MONITORING THE REPLENISHMENT OF CORAL TROUT (PISCES, SERRANIDAE) POPULATIONS

The replenishment of coral trout populations was monitored by daily sa mpling for pelagic juveniles using automated light-traps anchored at t hree sites close to the crests of two reefs (Arlington, Green) in the Cairns Region of the Great Barrier Reef. The water depth at each site was approximately 25 m; traps were suspended at 1 m and 20 m. A total of 237 presettlement coral trout were captured during the 1990/1991 sp awning season of which the majority were identified as Plectropomus le opardus. Four individuals were grown out to confirm generic identifica tion. Pelagic juveniles were collected from both depths but were consi stently more abundant at the surface. They were also consistently more abundant at Arlington than Green. Standard length was 16.8 +/- 0.2 (9 5% CL) mm with no trends in size among depths, reefs, sites or time. A ll presettlement trout were caught during a 17 day time window centere d around the new moon in November 1990. A similar pattern of replenish ment was generated by back-calculating the settlement dates of 36 juve niles collected from the reef at the end of summer. These back-calcula tions also estimated pelagic larval duration as 25.2 +/- 0.9 (95% CL) days, indicating birth dates near the previous new moon. This was cons istent with systematic observations on trout spawning at Scott Reef, w hich lies 40 km downstream of Arlington. Since Scott was obviously not the source of the fish captured in our study, synchrony between spawn ing and recruitment at this scale suggests that reproduction may be re gionally entrained, and that all replenishment in this season was sour ced from one period of spawning lasting about 2 weeks.