Mini Raman

Whale shark habitat assessments in the northeastern Arabian Sea using satellite remote sensing

One of the major requirements for the growing whale shark tourism industry is to identify potential areas of their aggregation for sighting. This would require baseline information on the occurrence of whale shark and the associated environment. In this context, the relationship between whale shark landings, phytoplankton concentration and sea surface temperature (SST) in the continental shelf and offshore regions of Gujarat coast were examined using satellite data from 1998 to 2000. Monthly images of chlorophyll-a (chl-a) concentration, an index of phytoplankton biomass and SST were derived for the eastern Arabian Sea from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and National Oceanographic and Atmospheric Administration-Advanced Very High Resolution Radiometer (NOAA-AVHRR), respectively. Whale sharks (Rhincodon typus) landing data were obtained from a survey conducted by Trade Records Analysis of Flora and Fauna In Commerce (Traffic)-India of the World Wide Fund (WWF)-India and the Central Institute of Fisheries Technology (CIFT), India. Mean chl-a concentration in the study area (between 20–22° N and 69–70° E) covering the continental shelf and adjoining offshore region of coast (depth > 25 m) was observed to be significantly higher (4.23 mg m−3 in February and 3.88 mg m−3 in March) compared to regions seaward of the study area (mean of 1.51 mg m−3 for February and 1.16 mg m−3 for March) and in southern latitudes of the eastern Arabian Sea (mean of 0.27 mg m−3 for February and 0.23 mg m−3 for March). The SST in the study area ranged from 23–26°C for February and March, whereas in the southern latitudes, it ranged from 27–29°C. The SST in regions outside the study area was marginally warmer by 0.5°C. A significant relationship between whale shark landings off Gujarat, chl-a concentration and SST was observed. Results presented in this study contribute to the idea that the combined use of ocean colour and SST images are an appropriate tool to identify potential areas of whale shark aggregation for sightings.

Locating tuna forage ground through satellite remote sensing

A technique has been developed to assess tuna habitat using satellite derived ocean colour, water transparency and sea surface temperature. The scientific approach is based on the food and feeding habit and the preferential temperature range of tuna. Time series chlorophyll-a images of Indian Remote Sensing Satellite P4-Ocean Colour Monitor (IRS P4-OCM) have been analysed to define the critical phytoplankton patch size, its persistence and the optimal water transparency depth. Sea surface temperature (SST) data derived from National Oceanic and Atmospheric Administration-Advanced Very High Resolution Radiometer (NOAA-AVHRR) have been analysed to study the optimum temperature range in the surrounding waters. Results based on hindcasting indicate that the tuna forage ground derived from satellite data yielded high catch of tuna (>2% hooking rate). Study reveals that a minimum time delay of 5–7 days is required for a phytoplankton patch to mature to the forage ground. In addition, water transparency for facilitating sight feeding should be about 25–30 m depth. The thermal limit and preferential temperature range observed is 26–30°C and 26–28°C, respectively. Preliminary results based on limited validation are highly promising. However, an experimental forecast is being planned to validate this approach.

Location of tuna resources in Indian waters using NOAA AVHRR data

Abstract The world demand for tuna resources is ever increasing and there is scope for better economic returns in terms of foreign exchange earnings. It is one of the least exploited resources of the Indian seas. Remote sensing based studies on the tuna environment began in the seventies in the Gulf of Guinea. This study helped to establish a fishing strategy during the eighties. But so far this has not been attempted in Indian waters. With the basic understanding that most of the species of tuna respond directly to temperature, a study using NOAA AVHRR data was carried out to locate tuna resources. Thermal data of NOAA AVHRR for eight dates in the 1989-90 season were analysed to generate sea surface temperature (SST) images. Catch per unit effort (CPUE) of tuna longline data acquired from the Fishery Survey of India pertaining to fishing conducted by chartered vessels, was plotted on the SST images. Yellowfin tuna (YFT) comprises the maximum catch plus small quantities of marlins. It was observed that al...

An approach to compute Photosynthetically Active Radiation using IRS P4 OCM

An approach was developed using the IRS P4 Ocean Colour Monitor (OCM) for the estimation of Photosynthetically Active Radiation (PAR). Birds clear sky model will be implemented using IRS P4 OCM data on an operational basis. IRS P4 OCM data (bands 6 and 8) were used in the present study to provide an instantaneous estimation of aerosol optical depth using the exponential approach. The transmittance coefficient required for aerosol optical depth was computed as a next step. This was further incorporated in Birds clear sky model with other atmospheric transmittance coefficients due to absorption from water vapour, ozone, and uniformly mixed gases. Spectral irradiance was integrated over photosynthetically active wavelengths (400–700 nm), which provides instantaneous photosynthetically available radiation (IPAR) at the sea surface. Ship cruise data (SK 171 and SK 186) for PAR using Satalantic radiometer was compared with IRS P4 OCM derived IPAR within 5.2% accuracy on average for two different monsoon seasons in the Arabian sea. The root mean square error between the observed PAR (in situ) and estimated (model‐derived) PAR was found to be 21.28 W m−2. This IPAR value is an important parameter for primary productivity estimation and will be used to compute primary productivity on an operational basis using IRS P4 OCM data.

