A PRELIMINARY STUDY OF THE BIOLOGY OF SILLAGO MACULATA Q. & G. IN SINGAPORE WATERS

Abstract

1. The study of certain biological aspects of the local whiting, Sillago maculata Q. & G. has been carried out within approximately two years. Among the aspects studied are maturity, food and feeding habits, low salinity tolerance, relative condition factor and growth. 2. In the taxonomic studies, it has been established that there are only two species of Sillago i.e. S. maculata Q. & & G. and S. sihama (Forsk.) in Singapore waters. The local distribution of the fish is briefly described. Both species are found in almost every sandy shore area around the island. 3. In the morphometric studies the length-weight relationships were established, the general equation being W = 0.006963L3.0995 where w is the weight of the fish in grams and L is the total length in cm. 4. The ecological conditions of Changi Beach where juveniles were collected were studied. 5. The gonad studies and the study of the seasonal distribution of the juveniles reveals that the local whiting spawns throughout the year. 6. The study of the food of the fish showed that very young juveniles fed mainly on copepods, older juveniles fed on bottom organisms and copepods, whilst the adult fish fed on bottom organisms, mainly polychaetes. A pattern of changing food preferences during their growth has been established. The study of feeding in relation to maturity showed that maturing fish generally fed heavily. 7. The seasonal variations in the relative condition factor of the juveniles and adults are described, the use of relative condition factors to find the size at first maturity has also been described. It is shown that females seem to reach first maturity at 160 mm. in total length and males at 140 mm. in total length. 8. The length-frequency data obtained from the materials collected throughout the period of investigation have been analysed. A polymodal pattern showing many growing breeds in the fish population throughout the year. The sex-ratio was studied according to size groups. It was found that according to size groups, younger groups consisted of more males and older groups had more females. 9. The growth of the fish was studied and the data were fitted to the Von Bertalanffy growth equation. The catabolic growth coefficient K was found to be 0.000909/day or 0.332/year. The asymptotic length L was found to be 242 mm. standard length. 10. Experiments on the acclimatisation of the fish to fresh water were carried out for both species and the results showed that young S. maculata could tolerate very low salinity (0.5%), but S. sihama could tolerate even fresh water for limited periods of time.