THE CULTURE AND BIOLOGY OF SOFT-SHELL TURTLE, TRIONYX SINENSIS WIEGMANN IN SINGAPORE

Abstract

This project was undertaken to study the culture and biology of soft-shelled turtle, Trionyx sinensis Wiegmann. Though they are endemic to China, Taiwan and Japan, they are cultured in preference to the native species T. cartilagineus. The survey of the three local farms showed that the culture as currently practised is still very traditional. Water conditions of the turtle ponds was described and showed that soft-shell turtles were quite hardy. They could survive in water with low dissolved oxygen content(> 0.1 ppm), high concentration of total nitrogen(< 543 ppm) and ammonia(< 126 ppm). Growth and reproduction was also included in this study. Growth rates of the immature, and adult turtles could be obtained from the following equations: immature Y = -3.75 + 19.69 X male Y = -7.08 + 22.8 X female Y = -20.6 + 26.0 X where body weight (Y) is expressed in grams and age (X) in months. It takes approximately 21 months for a turtle to reach marketable size of 500 g. Female turtle mature at a larger size of > 15 cm curvature carapace length and at an older age of 16.1 months. However, the males mature at > 10 cm and at an age of 9.3 months. Sexual dimorphism is pronounced. The male's tail is relatively longer than that of the female. Besides this, the body is dorso-ventrally thinner and the carapace is flatter. The reproductive cycles of the male and female turtles are continuous. Eggs are laid throughout the year. The mean clutch size is 10.1 + 0.5 eggs. There was no correlation between clutch size and turtle size. However, egg size is inversely correlated with clutch size and positively correlated with turtle size. Investigations on the stocking density of the turtles, temperature and handling effects on the eggs were also studied to improve the technique of culture here. The tolerance ranges of T. sinensis eggs was between 23 to 34°C. Incubation and hatchability were dependent on temperature. Turtle culturists could reduce the semi-natural incubation period of T. sinensis by 10 days if they were incubated at a constant temperature of 28°C without affecting the hatchability. Temperature has no effect on sex differentiation of the turtles. The egg viability could be increased provided that care should be taken to eliminate rough handling of the eggs and unnecessary movement especially rotation. This is because handling was found to have a detrimental effect on the eggs. The effect was greater when the frequency was increased and/or with the presence of position effect of inversion. Stocking density had significant effect on the growth but not the survival of adult T. sinensis. However, the mortality of the males was significantly higher than the females. Turtle culturists can use a stocking density of 6.6 turtles/m2 and below without adversely decreasing the growth rate.