New study reveals for the first time how environmental variability influences elasmobranchs’ distribution in the Western Indian Ocean (WIO) region

The Western Indian Ocean region (WIO) is considered one of four global hotspots for endemic and threatened elasmobranchs. The Mozambique Channel is one of the most important migratory corridors for marine megafauna in the entire Indian Ocean. 

A new scientific paper published in Frontiers in Marine Science, details the first study ever that examines spatial, temporal, and environmental factors influencing elasmobranch reef habitat use in southern Mozambique, a critical migration pathway for apex megafaunal species. The results yield important behavioral information highlighting specific reefs off the shores of Mozambique’s southern part of the Inhambane province as critical habitats for elasmobranch species. The findings on sharks’ and rays’ long-term movement and habitat use patterns can help inform conservation strategies.

“All the species monitored were significantly more abundant around very specific reefs in the surveyed region. It was also found that the numbers of different species’ presence depended on tidal regimes and ranges, moon illumination, and seasonal water temperature changes.”said Dr. Mario Lebrato, BCSS Chief Scientist and co-author of the report, led by Ph.D candidate Mr. Calum Murie, Director of Underwater Africa.

Long-term time-series studies are critical to truly understand and identify connectivity to environmental variability as well as habitat preferences.

Robust statistical analyses revealed that such spatiotemporal and environmental parameters influence the reef visits of species in closely related elasmobranch genera in similar ways. Analyzing visual scuba transects and video recordings on 16 reef sites over a four-year period, the team concluded that three mobula ray species formed a behavioral group, while other three shark species formed another.

Stingray (photo credit: BCSS/Orlando Miranda & Salvador Clovee)

Mobula raysvisits occurred most in the southern region of the study area, during specific months at time of full or new moons. Visits of reef manta rays (M. alfredi) and short fin devil rays (M. kuhlii) were recorded most frequently in October, while oceanic manta rays (M. birostris) visited most in May.

Meanwhile, shark visits were registered most often on a seasonal basis. Whitetip reef sharks (T. obesus) and grey reef sharks (C. amblyrhynchos) were spotted during warm water seasons, while zebra sharks (S. tigrinum), visited when water temperatures were the coldest.

Worryingly, out of 21 elasmobranch species encountered 76% are classified as either globally vulnerable, endangered or critically endangered under the IUCN Red List, with only one species being nearly threatened, one least concern, and three data deficient. None of them have been assessed for the regional conservation status.

Grey reef shark (BCSS/Orlando Miranda & Salvador Clovee)

Being able to group elasmobranch species based on the factors that influence their patterns of reef use is useful when considering how their communities should be managed. Prohibiting fishing of these species around the reefs where they gather and adjusting fisheries regulations respective to their movement and habitat use could greatly benefit the regional elasmobranch community.

Sharks and rays in the WIO region play a significant role in small-scale fisheries, providing food security and local livelihoods to vulnerable coastal communities who are highly dependent on the cash income, including from the sale of fins, and protein they provide. Inevitably, coastal fisheries regularly catch elasmobranchs, leading to overexploitation. Almost a quarter of shark and ray species in this region are classified as threatened on the IUCN Red List of Threatened Species.

Dusky shark (BCSS) and bull shark (BCSS/Daniel Escayola)

Unfortunately, a major obstacle to their successful management and protection is the current lack of ecological information for many species, which underlines the importance of this study. Population declines have been reported for manta rays in Mozambican waters, but few studies have focused on other elasmobranch species, even though the region is classified as one of the world’s Important Shark and Ray Areas.

Group of pelagic sharks (Carcharhinidae) (Jakob Owens/Unsplash)

BCSS is currently analyzing decadal time-series of megafauna sightings of over 50 species (including sharks and rays) in the Bazaruto Archipelago – Ocean Observatory, to understand patterns against weather, environmental change, and habitat pressure. Another recent study, published in Nature Scientific Reports with BCSS’ Ocean Observatory involvement in research logistics and data collection, yielded critical migratory information that highlights the importance of waters in the Inhambane province and the Bazaruto seascape as critical habitats for oceanic blacktips (C. limbatus) and bull sharks (C. leucas).

Such efforts to understand elasmobranchs’ movement and habitat use patterns allow regional managers to refine protection strategies and enhance policy on fishing regulation. Previous studies have demonstrated that such combined efforts can mitigate threats to these animals and local fishing communities alike, so these animals can reproduce and socialize in peace.

“Murie CJG, Oliver SP, Gavard L, Lebrato M, Brown J and Lawrence A (2024) Environmental factors modulate the distribution of elasmobranchs in southern Mozambique. Front. Mar. Sci. 11:1408727. doi: 10.3389/fmars.2024.1408727

More information:

Read the full paper published in Frontiers in Marine Science.

For questions about this article, please contact:

Dr. Mario Lebrato, Bazaruto Archipelago – Ocean Observatory Bazaruto Center for Scientific Studies (BCSS)
mario.lebrato@bcssmz.org

Bazaruto Center for Scientific Studies
Host of the first permanent Ocean Observatory focused on multi-ecosystem time series research in Africa, the Bazaruto Center for Scientific Studies (BCSS) was established in 2017 as in independent, non-profit organisation with a mission to protect and support the fragile ecosystems of the Bazaruto Archipelago, Mozambique. The research station is located on Benguerra Island, off the coast of Mozambique.
Scroll to Top

Stay in the know


Receive monthly updates about our research, projects, new papers and spectacular wildlife encounters, straight into your inbox.