The epizootics of vibriosis caused serious economic losses to farmers. Natural blooms of the pathogen can be prevented by sea cage management measures such as, changing the inner net of the cages, changing the location of the cages to relatively clean water (about 50 m apart) from the affected site and providing shade over the cages while the water temperature rises. Supplementation of the feed with immunostimulants and mineral mixture may be practised to improve the immune response against infection. Early diagnosis and sea cage management measures may prevent occurrences of the infection.

The impact of increase in temperature and light intensity on development and metamorphosis of hatchery produced larvae of silver pompano Trachinotus blochii (Lacepede, 1801) was studied. Two sets of larviculture tanks (Set 1 and Set 2) in triplicate were exposed to two different temperatures and varying light intensities. The growth of the larvae from day one to day 25 post-hatch (dph) and metamorphosis were studied. It was found that an increase of 2°C water temperature and increased light intensity significantly reduced the growth. The percentage of growth reduction gradually decreased and got stabilised towards the end of larviculture with a mean reduction of 15%. Further, a delay of three days in metamorphosis was also observed in Set 2. The reduced growth rate coupled with change in pigmentation of larvae can be considered as the resilience response of the larvae to combat temperature and light stress without compromising survival.

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h i g h l i g h t s• This study presents the first in-situ bio-optical data for the Coastal Waters Gulf of Mannar and Palk Bay.• High temporal and spatial variability in the absorption components were observed in the present work.• The biogeochemistry and phytoplankton biomass of the effect of Trichodesmium bloom on the Coastal Waters Gulf of Mannar and Palk Bay were studied. a b s t r a c tMarine cage aquaculture is gaining importance in India, due to its contribution as an alternate livelihood to coastal communities and also because of its export value. Water quality is the most important determinant for sustainable marine cage farming. Nutrient enrichment in coastal waters results in increased occurrence of algal blooms. A mariculture practice makes the coastal waters eutrophic due to increased input of nitrogen and phosphorous, ultimately leading to bloom. A phytoplankton bloom dominated by Trichodesmium species was detected outside mariculture cages located in Gulf of Mannar during August, 2015, which possibly interfered with fish gill function. High nutrient and chlorophyll a (Chl-a) (29.97 mg/m 3 ) concentration were observed during peak bloom period. Three groups of phytoplankton were identified -diatoms (24 species with, 14 centric species and 10 pennate species), dinoflagellates (10 species) and cyanobacteria (one species). Stable salinity condition and the depletion in nutrient concentration due to higher primary production might have triggered the bloom of Trichodesmium. In-situ bio-optical measurements were also made to understand the spatial and temporal variation and effect of bloom on the optical components. Our study is a preliminary step to understanding the in-situ bio-geochemical and bio-optical characteristics of coastal waters of Gulf of Mannar and Palk Bay, which could aid in the management of cage culture sites.

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