A PRELIMINARY STUDY OF THE BIOLOGY OF GERRES ABBREVIATUS (BLEEKER) IN SINGAPORE WATERS

Abstract

Nine species of the genus Gerres have been recorded by several workers between 1868 and 1959 from Singapore waters. However, during the period of investigation by the author, only five species have been collected from the local waters. Gerres abbreviatus (Bleeker) with a standard length of more than 15 mm can be distinguished from the other species in the field by the broader body height, in the absence of an elongation of the second dorsal spine, and the long pectoral fins which reach above the anal fin. Juvenile Gerres abbreviatus are found abundantly in sandy littoral zones of Singapore waters especially Tanah Merah beach. Ecological studies of this beach show that Gerres abbreviatus is a hardy fish which is able to tolerate wide fluctuations in temperature, salinity and dissolved oxygen content in the inter-tidal pools, when the littoral zone is exposed during low tide. Gerres abbreviatus is an active and heavy feeder which changes its food preference from zooplankton to benthic organisms as the fish grows from the juvenile stage to the adult stage. Most of the food consumed by the fish was found to be digestible. The length-weight relationship provides a useful means for converting length data to weight. There is no significant difference between the length-weight relationship of the male and female fish, and the combined equation for both sexes is W = 0.02885 L3.1549 where "W" is the weight of the fish in grams and "L", the body length in centimetres. The Relative Condition Factor was found to be unreliable for assessing the length at first maturity. The male fish matures at around 10.6 cm standard length and the female at around 11.1 cm. The male spawns at a standard length of about 10.6 cm and the female at about 12.8 cm. Spawning appears to be periodic and not continuous, and the fecundity varies from 23,759 to 65,698 eggs per fish. Growth study by the length-frequency method shows that the growth coefficient, K, is 0.00091 per day, and the asymptotic length, L, is 248 mm. Gerres abbreviatus kept in aquaria are found to be very susceptible to several diseases.