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

Abstract

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.