Zooplankton abundance and secondary production in the seas around Andaman-Nicobar Islands

Areas of remarkably higher concentrations of zooplankton standing stock were encountered in the northeastern (12°46'-13° 30 N and 93°03'-93°35' E) and southern (08°30'-09°30'N and 92o00'-92o41'E) regions the Andaman and Nicobar seas. The NE monsoon (October-January) was the most productive season followed by the premonsoon (February-May). The zooplankton population occurred in high abundance when the surface waters were characterised by low temperature and salinity. Occurrence of eggs and larvae of finfishes, pelagic tunicates, euphausiids, copepods, amphipods, foraminifers and lucifers in profusion accounted for the NE monsoon maximum. SW monsoon (June-September) was the least productive season. Monthly variations displayed high standing stock during November-February period and low during April-October with slight improvements in May-June and August-September. A comparison made on the standing stock of the southern and northern regions with respect to 10°N revelaed a rich population in the less saline waters north of 10°N to the west and east of islands contributing to the overall abundance of almost all the groups especially of the fish eggs and larvae, larval decapods, larvae of molluscs and mysids. The difference in biomass observed with reference to 93CE to the east and west was conspicuous such that the lowest 45.97 ml occurred in the eastern Bay of Bengal and the highest 54.44 ml in the western Andaman sea. Greater proliferation of zooplankton fauna encountered in the southwest, southeast and northwest regions of the island system during the NE monsoon was followed by the premonsoon maximum in the northeastern region. The neretic areas upto 50 m depth showed poor abundance, however, the 50-100 m depth zone was singularly rich recording further steady decline with increasing depth. The average values of secondary production estimated for the entire area of investigation was 4.8 gC/m2/yr. The peak period of the zooplankton population in the northern and southern sectors coincided with the maximum landings of the pelagic fishery resources of the Andaman-Nicobar Islands