The comparative ecology of Siganus lineatus and Siganus canaliculatus : two fishes of the Palau Islands (Pisces: Siganidae)

Includes bibliographical references.Includes illustrations and map.The abundance of S. lineatus and S.. canaliculatus in Palau presented an opportunity to examine the ecology of these two species of siganids. Topics related to both the basic ecology and to fishery related problems of these fish are emphasized. These topics include food habits, habitat, fecundity, sex ratios, size at sexual maturity, length-to-weight relationships, growth rate, schooling behavior and spawning activities. S. lineatus inhabits grass flats, areas beneath mangroves and areas over patch reefs. Adults feed nocturnally on the sea grass, Thalassia hemprichii, and on the sponge, Suberites sp. Juveniles feed on the epiphytes of mangrove roots. This species is sexually mature at 220 mm SL, and females were found to have fecundities ranging from 5.69 x 105 to 14.6 x 105 eggs per individual. Large juveniles had an average growth rate of 8.5 mm per month. The sex ratio of adults was observed to be 62% females to 38% males. Both adults and juveniles are strongly schooling fish. S. canaliculatus inhabits the grass flats where both adults and juveniles feed diurnally on the epiphytes of the sea grasses. This species is sexually mature at 170 mm SL, and females were found to have fecundities ranging from 4.61 x 105 to 7.20 x 105 eggs per individual. Small juveniles demonstrated a growth rate of 5 mm per week. The adult sex ratio was 68% females to 32% males. S. canaliculatus, as S. lineatus, shows strong schooling tendencies. The following conclusions are based on the results of this study. There is only slight overlap of the food resources of S. lineatus and S. canaliculatus. The sex ratio favoring females may be an adaptation for maximizing the number of offspring while under heavy predation pressures and/or while having limited food resources. The main restriction of using either of these species in a culturing program is the lack of a dependable supply of juveniles. The reported ciguatoxic aspect of these two fish is probably related to their feeding on the blue-green alga, Lyngbya majuscula.M.S. (Master of Science