THE ICHTHYOPLANKTON OF SELECTED ESTUARIES IN SARAWAK AND SABAH - COMPOSITION, DISTRIBUTION AND HABITAT AFFINITIES

The ichthyoplankton assemblages of 23 estuaries in Sarawak and Sabah w ere surveyed with particular reference to the culturally and commercia lly important clupeid Tenualosa toll. The species composition, distrib ution and density of all fish larvae were recorded, together with the physical characteristics of each estuary. A more detailed study was ma de of the Lupar and Lassa estuaries, which are the 'core area' for T. toli. The estuaries fall into two groups. The first consists of the es tuaries of the 'core area' from the Sebako in the west to the Lassa in the east, and the Labuk and Kinabatangan estuaries of eastern Sabah. They are large and deep, have middle-range salinities with no haloclin e, are highly turbid and have tidal ranges of >3.5 m and strong curren ts. There is little seasonal change in freshwater input and their wate rs are not peat-stained. Their total zooplankton biomasses (approximat ely 0.05 g m(-3) dry weight, excluding fish larvae) are an order of ma gnitude greater than biomasses in the second group of estuaries. The s econd group consists of all estuaries east of the Lassa as far as the Papar in Sabah. They are mostly smaller, shallower and have more varia ble salinities than the 'core area' estuaries, with marked haloclines and seasonal changes in freshwater inflow, lower turbidities, weaker c urrents, tidal ranges of <2 m and low overall zooplankton biomasses. T he composition of the ichthyoplankton is different in these two groups of estuaries. The assemblage in the first group ('core area') of estu aries consists primarily of taxa associated with estuarine and/or turb id water conditions, whereas those in the smaller estuaries of the sec ond group have mainly marine and clearer water affinities. Only the Go biidae are ubiquitous. Very few larvae of freshwater species were reco rded in any of the estuaries. The number of fish larvae was highly var iable, but the mean densities (0.01-9.23 m(-3)) were similar, and simi larly variable, to the densities reported for other tropical estuaries . The diversity of the ichthyoplankton assemblage in Sarawak and Sabah estuaries (56 taxa, 26 families) is lower than in most other tropical estuaries of the Indo-west Pacific. This is possibly because of their rigorous physical nature, particularly the very high turbidities and current speeds, or in smaller, less physically rigorous estuaries, the low biomass of zooplankton available as food for the larvae. (C) 1997 Academic Press Limited.