Inter-annual variability of thermal and chlorophyll fronts in selected parts of Eastern Arabian sea and their relation to marine fishery

Abstract

In the marine ecosystem, distribution of fish, as well as abundance, is determined by the presence of conditions favourable for their existence. Among various oceanic features, frontal zones are important sites promoting fish aggregation and these zones are found to be ideal habitats for fish. Therefore, areas with higher incidence of fronts are likely to be associated with higher fish production, than those with lower frontal incidence. An evaluation of this concept based on total thermal/chlorophyll frontal area of the eastern Arabian Sea (50- 80oE; 0 to 30oN), was performed for the period July 2002 to December 2016. This work utilized 8-day averaged satellite Sea Surface Temperature observations (SST) from MODIS/AQUA sensor as well as 8-day averaged satellite chlorophyll a concentration (Chl-a) from the Ocean Colour Climate Change Initiative (OC- CCI) project. Frontal identification was based on Cayula-Cornillon (1992) single- image edge detection algorithm on the satellite SST/Chl-a image in ArcGIS platform. Seasonal variation of thermal fronts showed maximum frontal coverage (0.5 – 0.6 million km2) during the winter period.Thermal fronts have their lowest incidence (0.2 – 0.3 million km2) during post and pre-monsoon period, and moderate existence during the summer monsoon (0.3 – 0.4 million km2). Chlorophyll fronts showed a slightly different pattern with highest frontal coverage (0.5- 0.6 million km2) during winter period followed by pre and post monsoon period (0.4 – 0.5 million km2). Chlorophyll fronts had their lowest incidence (0.2 – 0.3 million km2) during summer monsoon period.Possible physical reasons for the seasonal variation in thermal and chlorophyll front incidence are discussed. Analysis of inter-annual variability of thermal fronts indicated an increasing trend during 2015 and 2016 whereas chlorophyll frontal area showed a sudden drop in values from mid-2012 onwards. During this period, the usual seasonal variation in frontal zones is not observed. The mechanism responsible for this unusual behaviour was also analysed. Spatial variability of frontal zones in the Arabian Sea indicated that the pattern of variability in the west is different from that of the eastern Arabian Sea. The impact of thermal as well as chlorophyll frontal areaon fisheries during the inter-annual cycle is also discussed.