The leading cause of coral bleaching is climate change. A warming planet means a warming ocean, and a change in water temperature
Understanding the Current Coral Bleaching at Mnemba Island:
Earlier this year, the pristine coral reefs surrounding Mnemba Island experienced a severe coral bleaching event. This phenomenon impacted hard corals, soft corals, and even anemones, reaching depths of 30 meters. Unfortunately, this isn’t the first time coral bleaching has struck Zanzibar’s reefs. Similar events were recorded in 1998 and 2016. The 1998 event was particularly devastating, with coral mortalities reaching up to 80% on some Tanzanian reefs. As of now, the full extent of this recent bleaching is still under investigation, with ongoing monitoring efforts aimed at assessing the potential for recovery.
Coral bleaching occurs when corals lose their vibrant colours and turn white due to the breakdown in the relationship between the coral and their symbiotic algae, known as zooxanthellae. These algae not only give corals their striking colours but also provide essential food through photosynthesis. However, this delicate relationship is highly sensitive to stress, particularly rising sea temperatures. When ocean temperatures become too high, corals expel the algae, leading to bleaching. The primary driver of these increased sea temperatures is climate change. If sea temperatures return to normal quickly, corals can recover. But if the heat persists, corals may starve and die without the algae to provide sustenance.
We need to respect the oceans and take care of them as if our lives depended on it. Because they do
~ Dr Sylvia Earles, Marine Biologist and oceanographer.
The bleaching event at Mnemba was not isolated; it was part of a much larger mass bleaching event that affected all three major ocean basins—the Atlantic, Pacific, and Indian Oceans. This marked the fourth global coral bleaching event on record. Other countries along the East African coast, including Kenya, Comoros, Seychelles, Mauritius, and Reunion, also experienced significant bleaching, underscoring the widespread nature of this environmental crisis.
The coral bleaching had a profound impact on the OWB coral nursery near Mnemba Island. Out of 42 tables within the nursery, 36 were bleached. The corals on the artificial reef structures also suffered, with many new coral recruits being severely affected. Despite this, as sea temperatures begin to drop, we are starting to see some signs of recovery, offering a glimmer of hope.
OWB is actively collaborating with the Ministry of Blue Economy, particularly with the Director of Conservation, to understand the impact of this bleaching event on regional reefs. OWB is also developing a bleaching response strategy, which will include actions to reduce stress on the reefs, promote recovery, and build resilience against future bleaching events. The global scientific and diving communities have expressed shock and dismay at the severity of this event. With unprecedented sea temperatures recorded in the region, scientists and reef conservationists are closely analysing the bleaching data to assess the extent of the damage.
In response to the bleaching, OWB has been conducting extensive surveys of the reefs, coral nursery, and artificial structures. Many coral species, particularly branching types like Acropora and Pocillopora, have been significantly bleached, and their full recovery remains uncertain. Encouragingly, some corals are showing signs of resilience, sparking hope for the future. Moving forward, OWB plans to establish a deeper water coral nursery, providing a refuge for corals during heat stress events. This nursery will focus on growing climate-resistant corals that can be transplanted to artificial reefs, promoting long-term resilience.
While it is still too early to fully assess the long-term impact of the bleaching on the Mnemba reefs, there is concern that significant coral loss could lead to changes in fish populations. Such changes could, in turn, affect local fishers and tourism operators, potentially impacting livelihoods and the local economy.
Collaborations and Partnerships:
OWB is not tackling this challenge alone. We are working closely with the Ministry of Blue Economy to develop a robust strategy for addressing future bleaching events, which are expected to become more frequent and severe due to climate change. We are also engaging with other organizations involved in reef restoration around Zanzibar and with local dive operators to mitigate the impact of future bleaching by reducing local stressors on coral reefs, such as overfishing and unsustainable tourism practices.
This event serves as a stark reminder that as climate change continues, coral bleaching events will likely become more frequent and intense, posing a serious threat to the millions of people who rely on the goods and services provided by coral reefs. However, it’s crucial to remember that coral bleaching does not always result in coral death. If the stressors driving bleaching can be mitigated, corals have the potential to recover. This requires not only global efforts to combat ocean warming but also local initiatives to ensure best practices in coral reef management. Effective collaboration among governments, NGOs, communities, and the private sector will be essential in ensuring that coral reefs can survive and thrive in our changing climate.
The situation at Mnemba House Reef highlights the urgent need for both immediate action and long-term strategies to protect our vital coral ecosystems. Through continued monitoring, research, and collaboration, we can work toward a future where coral reefs remain resilient in the face of environmental challenges.
Wild Impact is a Public Benefit Organisation registered in South Africa. Reg. No.: 930002115