Lunar spawning in Siganus canaliculatus

A major spawning of the seagrass rabbitfish Siganus canaliculatus occurred 4 days after the new moon in both May and June 1993, and 7 days after the new moon in 1994. The gonadosomatic index (I(G)) and serum vitellogenin (VTG) levels fluctuated according to the lunar cycle; I(G) and VTG levels showed peaks at around the new moon and the waning moon, respectively, suggesting that spawning of this species is synchronized with the lunar cycle. Vitellogenic oocytes appeared on day 2 after the first spawning and were fully mature on day 30. When a greater percentage of the most advanced oocytes attained the tertiary yolk stage, they formed a batch and separated from the adjacent group of smaller pre-vitellogenic oocytes, indicating that S. canaliculatus is a multiple spawner with an ovary belonging to the group-synchronous type of oocyte development. Batch fecundity, assessed using batches of oocytes at and after the tertiary yolk stage, ranged from c. 0·52 to 2·56 million eggs. The relationship between batch fecundity (F) and fork length (L) can be represented as F = 0·0536854L⁵·⁷²⁹².