Swim bladder inflation in larvae of cultured sand whiting, Sillago ciliata Cuvier (Sillaginidae)

Abstract

Failure of the swim bladder to inflate can cause mortality in intensively cultured marine fish larvae. Surface access and light intensity are two key factors which influence inflation. Sand whiting ( Sillago ciliata) have a diel pattern of nocturnal swim bladder inflation in the wild. The objectives of this study were to determine when initial swim bladder inflation starts in cultured S. ciliata larvae, if surface access is required, and the effects of light. S. ciliata was found to inflate the swim bladder at night and completely deflate it during the day. This pattern starts on day 4, when the mean larval length is 2.8 mm and coincides with the completion of yolk sac and oil globule absorption, the start of exogenous feeding, and heavy larval mortality. Maximum swim bladder volume in 5-day-old larvae peaks 2 h after dusk and then gradually declines before dawn. The diel pattern of nocturnal inflation continues until day 17 when increasing numbers of larvae (mean length 6.4 mm) retain partially inflated swim bladders during the day. Experiments with 12-, 15- and 19-day-old larvae, siphoned into 2-litre beakers at midday, show that larvae respond to darkness by inflating their swim bladders. Placing oil on the surface of darkened beakers did not prevent swim bladder inflation in larvae from day 12. Swim bladder volumes were small and not significantly different for larvae in beakers with low light (413 lx) and high light (1360 lx). However, significant differences in feeding were observed with larvae, under high light, eating more rotifers than larvae in low light. Larvae did not feed in the dark. It is proposed that nocturnal swim bladder inflation allows S. ciliata larvae to conserve energy at night when they are not feeding. Although S.ciliata larvae have a different swim bladder inflation pattern from that of most species, it is suggested that it will present few additional culture problems. A light intensity of 1000–1500 lx, and a constant photoperiod are recommended to maximise swim bladder inflation.