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Three incidences of bloom (Noctiluca scintillans) has been recorded in Gulf of Mannar (GOM) during 2019(September) 2020(September& October) and 2021 (October). On 10th September 2019, a deep green bloom started along the coast of Gulf of Mannar stretching from Kunthukkal(9.2579° N, 79.2213° E) to Vedalai (9.2723° N, 79.1040° E) (about 10 kms) and algal crashing started on 11th and 12th September, 2019, resulting in mass mortality of about twenty wild fish groups mostly associated with coral reefs. The ichthyo-diversity affected during the bloom period have been identified and quantified. Noctiluca cell density was high ranging from 1.33 x 103cells/L to 1.24 x106cells /L with slight year to year variations. High chlorophyll concentrations (> 7.5 mg /m3) were measured during the bloom event at the cage site. Remote sensing images acquired from MODIS –Aqua 4 km data during the bloom period indicates very high concentration of chlorophyll in most of the northern areas of GOM indicating the presence of algal bloom. Low concentration of dissolved oxygen (less than 3.5 mg/L) was measured during the bloom in all the three years. Concentration of ammonia in surface and sea bottom waters was quite high 7.54 & 18.86µg-at/L. Wild fishes in the coral reefs and fishes cultured in cages (Cobia and Silver Pompano), were found dead due to acute hypoxia and anoxia. Decay of the bloom led to enormous ammonia production, sudden reduction of dissolved oxygen in the water and, eventually stress, shock and mortality of all fishes.

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