlife Sanctuary in Mozambique.
Fitting the tags to specimens in the Vilanculos sanctuary and Bazaruto Archipelago National Park, researchers aim to map the two wedgefish species’ distribution sites and gather information on the threats that may have led to their steep declines.
“Once we retrieve and analyze the data from the tags, we will be able to provide a better indication of the high-use zones and areas of critical habitat (e.g. nursery areas, feeding grounds) for wedgefish in and around these protected areas,” said Stephanie Venables, a senior scientist at the MMF. “This information can be used to guide the design and implementation of future management measures and protection strategies for these species.”
Both fish species are rays and members of the Rhinidae family, collectively known as wedgefishes. The family only consists of 11 species across three genera, with the bowmouth guitarfish the only species in its genus of Rhina.
While the use of tags is standard practice by marine biologists when studying animals including sharks, rays and cetaceans, among others, this project is the first in Mozambique to use the combination of both acoustic and satellite tags to “ground-truth” the location of the species.
Scientist attribute steep declines of wedgefishes in part to the shark-fin trade.
“They are targeted throughout the world for their large, and therefore high-value, fins that are sold in the shark-fin trade,” Venables said, adding that “from the available fisheries data, some species have suffered significant regional population declines.”
Venables cited the bottlenose wedgefish and the whitespotted wedgefish (Rhynchobatus djiddensis), whose populations in the Arabian Sea have plunged by 50-80% over the last 30 years. She also pointed to large-scale declines in Pakistan and Indonesia.
Their presence in shallow nearshore waters makes these wedgefishes an easy target for aggressive coastal fisheries and bycatch. Moreover, wedgefish have low reproductive rates as a result of their slow growth, late maturity, and long gestation periods.
Very little is known about the life history characteristics of wedgefishes, Venables said, adding there’s a general lack of age and growth data, including when the species reach maturity.
“But based on a study on a closely related species, female age at maturity for all larger wedgefish is estimated at 7 years, and maximum age estimated at 19 years,” she said.
Although gestation period for both species being tagged remains unknown, Venables said, the scientists only know that bowmouth guitarfish have litter sizes of two to 11 pups, and bottlenose wedgefish between seven and 19 pups. Researchers estimate the generation length at 15 years for both species.
For the project, the researchers began by fitting the specimens with an acoustic tag, a small transmitter that emits a unique acoustic signature. This signature can be detected by a collection of underwater acoustic receivers already in place whenever they come within a few hundred meters. These receivers stock the information from tags, allowing researchers to collect them every few months to download the data.
Researchers will be able to glean information whenever the animals are in Mozambique’s acoustic array, which spans from the Bazaruto Archipelago to Ponta do Ouro on Mozambique’s southern border.
The batteries on the acoustic tags last up to five years, providing long-term insight into the rays’ movement patterns.
To compliment the acoustic tags, researchers also attached pop-up satellite archival tags (or PSAT/miniPAT tags) to the specimens’ dorsal fins. The satellite tags provide a wealth of information, recording depth, temperature and light-based geolocation, a feature used to determine location based on sunlight.
“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,” said Andrea Marshall, MMF co-founder and co-leader of the project. “This will help to identify areas of critical habitat that must be prioritized for protection.”
These tags are programmed to stay attached to the wedgefish for six months, before detaching, floating to the surface, and transmitting a summary of the archived data back to scientists via the ARGOS satellite network.
Although scientists have documented a number of sites where the species visit regularly through underwater surveys, little is known about how far they move along the coast or offshore. Scientists also don’t know whether they stay in the area year-round or only visit seasonally.
“[And] we do not know where they go when they are not on the reefs,” Venables said. “Our study is designed to provide baseline information on movement and habitat use, so we can better understand the spatial ecology of these species and identify specific areas to be prioritized for protection.”
While bottlenose wedgefish in Mozambique face a lot of fishing pressure, there are populations of the species off the Australian coast that experience very little exploitation. Bowmouth guitarfish, on the other hand, are particularly rare throughout their entire range, with sightings generally infrequent.
Few management plans exist to protect declining populations of the species worldwide, and moreover very little is known about their biology and ecology as they continue to be killed, even when caught as bycatch, for their fins.
“Wedgefish could easily be compared to the pangolin of the ocean,” said Gilroy from the Vilanculos sanctuary. “They are rare, elusive, critically endangered, and fished intensively for their fins. Just like pangolins, they require urgent protection.”
David Ebert, director of the Pacific Shark Research Center at the Moss Landing Marine Laboratories in California, commended the tagging project, noting that it shines a light on conservation of the species.
“I think it is a very timely and important research project,” he said.
The MMF also announced it will continue to deploy additional tags in the coming months to be used in this first-of-its-kind project to better understand, and protect, the wedgefish family.
Banner image: A researcher filming a bowmouth. Image courtesy of Marine Megafauna Foundation.
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