Devastating impact of Noctiluca scintillans ((Macartney) Kofoid & Swezy, 1921) bloom on marine fishes in the Gulf of Mannar, India

The Gulf of Mannar, located on the southeast coast of India experienced a severe harmful algal bloom (HAB) caused by the dinoflagellate Noctiluca scintillans, in October 2021. This bloom had a devastating impact on marine fisheries and biodiversity affecting the regions from Pamban to Mundal. This study revealed significant temporal variations during the occurrence of the N. scintillans bloom. The average sea surface temperature (SST) ranged from 28 to 29 °C, while salinity varied between 38 and 36.5 psu. Chl-a levels ranged from 8.12 to 17.5 mg/m3. The cell density scintillans ranged from 3.27 × 106 to 2.83 × 103 cells/L across all stations. Post bloom observations revealed mass mortality of marine organisms including finfishes, shellfishes, sea anemones, sea hares, sea horses, and polychaetes. Fish mortality was significant, with a total estimated weight of 7.2 tonnes among these 0.96 tons of commercially important fish valued at US$ 5.5 M. The bloom had a particularly detrimental effect on two damselfish species Dascyllus reticulatus and D. trimaculatus. The intense N. scintillans bloom killed 0.83 tons of damselfish, equivalent to 9 million fishes. The increasing frequency of HAB incidences in the region is likely attributed to environmental changes. Severity of the blooms highlights the urgent need for continuous monitoring and early-warning systems to mitigate future impacts on marine biodiversity. Developing effective management and conservation polices is essential for addressing these ecological threats.