Ma. Samoilys, PERIODICITY OF SPAWNING AGGREGATIONS OF CORAL TROUT PLECTROPOMUS-LEOPARDUS (PISCES, SERRANIDAE) ON THE NORTHERN GREAT-BARRIER-REEF, Marine ecology. Progress series, 160, 1997, pp. 149-159
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.