The Ichthyoplankton of Selected Estuaries in Sarawak and Sabah: Composition, Distribution and Habitat Affinities

Author： Blaber S.J.M. Farmer M.J. Milton D.A. Pang J. Boon-Teck O. Wong P.

ISSN： 0272-7714

Source： Estuarine, Coastal and Shelf Science, Vol.45, Iss.2, 1997-08, pp. : 197-208

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Abstract

The ichthyoplankton assemblages of 23 estuaries in Sarawak and Sabah were surveyed with particular reference to the culturally and commercially important clupeid Tenualosa toli . The species composition, distribution and density of all fish larvae were recorded, together with the physical characteristics of each estuary. A more detailed study was made 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 estuaries 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 halocline, are highly turbid and have tidal ranges of >3·5 m and strong currents. There is little seasonal change in freshwater input and their waters are not peat-stained. Their total zooplankton biomasses (approximately 0·05 g m -3 dry weight, excluding fish larvae) are an order of magnitude greater than biomasses in the second group of estuaries. The second group consists of all estuaries east of the Lassa as far as the Papar in Sabah. They are mostly smaller, shallower and have more variable salinities than the ` core area ' estuaries, with marked haloclines and seasonal changes in freshwater inflow, lower turbidities, weaker currents, tidal ranges of <2 m and low overall zooplankton biomasses.The composition of the ichthyoplankton is different in these two groups of estuaries. The assemblage in the first group (` core area ') of estuaries consists primarily of taxa associated with estuarine and/or turbid water conditions, whereas those in the smaller estuaries of the second group have mainly marine and clearer water affinities. Only the Gobiidae are ubiquitous. Very few larvae of freshwater species were recorded in any of the estuaries. The number of fish larvae was highly variable, but the mean densities (0·01-9·23 m -3 ) were similar, and similarly 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.