Vanishing giants: The Indian Ocean’s biggest fish need saving (commentary)

I have spent much of my life studying predatory fishes in East Africa’s coastal waters, but never have I been more concerned about their future. The disappearance of the western Indian Ocean’s largest reef fishes is not just an ecological tragedy, it is a crisis that threatens entire coral reefs in this region.

These species — sharks, large groupers, and the humphead parrotfish (Bolbometopon muricatum) and humphead wrasse (Cheilinus undulatus) — are vital to maintaining the balance of coral reef ecosystems. Without them, we risk cascading consequences that harm food security, coastal livelihoods and biodiversity.

My latest research, spanning 27 coral reef sites across seven countries and done in collaboration with scientists from the U.K. and France, paints a stark picture. The population study, which used visual surveys via scuba, recorded many “zero sightings” instances — where once-abundant predatory sharks, tunas, barracudas, giant groupers (Epinephelus lanceolatus) and other large-bodied groupers, as well as the humphead parrotfish and humphead wrasse, were missing.

Overfishing and insufficient protected area management have driven many of these species toward local extinction. Even in marine reserves where protection is legislated, enforcement is often too weak or the size of the reserves is too small. Some of these large-bodied fishes, which include the largest fish in the world, are now globally endangered on the IUCN Red List of Threatened Species.

Growing up along the coast of Kenya, I developed a fascination with marine life that eventually led me to study these reef giants in the Indian Ocean. For more than two decades, I have worked to understand the populations of these species and how we might protect them. While the decline I have witnessed is alarming, I firmly believe there is still hope — if we act now.

Underwater crisis

Large predators play a crucial role in regulating marine ecosystems. They control prey populations, prevent outbreaks of reef-damaging species, and, due to their size (which represents a high accumulation of biomass), signal the productivity of a coral reef. They help maintain the delicate balance of coral reefs, contributing significantly to energy flows through marine ecosystems.

Across the western Indian Ocean, many of these large fish species that once thrived in these waters are now either critically depleted or absent. Even remote regions like the Chagos Archipelago, where protection from fishing is strong, shark abundance is far lower than it should be. Particularly concerning is the near total absence of species like the humphead wrasse, humphead parrotfish and giant grouper. We’re seeing local extinctions unfold in real time, with a guaranteed domino effect.

Ineffective protected areas and overfishing due to lack of enforcement and continued use of destructive fishing equipment are driving these declines in the western Indian Ocean. Many large-bodied species are particularly vulnerable because they grow slowly and take years to reproduce, like groupers, or produce very few young per year, like sharks. Gill nets, which are widely used across the region, are unselective and will capture everything within their mesh size. The largest mesh size gill nets, the size of a large cooking pot lid, are particularly effective at snagging sharks and rays. Even within designated marine protected areas (MPAs), illegal fishing often goes unchecked due to a lack of enforcement.

Compounding the problem is climate change. Rising sea temperatures are leading to coral, death which alters the structure and benthic habitats of coral reefs, with negative impacts on reef fish populations. Growing coastal populations are also increasing the demand for seafood, putting even more pressure on already dwindling fish stocks.

The result is a mix of anthropogenic stressors with the potential to wipe out large-bodied predators and other reef fishes in the western Indian Ocean.

Turning the tide

Despite these challenges, I remain convinced that we can reverse the decline of the Indian Ocean’s big fish. Marine species and ecosystems show great potential for recovery when given the chance. The key is in creating the right conditions for their survival.

