The Good, The Bad and The Algae

“The sea is dark green all the way from Praslin to Cousin”, Christopher Mahoune, a senior Warden on Cousin Island special Reserve told me at round 5 pm on Friday 23rd of October. Christopher said there were dead fish washing up on the beaches of Cousin. I immediately asked our Science Officer Cheryl Sanchez to start listing the species affected and to take samples and observations of the water. The Cousin staff also observed bioluminescence in the water and my first thought was that an algal bloom was taking place. Subsequent conversations with other people confirmed the same phenomenon was taking place on Mahe and as far as Fregate Island. 

Those Blooming Plankton

Algal blooms are rare in Seychelles which is famed for crystal clear waters. Indeed our sea is clear because it generally lacks nutrients. Sometimes events that are man-made, natural, or caused by climate change can cause an upsurge in nutrients which may lead to algal blooms. For about three days in August 2003 the sea around the inner Seychelles islands including Praslin, La Digue and Mahe turned a green color. This algal bloom was caused by an increase in naturally occurring phytoplankton in the sea. The phytoplankton were caught in bays, where they used up all the nutrients in the water and died.

Plants That Kill

Phytoplankton are microscopic plants that float in the sea and form the base of many marine food webs. Phytoplankton is made up of various organisms known as blue-green algae, dinoflagellates, diatoms and green algae. Phytoplankton or algal blooms are defined by experts as those, which are noticeable, particularly to general public, directly or indirectly through their effects such as discoloration of the water, foam production, fish mortality, or toxicity to humans.

Red Doesn’t Mean Dead

Not all algal blooms are toxic. A species that is very common in the Western Indian Ocean is a blue green algae with the scientific name of Trichodesmium. They can form very large blooms with a reddish brown color. They have been mistaken for the infamous “red tides” which occur in some other countries and are extremely toxic. Trichodesium blooms are not toxic but if trapped in bays or enclosed areas they deteriorate and can use up all the oxygen causing death in fish and some other marine organisms.

The HABs And The HAB Nots

Toxic algae do occur in our area. Toxic algal blooms are internationally known as HAB (Harmful Algal Blooms). Several species of naturally toxic algae have been collected from the Seychelles including Alexandrium, Dinophysis and Gymnodium. However, blooms of these algae have not been recorded in our waters. The International Oceanographic Commission (IOC) has produced a very good manual on the harmful algae of the Western Indian Ocean and this has been of great use to many of the countries. The Seychelles has benefited from research and training in HABs through a worldwide programmme of the IOC. In the 1990’s technicians from several agencies including the Seychelles Fishing Authority were trained by Swedish and other experts. The Swedish International Development Agency (SIDA) provided equipment as well. The best facility in the region is located in La Reunion where world class toxicologists, like my colleague Jean Pascal Quod, work.


The authorities say they have identified the plankton species. But we don’t know yet whether the fish kills were caused by lack of oxygen or toxicity. All the dead fish encountered by Nature Seychelles scientific staff had a death rictus of open mouth and bulging eyes. No skin lesions were observed. However, most fish showed expanded abdomens and had a very bad smell suggesting bacterial decomposition had started faster than normal. Nature Seychelles science staff identified fish species from 23 Families as well as octopus. Over the two days some 400 individuals of convict surgeonfish were encountered on the beaches of Cousin Island– a very significant and alarming death toll.

Don’t Eat That!

Sarah Friar-Torres, Nature Seychelles chief scientist with the Reef Rescuers team, has told me that people exposed to green water or to sea spray during the event did not report coughing or itchy eyes. Sarah also heard of scattered (but unconfirmed) reports of fishermen collecting the dead fish with the purpose of eating them; one dog at the Anse Kerlan beach in Praslin was said to have eaten a dead fish and allegedly got sick from it. The water seemed cold according to various people, although the temperature was not taken, perhaps suggesting that deeper waters bringing nutrients to the surface had been churned up. Sarah looked for satellite images in the visual spectrum but those didn’t show anything out of the ordinary. She is now searching for chlorophyll sensor satellite images.

Corals Bleach

A very important observation has been made by veteran Seychellois diver Michael Gardette. His staff diving with students at Conception Island at 10 m depth on Monday 26th found all the corals bleached. Since this is a location where many fish showed up dead during the weekend it may be that the corals were affected by the algal bloom. Indeed, algal blooms can cause rapid changes and declines in the structure of coral reefs. A study in 2008 in the Gulf of Oman, Indian Ocean found similar effects as observed during severe bleaching events

Could it happen again?

Virtually every coastal country in the world is affected by harmful algal blooms. The frequency, locations, and economic impact of HABs have all increased in recent decades. This has been in parallel with, and sometimes a result of increasing development on the coastal zone. Algal blooms are also associated with natural phenomena such as cyclones and events such as the El Niño-Southern Oscillation (ENSO). Algal blooms could become a recurring phenomenon in Seychelles. Whilst it is impossible to build the capacity to deal with every type of issues and events, it is vital that we do have the capacity to understand how to deal with such events and who to contact in a timely manner for assistance

By Dr. Nirmal Jivan Shah

CEO, Nature Seychelles. 

Blue Economy Knowledge Centre,

This email address is being protected from spambots. You need JavaScript enabled to view it. 

Download this file (Algal Bloom Report - Cousin Oct 2015.pdf)Algal Bloom Report[Nature Seychelles' Report on the algal bloom around Cousin Island Special Reserve]894 kB

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