10.1016/0967-0653(95)91766-w

Abstract

The reproductive biology of the serranid fish, Plectropomus leopardus, was studied from samples collected from mid shelf reefs in the central Great Barrier Reef and mid shelf reefs and waters adjacent to Lizard Island in the northern Great Barrier Reef. For the two locations, a spawning period was observed from September through November, during which multiple spawning occurred. An inverse relationship between fat and gonad weight was observed for the coral trout, indicating that these deposits of mesenteric fat are probably being used in the processing of gonad products. The mode of sexual development, monandric protogynous hermaphroditism, was confirmed through histological analyses of gonad material. The sex structure of the sampled population was analyzed based on age and size information. The size and age of first reproduction for females was 32 to 36 cm FL and 2 to 3 years of age. Size and age distribution of females overlapped with size and age distribution of males over a wide range, indicating that sex change can occur over a broad range of sizes and ages but females were significantly smaller and younger than males. While at the present point it is not clear how sex change is determined for the coral trout, the variability observed in the size and age in which sex change occurs and in the process of transition itself suggests that behavioral processes could be involved. The common coral trout Plectropomus leopardus (Lacepede 1802) is the most commercially exploited species of Serranidae on the Great Barrier Reef, with around 1,200 tons caught annually by the commercial fishery (Trainor, 1991). In spite of its importance, the reproductive biology of the common coral trout has been studied only for a population on the southern Great Barrier Reef (Goe­ den, 1978). Like all epinepheline serranids that have been studied until now (Sha­ piro, 1987), the coral trout is a protogynous hermaphrodite (Goeden, 1978). Man­ agement of a fishery is considerably complicated by sequential hermaphroditism, as males are likely to be removed selectively (Shapiro, 1987; Bannerot et aI., 1987). The responses of these populations to fishing pressure are largely unknown. Mechanisms of social induction of sex change have been suggested for epinephe­ line serranids as an alternative to genetically programed mechanisms, in which sex-change would occur at a certain size or age (Shapiro, 1987). For the coral trout, however, no information on population structure or behavior is available at the moment to assess these questions. In the present work, the reproductive biology of P. leopardus was studied from data collected in two areas of the Great Barrier Reef. Gonads were analyzed to determine the mode of reproduction and spawning season. The sex, size and age structures of these populations were analyzed to determine the age and size of first reproduction, distribution of sexes and range of occurrence of sex-change.