Blue carbon stock of the seagrass meadows of Gulf of Mannar and Palk Bay, off Coromandel Coast, south India, were computed from the organic carbon content and dry bulk densities of sediment core taken from the seagrass meadows of these two ecosystems. The Gulf of Mannar (GoM) and Palk Bay (PB) harbour 13 seagrass species dominated by Cymodocea serrulata and Syringodium isoetifolium. The soil carbon density of both GoM and PB were higher in subsurface cores. The blue carbon stock of seagrass meadows of the GoM was estimated as 0.001782 Tg and that of PB as 0.043996 Tg. The estimated value of blue carbon stored in seagrass meadows of GoM was 17820 US$ and that of PB was 43,99,682 US$. The results of this study are discussed in the light of climate change mitigation, emphasising the need to conserve these underwater meadows.

Impact of microplastic pollution on the marine environment and its biota is a major concern globally. Gulf of Mannar (GoM) and Palk Bay (PB) are two important biodiversity hotspots along the south-east coast of India. However, in the recent years the intense tourism and fishery activities have made the ecosystem and biota of these two ecologically significant coastal zones vulnerable to microplastic contamination. Hence, a comparative study on the seasonal distribution of microplastics in the surface waters, sediments, and in commercially important fishes were conducted to evaluate the threats imposed by microplastics on the environment and biota of both these coastal ecosystems. Microplastic distribution in the surface waters and sediments of GoM and PB exhibited conspicuous seasonal variation and showed positive correlation with the seasonal current patterns. In the surface waters of GoM, microplastics were abundant during the South West Monsoon whereas in PB it was high during the Early Winter Monsoon period. In the sediments of PB, microplastics was more during Spring Inter-monsoon and South West Monsoon whereas in GoM it was high during Late Winter Monsoon. Fragments, of size 1 to 5 mm formed the abundant microplastic type in the surface waters whereas in the sediments, fragments of size < 1 mm dominated. Of the gut content of the 613 fishes belonging to 12 families examined, microplastics were more in pelagic than demersal fishes with higher incidences in the family Clupeidae (42%). Among the fishes studied, microplastic ingestion was more in Selaroides leptolepis (27.77%), Sphyraena sp. (14.28%), Pelates quadrilineatus (12%), Caranx sp. (10.34%), and Sphyraena barracuda (10%).

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.

The needlefish both full beak and halfbeaks support a good fishery in Tamil Nadu and exploited by drift gillnet (Mural valai). In this study, we were focused to document the conversion of Mini trawls (Thallumadi) into light fishing craft targeting Belonids and Hemiramphids along Palk Bay. The detailed survey was conducted in the six landing centres along Palk Bay. The fishing was mainly targeted to catch Tylosurus crocodilus from 2000 to 0300 hrs at the distance of 3-4 NM in the depth of 5 m. After reaching the fishing ground, the fishes are usually aggregated using LED lights. The fishermen use the scoop net to collect the fishes. Catch per unit effort (CPUE) ranged from 36 to 40 kg/Unit and the fishery was dominated by T. crocodilus (85%) followed by Hemiramphus far (9%) and Ablennes hians (6%). The net operating income for this fishery is higher than the mini trawl fishery.