Surveys suggest that coral cover around the tiny island of Mnemba has declined by nearly 90% over the last two decades owing to climate change and 'overtourism'. A group of South African scientists are trying to reverse the trend.

Listen to this article 7 min Listen to this article 7 min Spectacular coral reefs, clear blue seas and white sand beaches are some of Zanzibar's major drawcards, helping to attract more than 100,000 tourists every year.

But the financial benefit of this increasing tide of "overtourism" is putting further strain on the colourful reef attractions that are already getting battered by the impact of climate change.

South African marine scientist Dr Camilla Floros notes that there was an "unprecedented warming event" along sections of the East African coastline earlier this year, when sea water temperatures rose to 31°C to a depth of 30m.

These unusually high temperatures, which lasted for nearly a month, are bad news for many corals that can only thrive within a narrow temperature band.

Floros says that if such unusually high temperatures persist for more than two weeks, the corals start to "bleach".

(Healthy corals derive their colour from the algal species that live in their tissue - but when water temperatures get too high the stressed coral polyps start to expel the algae, exposing their white skeletons).

Simultaneously, the corals also lose their ability to obtain food from the algae, eventually killing the coral...