Standing stock and composition of macrozoobenthos in the south-eastern Arabian Sea: A revisit after a decade

Abstract

Changes in macrozoobenthic standing stock and composition along the south-eastern Arabian Sea continental shelf (30-200 m) are investigated, based on data collected from the same sites during the same season 14 years apart (winter monsoon, 1998 and 2012), using the same platform, sampling gear, methods etc. During both surveys, polychaetes dominated among the macrozoobenthos, followed by crustaceans and molluscs. A decline in macrozoobenthic density and biomass with increasing depth was noted in both surveys. Macrozoobenthic density increased significantly between 1998 and 2012, especially in the inner and mid shelf (30-50 m), due to the increase in density of polychaetes and molluscs, while the density of crustaceans decreased. The mean individual body weight of polychaetes decreased between 1998 and 2012, most notably in the inner and mid shelf (30-50 m). This reflected an overall increased dominance of small-sized opportunists such as spionids, magelonids, paraonids, cirratulids and pilargids in 2012 across the entire shelf. Such an increase in dominance of opportunists and decline in abundance of sensitive taxa is indicative of long-term environmental stress and anthropogenic pressure. However, no significant changes were noted in the measured environmental variables which are known to influence benthic faunal distribution in the region, such as sediment nature and oxygen availability. The observed changes may rather be indicative of the long-term effect of intense bottom trawling, which is practiced in the study area year-round. This is the first study of its kind for soft-sediment shelf benthos from the tropical belt.