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 behaviour of a serranid, the common coral trout Plectropomus leopardus, on the Great Barrier Reef, Australia, using underwater visual census surveys. The study was conducted at Scott and Elford Reefs, 2 mid-shelf reefs off Cairns, for 4 and 3 yr, respectively. At each reef, coral trout aggregated and spawned at the same 'primary' aggregation site in each year. Primary sites are defined as those with the largest aggregations of coral trout. Spawning activity, though not Limited to, was concentrated 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 August-December, 1991 to 1993. In 1990 only 2 aggregations were detected at Scott Reef, both around the new moon. The largest aggregations were recorded at the primary site at Scott Reef: numbers of fish at the aggregation site rose from an average pre-spawning density of 3.9 fish 1000m(-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 (forklength) at both reefs in all years, with a maximum range of 16 to 80 cm FL at Scott Reef, and 16 to 65 cm FL at Elford Reef. The onset of the 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 sites throughout the day. The aggregations appeared to disperse in the morning and re-establish after 13:00 h. Coral trout spawned in pairs predominantly on flooding tides and when current flow at the spawning sites was minimal. Ninety-four spawning rushes were observed which only occurred during a 33 (+/-4 SE) min period spanning sunset. The spatial and temporal predictability of P. leopardus spawning aggregations makes them vulnerable to overfishing, but also amenable to specialised management through seasonal and/or spatial closures.