https://www.sundaytimes.lk/241110/news/sudden-algal-bloom-dealt-death-blow-to-trincos-triggerfish-577483.html published on SundayTimes on 10.11.2024
Thousands of dead triggerfish found washed ashore on Trincomalee beaches likely died due to factors associated with an algal bloom, according to scientists.
Trincomalee is renowned for its serene beaches, but certain areas recently turned foul-smelling due to thousands of decaying fish washing ashore and floating in the shallow bays. Onlookers speculated various causes, from pollution and temperature changes to an impending natural disaster or even a bizarre case of mass fish suicide.
The fish die-off occurred between October 29th and November 1st, coinciding with Halloween week, sparking eerie rumors of supernatural occurrences. The affected species, the Red-toothed Triggerfish (Odonus niger), is named for its distinctive red teeth visible when the mouth is closed. The sight of these fish might easily inspire Halloween-themed stories, perhaps invoking images of Dracula-like creatures.
However, the mass death of triggerfish in Trincomalee is linked to a natural phenomenon – a sudden algal bloom, explained Dr. Sujeewa Athukoorala, a Senior Scientist at the National Aquatic Resources Research and Development Agency (NARA). Analysis of satellite imagery from the affected period revealed a high-density algal bloom in Trincomalee’s waters. The thick algal layer likely caused oxygen depletion in the fish’s habitat. Additionally, the dense algae can clog the gills of triggerfish, impairing their respiration. This combination of low oxygen and gill blockage led to the mass mortality event, according to Dr. Athukoorala.
Sri Lanka experienced heavy rainfall throughout October, and the Mahaweli River, which flows into the sea at Trincomalee, likely brought large volumes of freshwater and nutrients into the marine environment. Torrential rain often carries excess nutrients, potentially triggering algal blooms, Dr. Athukoorala told the Sunday Times.
The Red-toothed Triggerfish typically inhabits seaward coral reefs with strong currents, often forming aggregations while feeding on zooplankton and sponges. Juveniles are found in isolated rubble patches or crevices that provide suitable shelter. The species can grow up to 50 cm, with an average length of 30 cm. “During this mortality event, the sampled fish were juveniles, measuring between 10 to 15 cm in length,” noted Dr. Athukoorala.
Triggerfish called as ‘pothubari’ in Sinhala are more environmentally sensitive than many other marine species, making them particularly vulnerable to changes in their surroundings. A similar mass mortality event involving mainly triggerfish occurred in the Maldives in 2016, linked to high sea temperatures, coral bleaching, and subsequent algal blooms, resulting in oxygen depletion.
In the case of Trincomalee’s fish die-off, temperature data from the surrounding area indicated an average of 30°C during the three-day period. Since no sudden spikes in temperature were observed, a temperature change is unlikely to have caused the mass mortality, Dr. Athukoorala confirmed to the Sunday Times.
Red-toothed Triggerfish (Odonus niger), is named for its distinctive red teeth visible when the mouth is closed © Waikikiaquarium.