Saving the WIO’s Reef Giants

SAVING

the WIO's Reef giants

Urgent Conservation Action Needed for WIO's Large-Bodied Fishes

Our ‘long swim’ study on the protection status of the western Indian Oceans large-bodied fish was published online in January 2025 in Conservation Biology. The study, led by CORDIO Director Melita Samoilys, was conducted with a team of international researchers and spanned seven countries. Key findings reveal significant threats to species like sharks, Giant groupers, Humphead parrotfish and green Humphead wrasse, which are crucial players in coral reef ecosystems. With overfishing and climate change mounting pressure, even marine protected areas (MPAs) are struggling to safeguard these populations. This study calls for urgent, species-specific protections and enhanced regional collaboration for effective conservation.

Background

Large-bodied reef fishes, such as predator sharks and the Giant grouper, and the Humphead parrotfish and Humphead Wrasse, play a critical role in supporting coral reef productivity and maintaining ecological balance. They help regulate populations of lower trophic groups, preserving the health and structure of reef ecosystems. However, these species are in decline globally, particularly in the Western Indian Ocean (WIO), where monitoring and conservation efforts face challenges due to limited resources. Key threats include overfishing, illegal fishing gears and ineffective protected areas. The WIO, a biodiversity hotspot, also lacks sufficient population data for these species, with implications for conservation assessments, such as the IUCN Red List, and regional biodiversity goals. Certain large-bodied species, like the Humphead wrasse and Green Humphead parrotfish, are already listed as vulnerable or endangered, yet comprehensive data from the WIO are scarce, hindering effective management and awareness of their population status. Climate change and rapid population growth in the region further compound the threats facing these fishes, particularly as artisanal fisheries become more prevalent.

Method

To address data gaps on large-bodied fish populations in the WIO, the researchers developed a rapid survey method using underwater visual census (UVC) techniques. They conducted timed long swims on SCUBA to estimate fish densities and biomass across seven regions, including protected and unprotected sites. Survey locations spanned 27 sites in seven countries, covering areas with various levels of marine protected area (MPA) enforcement, from unmanaged sites to those with strong protection and restricted access, such as the Chagos Archipelago and Iles Glorieuses. The team assessed population densities of ten targeted fish groups—both pelagic, demersal and large bodied single species—focusing on those over 55 cm in length. They also compared data across protection levels using zero-inflated negative binomial (ZINB) modeling and other statistical analyses to evaluate the effectiveness of the protection index on fish densities.

Click the PDF below to read the full paper.

https://doi.org/10.1111/cobi.14430

Key Findings

The study revealed a concerning local decline of large-bodied reef fishes, including sharks, Giant grouper, Humphead parrotfish, and Humpead wrasse, across coral reefs in the Western Indian Ocean (WIO). Only one reef in northern Mozambique showed the presence of reef sharks, while the Chagos Archipelago, where protection from fishing is strong, lower than normal shark biomass was recorded. This indicates a broader trend of depletion across several important species and suggests that regional protections may not be effective enough to curb these losses.

Similarly, populations of pelagic species like tuna and barracuda appear depleted, with multiple zero observations outside Chagos. Tuna, being highly valuable in commercial fisheries, faces intense fishing pressure and the effects of climate change, while limited data on barracuda restricts a clear understanding of their population dynamics in the WIO. The survey found that three particularly vulnerable species—the Humphead parrotfish, Humphead wrasse and Giant grouper—were rarely observed, underscoring their precarious state in the WIO. In many WIO countries, limited awareness and sparse Red List data on such species impede regional conservation efforts.

Marine protected areas (MPAs) were determined to vary in effectiveness. Only remote, highly protected areas, such as parts of the Chagos Archipelago, showed higher biomass levels for some large-bodied species. Even so, protection effects were inconsistent; species like the Humphead wrasse did not appear to benefit significantly from current levels of protection. This highlights the limitations of isolated protections and calls for more comprehensive management strategies.

Recommendations

Declines in large-bodied reef fish populations carry significant ecosystem consequences, as these species play essential roles in coral reef health and resilience. Effective MPAs show greater benefits when they are complemented by strong surrounding fisheries management practices. For instance, gillnetting, a widely used and unselective fishing method in the WIO, captures various reef-associated species, including vulnerable species like snappers, groupers and parrotfish. A reduction in gillnet usage, particularly in MPA buffer zones, could help alleviate pressure on these ecosystems.

This study calls for urgent, species-specific protections for the Humphead parrotfish, Humphead wrasse, and Giant grouper. Expanded protections and enforcement efforts in high-level MPAs, alongside targeted restrictions, are critical steps for effective conservation. Integrating monitoring methods like the timed UVC swim into standard coral reef assessments, as well as increasing the use of BRUVs, would improve data collection on large-bodied species across the region. Countries in the WIO must further increase local awareness of the Red List and prioritize data collection for regional endangered species.