• First, we need to expand and strengthen MPAs: my research confirms that the most remote and well-enforced ones still support viable predator populations. However, conservation efforts must go beyond isolated protected zones. We need a network of well-managed MPAs, or in the case of reef sharks, large sanctuaries, to provide these species with the space to recover, and then thrive. In places like the Solomon Islands in the Pacific, reef sharks are abundant due to gill net bans, other fishing restrictions, large closures and sanctuaries. Scientists predict that these measures can, for example, return the depleted reef shark populations in Madagascar to abundance.
• Second, fisheries management can be improved through implementing science-based interventions such as modifying fishing equipment through inclusive and participatory approaches with local fishing communities so they’re central to decision-making processes. This also helps promote compliance with fishing regulations and spurs enforcement patrols. Only in this way will sustainable fishing methods be achieved to both protect marine ecosystems and the livelihoods of coastal communities.
• Third, better data collection is essential. Many species are disappearing before we even fully understand their populations. Incorporating monitoring techniques suitable for large-bodied reef-associated fishes — such as timed underwater visual surveys and baited remote underwater videos (BRUVs) — can help fill these critical knowledge gaps.
• Our new research shows that the rapid timed scuba swims are effective for estimating populations of large-bodied fishes and, importantly, can be easily added to the global standard coral reef monitoring transect method in a mere 10 minutes. The addition of BRUVs for surveying predatory fishes on deeper reefs is also excellent, if video equipment is available.
• Finally, awareness is crucial. Many people are unaware of how rapidly these species are vanishing. Conservation efforts are known to be far more effective when local communities, fishers and policymakers understand the urgency of the situation and are actively involved in solutions.

The decline of the Indian Ocean’s big fish is not just an ecological issue — it is a crossroads. Do we stand by as these species disappear, throwing marine ecosystems into chaos, or do we take meaningful steps to protect them?

This issue isn’t just about protecting fish, but also maintaining a healthy ecosystem, supporting a crucial food source for millions, and sustaining the livelihoods of many coastal communities. If we act now, we can still turn the tide.

 

Melita Samoilys is a coral reef ecologist and director of Coastal Oceans Research and Development in the Indian Ocean (CORDIO), a marine research and conservation organization based in Kenya. She is also a member of three IUCN Specialist Groups (Sharks; Snapper, Seabream and Grunt; Groupers and Wrasses) and wishes to thank her CORDIO colleague Timothy Allela for his help editing this commentary.

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Citations:

Obura, D., Gudka, M., Samoilys, M., Osuka, K., Mbugua, J., Keith, D. A., … Zivane, F. (2021). Vulnerability to collapse of coral reef ecosystems in the western Indian Ocean. Nature Sustainability, 5(2), 104-113. doi:10.1038/s41893-021-00817-0

Samoilys, M., Osuka, K. E., Roche, R., Koldewey, H., & Chabanet, P. (2025). Effects of protection on large‐bodied reef fishes in the western Indian Ocean. Conservation Biology. doi:10.1111/cobi.14430

Samoilys, M. A., Osuka, K., Maina, G. W., & Obura, D. O. (2017). Artisanal fisheries on Kenya’s coral reefs: Decadal trends reveal management needs. Fisheries Research, 186, 177-191. doi:10.1016/j.fishres.2016.07.025

Karkarey, R., Kelkar, N., Lobo, A. S., Alcoverro, T., & Arthur, R. (2014). Long-lived groupers require structurally stable reefs in the face of repeated climate change disturbances. Coral Reefs, 33(2), 289-302. doi:10.1007/s00338-013-1117-y

MacNeil, M. A., Chapman, D. D., Heupel, M., Simpfendorfer, C. A., Heithaus, M., Meekan, M., … Cinner, J. E. (2020). Global status and conservation potential of reef sharks. Nature, 583(7818), 801-806. doi:10.1038/s41586-020-2519-y

Obura, D. O., Aeby, G., Amornthammarong, N., Appeltans, W., Bax, N., Bishop, J., … Wongbusarakum, S. (2019). Coral reef monitoring, reef assessment technologies, and ecosystem-based management. Frontiers in Marine Science, 6, 580. doi:10.3389/fmars.2019.00580

Osuka, K. E., Stewart, B. D., Samoilys, M. A., Roche, R. C., Turner, J., & McClean, C. (2021). Protection outcomes for fish trophic groups across a range of management regimes. Marine Pollution Bulletin, 173, 113010. doi:10.1016/j.marpolbul.2021.113010