Phytoplankton bloom due to cyclone Sidr in the central Bay of Bengal

The evidence of phytoplankton bloom in response to the cyclone Sidr in the central Bay of Bengal (BoB) basin has been reported. Satellite observations of surface chlorophyll-a concentration (Chl-a) from MODIS-Aqua, sea surface temperature (SST) from MODIS-Aqua and TMI were studied for before, during and after cyclone periods. The mixed-layer depth (MLD) derived from the nearest Argo float that was closest to the cyclone track along with the float temperature and salinity (T/S) profiles near the cyclone track were studied to further complement the satellite observations. Nearly 900 km long phytoplankton bloom was observed along the cyclone path. Along the path of cyclone Sidr, the average increase in Chl-a due to Sidr during the cyclone and after cyclone period was about 285 per cent and 150 per cent respectively, from the before-cyclone period. The mean decrease in SST from before cyclone period was about 1.75°C and 1.27°C between during and after cyclone period respectively, with maximum cooling of 3.2°C observed during cyclone period. The Argo float observations indicate deepening of MLD by 15 m, during the cyclone and shoaling of MLD after the cyclone by about 35 m. Upward Ekman pumping due to cyclonic winds caused upwelling of nutrient rich subsurface waters, which led to the bloom in the BoB

Oceanographic preferences of yellowfin tuna (Thunnus albacares) in warm stratified oceans: A remote sensing approach

Yellowfin tuna (Thunnus albacares, Bonnaterre, 1788; YFT) face extensive exploitation pressure worldwide owing to the attractive economics of these fisheries. A better understanding of how oceanography influences the distribution of YFT (or highly migratory species more generally) will improve the ability for fisheries management policies to achieve conservation (e.g., maintain a stocks reproductive capacity), economic (e.g., maximize gross domestic product), and social (e.g., optimize successful fishing opportunities) objectives. Such ecosystembased linkages, when combined with real-time remote sensing data, provide the predictive framework for enhancing favourable fishing opportunities and limiting excessive harvest. We examined factors influencing YFT distribution and behaviour using pop-up satellite archival tags and remotely sensed oceanographic data from two regions in the northern Indian Ocean. Tagged individuals did not exhibit significant deep diving or diurnal behaviour, and preferred ambient temperatures of 26–30°C and 25–29°C in the Arabian Sea (AS) and the Bay of Bengal (BoB), respectively. In general, tagged YFT were found to be in waters with sea surface temperatures of 26–29°C, 60% of the time andwithin a sea surface height-anomaly of ±6 cm 70% of the time. YFT avoided moving below the relatively shallow oxycline depth, which is indicative of the stratified waters of the AS and the BoB. Low dissolved oxygen levels are likely a limiting factor for tuna movement given their high oxygen demand. The northern Indian Ocean provides an opportunity to study climate impacts on fish distribution and movement, and our findings provide a basis for understanding how habitat and migratory patterns may be altered under climate change