Underwater ‘Listening Stations’ Track Reef Manta Rays In Mozambique
A new study by researchers at the Marine Megafauna Foundation (MMF) has used acoustic telemetry to reveal critical information about the habitats and movement patterns of reef manta rays (Mobula alfredi) in the waters around Mozambique. The study, conducted with scientists from the MMF, University of Western Australia and Utrecht University was published on 23 January in the journal Marine Ecology Progress Series.
Tracking the movement of manta rays is not an easy task - they can range over thousands of kilometres, dive hundreds of metres deep and don't need to surface to breathe. Understanding where they go and how long they spend at any given destination, however, is vital for establishing protections for reef manta rays, which are listed as ‘vulnerable’ and decreasing on the IUCN Red List of Threatened Species.
Tracking the movement of aquatic animals is accomplished through the use of telemetry, or 'tagging'. A transmitter is fitted to the animal which can relay data to either a satellite or underwater receiver - also known as a 'listening station.' Since manta surface only rarely and their tags would be unable to communicate regularly with a satellite, acoustic tagging is the preferred method of tracking.
The researchers tracked 42 reef manta rays were between 2010 and 2014, using an array of underwater listening stations placed at 14 sites over a 250km stretch of Mozambique's southern coastline, from the Bazaruto Archipelago to Tofo Beach and Závora. The sites ranged from cleaning stations, which manta rays are known to regularly visit, feeding areas, and reefs that were thought to be used by the manta for navigation. Each tag emits a unique acoustic signature, which is detected and logged by the listening stations whenever a tagged animal comes within a few hundred meters. Researchers would collect the receivers every few months to download and analyse the stored data.
MMF scientists have been studying the manta rays around Mozambique for over a decade using a range of techniques, including photo identification and aerial surveys. Tracking a manta's movement by photo ID presents an obvious challenge as somebody has to see the manta and take a photograph of the spot-patterns on its underside - a process that is inevitably somewhat hit-and-miss.
'Long-term photo ID studies have provided excellent preliminary data about the manta rays that visit this coastline, however, acoustic telemetry allowed us to continuously monitor multiple individuals for periods of up to 2 years, providing us with more detailed movement data,' explained lead author Stephanie Venables, a researcher with the Marine Megafauna Foundation and PhD candidate at the University of Western Australia. 'Understanding how manta rays use these waters, how far they move and where they spend the majority of their time is vital information for designing effective protection measures,' she added.
The researchers found that cleaning stations, where small reef fish remove parasites from the mantas, proved to be an important habitat for the tagged animals, with 95 per cent of all detections recorded at such locations. The study also found that manta rays follow something of a routine, visiting cleaning stations more frequently during the day, for an average time of 26 minutes. One-quarter of the tagged individuals stayed for over three hours at the cleaning stations, with the longest visit spanning over eight hours. The manta returned to favoured sites on multiple occasions - in some cases after 7 months - a behaviour known as 'site affinity'
The study also found that during the night, the manta rays were almost entirely absent from the monitored reefs, with the exception of one site where feeding behaviour was observed. 'It’s likely that mantas are moving into deeper, offshore waters outside of the detection range of the acoustic array at night. We expect that these waters are feeding grounds for the manta rays,' said Daan van Duinkerken, MMF researcher and co-author on the study.
Mozambique's manta ray population is known to be threatened by human activity. A 2013 study by the MMF documented an extremely worrying 88 per cent decline in reef manta ray sightings over a single decade in the Inhambane Province, highlighting the need for effective conservation measures in areas where the manta aggregate.
'Manta rays face a number of human-induced threats throughout their range including targeted fishing (for their gill plates), incidental capture (by-catch) and entanglement in fishing gear,' said Stephanie Venables. In Mozambique, gill nets — large nets that stretch from the seabed to the surface and can be hundreds of metres in length — are a major threat to many marine species including sharks, rays and turtles, which become easily entangled in the mesh.
The researchers used the tagging data from the study to identify areas that are most frequently used by manta rays, crucial information for estimating the species' range and designing 'spatial management' initiatives such as marine protected areas (MPAs). The team found that three-quarters of manta ray habitat around Mozambique is unprotected. 'Offering protection to a mobile species across its entire range is both logistically and financially demanding,' said Dr Andrea Marshall, co-founder and principal scientist of MMF’s global manta ray program. 'Protecting core habitat areas, where the animals spend most of their time, is a good starting point. These typically include feeding and breeding grounds, and in the case of manta rays, cleaning stations.'
The study found that manta rays move widely between the studied high-use sites and the scientists believe that restricting the use of indiscriminate fishing gear, like gill nets, would greatly lessen the risk to manta rays - and other species - as they travel. 'We hope this knowledge can be used to develop more effective protection for manta rays in Mozambique, in the areas where they need it most,' concluded Dr Marshall.
The original version of this article is published on the MMF website
The study entitled 'Habitat use and movement patterns of reef manta rays Mobula alfredi in southern Mozambique' was published in the journal Marine Ecology Progress Series on 23 January 2020 and is available here: https://doi.org/10.3354/meps13178.
MMF’s field research in Mozambique is supported by Peri-Peri Divers, Casa Barry Lodge, Tofo Scuba, Pestana Bazaruto, Big Blue, Zavora Lodge, J. Wright and Y. Tibirica, Pomene Lodge, Paindane Dive and D. Steuber.