By Dave van Beuningen
July 29, 2019
The Western Indian Ocean (WIO) is a hotspot for sharks and rays. It’s home to over 220 species, including many that are found nowhere else on earth. Today, though, they are facing tremendous threats, primarily from fisheries. Just over a quarter of the species in this area are classified as either Critically Endangered, Endangered or Vulnerable by the International Union for the Conservation of Nature (IUCN).
And that’s just the ones we know about. Many of the sharks and rays in the WIO region are understudied. In fact, we don’t have enough information on 27% of the species there to even determine whether or not they’re in trouble.
This presents a considerable challenge for conservation. To adequately protect these animals, we need to know more about where they live and how they interact with their environment, such as which shark and ray species use certain habitats during different times of year. We’ve set out to collect this data using a common sampling technique called baited remote underwater video (BRUV).
A BRUV system consists of a metal frame with a bait canister at one end, and then either one (mono-BRUV) or two (stereo-BRUV) cameras facing the bait canister. The bait canister is filled with mushed up fish, and the BRUV is lowered to the seabed where it rests for an hour. The bait releases a strong scent, which attracts nearby sharks and rays, and the cameras film whatever swims past. Through the BRUV sampling technique, scientists are able to determine the different shark and ray species present in an area during different times of the year.
At the Wildlife Conservation Society (WCS), we’ve been conducting BRUV surveys since 2017 in four different countries in the WIO — Kenya, Tanzania, Mozambique, and Madagascar. So far, the results tell an unpleasant story. We see a low abundance of sharks and rays and a low level of diversity in many of the areas we’ve sampled.
In northeast and southwest Madagascar, Kenya, Zanzibar and Pemba (northern Mozambique), sharks and rays were recorded in fewer than 10% of our BRUV deployments. Things were only slightly better in northwest Madagascar (16%). Still this is considerably lower than what we would expect to see on a long stretch of coastal reef like this. One exception was southern Mozambique. Thirty-five percent of our deployments recorded a shark or ray there.
Even this pales in comparison to well protected areas, such as the iSimangaliso Wetland Park MPA (South Africa), and more remote areas, such as the Aldabra Atoll (Seychelles), which recorded at least one shark or ray in 61% and 91% of deployments, respectively.
The results from these BRUV surveys indicate that something is seriously wrong in much of the WIO. In a healthy marine ecosystem, one would expect to find a high abundance and diversity of shark and ray species. This is because, if the reefs are healthy, they will provide a high abundance and diversity of prey species. A lack of sharks means either their prey has been fished out and they have moved elsewhere, or the sharks themselves are being fished out. In this way, sharks can act as indicator species, providing information on the status of the ecosystem and its food web.
Our hope is the BRUV surveys we’re conducting can contribute valuable data toward improved conservation, particularly in the creation of Marine Protected Areas (MPAs). Knowing where different species go during important moments in their lifecycle — such as during breeding events or while juveniles inhabit nursery areas — can contribute to the design of more effective MPAs.
Our next step is to implement long-term BRUV monitoring in selected areas in each of the four countries, to collect information on seasonal trends in abundance and diversity. This information will go a long way to informing management and improving MPA design and suitability for sharks and rays in the WIO region.
Dave van Beuningen is the Shark and Ray Conservation Assistant for the Wildlife Conservation Society’s Madagascar & Western Indian Ocean Shark and Ray Program. Rhett Bennett, WCS Shark and Ray Conservation Officer, also contributed.