Modulation in Ocean Primary Production due to Variability of Photosynthetically Available Radiation under Different Atmospheric Conditions

The rate of photosynthesis primarily depends on nutrients and photosynthetically available radiation (PAR) at sea surface. Several ship cruises were carried out to measure optical, biological, and atmospheric parameters in the Arabian Sea and their variability were studied. An analytical nonspectral photosynthesis-irradiance model was used to estimate euphotic primary production (EuPP) to study its variability during cruise periods. PAR was estimated using COART model using in situ measured aerosol optical depth (AOD) to compare with in situ measured PAR. In order to understand the variability of PAR under different types of aerosol and different aerosol loading, a simulation study was carried out using COART model. EuPP was estimated for various PAR values under different aerosol loading and cloud coverage conditions. Sensitivity analysis showed that for maritime, maritime polluted, and desert aerosols, the ratio PAR/PAR0AOD has attenuated to about 11–25%, whereas it has attenuated to 44% for urban aerosol type. PAR/PARclear  sky was reduced by ~57% for high aerosol loading and for overcast sky. The decrease in EuPP under various aerosol loading and cloud coverage was observed to depend on the photoadaptation parameter. EuPP/EuPPclear  sky was reduced by 38% for maximum maritime aerosol loading and for overcast sky.

Seasonal variability in bio-optical properties along the coastal waters off Cochin

Abstract Strong seasonal upwelling, downwelling, changes in current patterns and the volume of freshwater discharge from Cochin Estuary defines the coastal waters off Cochin. These coastal waters were investigated through monthly sampling efforts during March 2015 to February 2016 to study the seasonal and spatial variability in bio-optical properties for the four different seasons mainly Spring Inter Monsoon (SIM), South West Monsoon (SWM), Fall Inter Monsoon (FIM) and Winter Monsoon (WM). The Barmouth region is the meeting place where freshwater from Cochin Estuary directly enters to the sea through a single narrow outlet, was dominated by highly turbid waters during the entire period of study. Among the four seasons, chlorophyll a (Chl_a) concentration showed a high value during SWM, ranged from 2.90 to 11.66 mg m−3 with an average value of 6.56 ± 3.51 mg m−3. During SIM the distribution of coloured dissolved organic matter (CDOM) is controlled by decomposition of phytoplankton biomass and the river discharge, whereas during SWM the temporal distribution of CDOM is controlled only by river discharge. The highest value for CDOM spectral slope (SCDOM) was observed during SWM, ranged from 0.013 to 0.020 nm−1 with an average value of 0.015 ± 0.002 nm−1. During WM, the high SCDOM with lower aCDOM (443) indicates the photo-degradation affects the absorption characteristics of CDOM. The observed nonlinearity between Chl_a and the ratio of phytoplankton absorption aph (443)/aph (670) indicating the packaging effect and changes in the intercellular composition of pigments. During the study period, aph (670) was strongly correlated with Chl_a than aph (443), which explains the accessory pigment absorption dominating more than Chl_a in the blue part of the spectrum. Similarly, the results obtained from seasonal bio-optical data indicating that Chl_a significantly contributes light attenuation of the water column during SIM, whereas detritus (ad) significantly contributes light attenuation during SIM and WM. During the study period, the relative absorption of detritus materials dominates the relative absorption of phytoplankton and CDOM at 443, 555 and 670 nm wavelengths.

Assessment of apparent and inherent optical properties in the northeastern Arabian Sea using in situ hyperspectral remote sensing

In-situ measurements of the bio-optical properties of the seawater are important to develop algorithms for seawater constituent estimation using satellite remote sensing. A data collection campaign was conducted for bio-optical characterization of the open and coastal waters of the Arabian Sea during April 15-29, 2006. Bio-optical measurements were made using the Satlantic hyper-spectral underwater radiometer (Hyperpro-II) for 13 sampling stations include oligotrophic, Trichodesmium bloom dominated and coastal waters in 400-800 nm spectral range. For open oceans stations 1% light was available at 50 to 70 meter depth, whereas, for coastal waters it varied from 18 to 35 meter. The deep chlorophyll maxima (DCM) was observed at 30 to 42 meter depth during the bloom conditions with surface chlorophyll-a concentration ranging between 0.1 to 0.85 mg m-3 whereas, for open ocean and non-bloom conditions the DCM depth varied from 35 to 60 m with surface chlorophyll ranging between 0.05 to 0.12 mgm-3. Particulate back scattering coefficient at 700-nm vary from 0.0011 to 0.0031 for bloom waters and 0.00046 to 0.0012 for open ocean waters. The normalized water leaving radiance computed from these spectra in the spectral bands of IRS-P4, OCM bands were examined. The global ocean chlorophyll-2 (OC2), and 4 (OC4) algorithms performed reasonably well for open ocean waters, however both the algorithms overestimated chlorophyll concentration for bloom dominated waters.