PERIODICITY OF SPAWNING AGGREGATIONS OF CORAL TROUT PLECTROPOMUS-LEOPARDUS (PISCES, SERRANIDAE) ON THE NORTHERN GREAT-BARRIER-REEF

I investigated seasonal, lunar and diel patterns in the spawning behav iour of a serranid, the common coral trout Plectropomus leopardus, on the Great Barrier Reef, Australia, using underwater visual census surv eys. The study was conducted at Scott and Elford Reefs, 2 mid-shelf re efs off Cairns, for 4 and 3 yr, respectively. At each reef, coral trou t aggregated and spawned at the same 'primary' aggregation site in eac h year. Primary sites are defined as those with the largest aggregatio ns of coral trout. Spawning activity, though not Limited to, was conce ntrated at primary sites. The aggregation site at Scott Reef measured 1700 m(2) in area, and that at Elford Reef 3200 m(2). Maximum numbers of coral trout recorded in these sites were 128 fish at Scott Reef and 59 at Elford Reef. Coral trout aggregated and spawned at the primary sites around 3 consecutive new moons each year within the period Augus t-December, 1991 to 1993. In 1990 only 2 aggregations were detected at Scott Reef, both around the new moon. The largest aggregations were r ecorded at the primary site at Scott Reef: numbers of fish at the aggr egation site rose from an average pre-spawning density of 3.9 fish 100 0m(-2) to aggregation densities ranging from 37.1 to 75.3 fish 1000m(- 2). The median size class of aggregating fish was 41 to 45 cm FL (fork length) at both reefs in all years, with a maximum range of 16 to 80 c m FL at Scott Reef, and 16 to 65 cm FL at Elford Reef. The onset of th e spawning season corresponded with a rise in water temperature (> 24. 00 degrees C) after the austral winter. Spawning aggregations occurred for an average of 5 d, however aggregations were not found at the sit es throughout the day. The aggregations appeared to disperse in the mo rning and re-establish after 13:00 h. Coral trout spawned in pairs pre dominantly on flooding tides and when current flow at the spawning sit es was minimal. Ninety-four spawning rushes were observed which only o ccurred during a 33 (+/-4 SE) min period spanning sunset. The spatial and temporal predictability of P. leopardus spawning aggregations make s them vulnerable to overfishing, but also amenable to specialised man agement through seasonal and/or spatial closures.