Exploration of fishery resources through integration of ocean colour with sea surface temperature: Indian experience

Abstract

Exploration of fishery resources using remote sensing technique is based on the principle of identification of feeding grounds where fish tend to accumulate. It has been proven that thermal or colour gradients revealed by oceanic fronts indicate sites of high biological productivity. Some constraints in using SST (sea surface temperature) gradients for locating fish in the Indian waters were experienced such as the narrow range of SST and difficulty in detecting gradients, particularly in summer. Two approaches were developed and validated in the coastal waters of the west coast of India. In the first approach, SST contours (using NOAA AVHRR) were overlaid on chlorophyll image (from IRS P4 OCM) of corresponding date. This enabled identification of common frontal structures from the composite product. These sites were selected as priority fishing zones for the trial forecasts. Besides, ocean colour images were found to provide information on additional productive areas not found from SST images alone and hence, a second approach made exclusive use of patterns of ocean colour. Merits of ocean colour arose from penetration of visible radiation below surface up to one attenuation depth and from the frequent repeat cycle of the satellite data. The improvements with use of ocean colour include capability of prediction of oceanic features, exploitation of knowledge of the history of the features, identification of biological fronts in the deep sea waters etc. Also, such features as non-toxic winter blooms and internal waves were identified in the deep waters of the Northern Arabian Sea using chlorophyll images and the response of fish to these features was studied. This paper highlights how ocean colour improves our ability to locate areas of high abundance of fish. Because the time taken in information extraction from satellite data is a critical factor, on-line reception of OCM and AVHRR data was arranged. Fishery forecasts were generated using the integrated approach within 24 hours of satellite over pass, and disseminated to collaborating agencies for follow up fishing operation. The validation experiment for the forecasts was carried out for three years covering different seasons during 1999-2001. It was found that the forecasts were superior in terms of rate of success and magnitude of fish catch. Summary of feedback received indicated 70-90 % success rate (reliability) of the forecasts and 70-200 % increase in catch. In comparison with this, earlier SST-based approaches for the forecast yielded 50 % success rate and 40-50 % increase in catch. The integrated approach is currently being used to generate nation-wide fishery forecasts. In addition to this, cost-benefit analysis for the satellite fishery forecasts was also attempted. It was observed that the benefit:cost ratio increased from 1.27 to 2.12 for bottom trawling and 1.3 to 2.14 for gillnet fishing with the use of satellite forecasts.