St John et al.: Diet of Plectropomus leopcirdus on the Great Barrier Reef 
181 
dance of P. leopardus in some parts of the GBR, particular- 
ly close to centers of human population (Craik, 1981). Fur- 
thermore, fishing pressure is expected to intensify owing 
to increases in both commercial and recreational demand 
for this species. Maintaining grouper stocks on coral reefs 
depends on careful management of these fisheries. 
Strategies of coral reef management include closing 
reefs to all forms of exploitation. This strategy is designed 
to protect reef-fish stocks and habitats (Williams and 
Russ, 1994), which enables populations of reef fishes to re- 
gain or maintain natural levels of abundance (see review 
by Roberts and Polunin, 1991). Since the establishment of 
the Great Barrier Reef Marine Park (GBRMP) in 1981, the 
major management strategy has involved partitioning of 
reefs into six main zones that permit different levels and 
types of fishing. 5 Actual fishing pressure on reefs in these 
zones, however, has not been measured in many areas of 
the marine park (Williams and Russ, 1994). 
Most concerns about the impact of fishing have focused 
on the reduction of stock abundance (Russ, 1991). Both 
direct and indirect trophic effects on the structure of cor- 
al reef-fish communities by the removal of piscivores re- 
main poorly understood (Hixon, 1991; Russ, 1991; Steneck, 
1998) . Some authors have argued that the removal of large 
piscivorous fishes leads to compensatory increases in over- 
all abundance or changes in relative abundance of prey of- 
ten termed “prey release” (Beddington and May, 1982; Go- 
eden, 1982; Beddington, 1984; Grigg et al., 1984; Koslow 
et al., 1988). Russ (1991) and Jennings and Lock (1996), 
however, argued that the evidence for “prey release” on 
coral reefs is limited and equivocal. In 1978 Goeden sug- 
gested that depleting the abundance of P. leopardus may 
irreversibly alter the structure of the community of coral- 
reef fishes on the GBR, and yet, 20 years later, nothing is 
known about the trophic impacts of this fishery. 
Coral reefs with different fishing histories represent 
valuable large scale manipulations of predator densities 
(Jennings and Polunin, 1997). In contrast to results of 
smaller-scale predator-removal experiments on site-at- 
tached coral reef fishes (Caley, 1993; Hixon and Beets, 
1993; Carr and Hixon, 1995), large-scale “experiments” of 
predator removal by fishing have not demonstrated that 
predators play an important role in structuring fish com- 
munities. Several studies have compared prey populations 
on fished and unfished reefs but have found no evidence for 
corresponding changes in diversity or biomass of potential 
prey species (Bohnsack, 1982; Russ, 1985; Jennings and Po- 
lunin, 1997; Russ and Alcala, 1998a, 1988b). These studies, 
however, were hampered by a lack of dietary information 
necessary to determine exactly which prey fishes are im- 
portant in the diet of the fished species. For coral reef pisci- 
vores, commercially important or otherwise, quantitative 
studies on food consumption, feeding strategies, and com- 
prehensive lists of prey species are rare (but see Norris, 
1985; Norris and Parrish, 1988; Sweatman, 1984; St John, 
1999) . Studies of predator-prey relationships on coral reefs 
5 1987. Central Zoning Information Pamphlet. GBRMPA, PO 
Box 1379, Townsville, Queensland 4810, Australia. Fold-out pam- 
hlet with no pagination. 
cannot progress further without knowledge of the prey con- 
sumed (Jennings and Polunin, 1997). 
Information on the trophic biology of groupers is impor- 
tant for protecting the stocks and ensuring a healthy fish- 
ery for the future. Knowledge of the breadth of the diet 
and the specific habitat of important prey will indicate the 
resilience of stocks to changes in prey abundance or habi- 
tat destruction. Furthermore, baseline information on the 
diet of unfished populations is needed as a comparison for 
diets of fished populations. Such information should in- 
clude some measure of spatial variation in regard to the 
diet. Fishing activities may alter the trophic ecology of 
the species targeted by altering the structure or the be- 
havior of the wild population or both. For example, remov- 
ing larger individuals of P. leopardus may alter the diet 
of the population because larger predators eat larger prey. 
St John (1999) found that the composition of the fish diet 
of P. leopardus on the GBR varied with size until preda- 
tors attained 35 cm (TL). If this result applies to other 
geographic areas, fishing could affect the trophic impact of 
P. leopardus populations where legal minimum size limits 
are less than 35 cm (TL) or do not exist (e.g. Okinawa, Ja- 
pan). On the GBR, however, such effects would be minimal 
because the legal minimum length is 38 cm (TL). 
Fishing may also affect the behavior of piscivores. Line 
fishermen use baited hooks and throw bait into the water 
(termed burleying) to attract leopard coralgroupers to 
their fishing sites on the GBR. The use of bait to catch 
these fish could affect feeding behavior or feeding-related 
patterns of movement of P. leopardus in fished reefs and 
may have short- or long-term effects on the feeding ecology 
of P. leopardus. 
Compared with commercially important fishes in many 
other ecosystems, relatively little is known about the diet 
of adult P. leopardus on the GBR, despite the huge num- 
bers of P leopardus fished from the reefs on the GBR 
annually (but see Choat, 1968; Goeden, 1978; Ivingsford, 
1992; St John, 1995, 1999). In all the existing studies, most 
samples were collected by spear, presumably to avoid pos- 
sible dietary biases caused by line fishing methods. Thus, 
nothing is known about the trophic impacts associated 
with line fishing, which uses baited hooks at local fishing 
sites and which is by far the most common method used 
to catch P. leopardus. A comparison of stomach contents 
between P. leopardus caught by spear and those caught by 
hook and line at the same reefs will provide information 
on the selectivity of line fishing and trophic impact of this 
species. 
Our study is the first to examine the diet of a large com- 
mercial species of coral reef fish at reefs in open and closed 
fishing zones. The feeding ecology of P. leopardus , a major 
coral reef piscivore, was examined at reefs zoned open and 
closed to fishing for eight years in the Cairns section of the 
GBRMP. Although the primary aim of this study was to de- 
scribe and compare the diet off! leopardus on reefs in two 
fishing zones, we pooled reefs and compared the diet by lo- 
cation to assess the natural spatial variability in the diet of 
P. leopardus in this region. Also, we assessed the size struc- 
ture and diet of P. leopardus caught by two fishing meth- 
ods: nonselective spear fishing (described in the “Materials 
