FISHERY BULLETIN: VOL. 86, NO. 4 



similar experimental areas to employ as replicates 

 and in selecting control reefs which are equivalent 

 to the premanipulation condition of experimental 

 reefs. In this experiment juveniles on one reef, 

 selected as the control site, were left at natural, high 

 densities, while those on another, selected as the 

 treatment site, were reduced to approximately 25% 

 of their original, natural density. A third reef, also 

 left at its natural high density, was monitored at a 

 lower level to examine the degree of representative- 

 ness of the control. 



METHODS 



Reef Study Sites 



Three limestone patch-reefs at Seven Mile Beach, 

 Western Australia (lat. 29°08'S; long. 114°54'E), 

 designated as reefs I, III, and V (Fig. 1), were used 

 as the study sites. These three test reefs are typical 

 of those at Seven Mile Beach in terms of both their 

 structure and biota. Observations by divers also in- 

 dicated that the size structure of P. cygnus on each 

 reef was similar. The patch-reefs occupy a lagoon 

 environment between the beach and a limestone bar- 

 rier reef approximately 400 m offshore. Each patch- 

 reef is surrounded by a calcareous, sandy substrate 

 and areas of limestone, which both support exten- 



sive seagrass beds of mixed species composition, the 

 primary feeding areas for juvenile P. cygnus (Cobb 

 1981; Joll and Phillips 1984). Seagrass species of the 

 genera Amphibolis, Heterozostera, and Halophila 

 dominate in these beds. The reefs themselves are 

 covered by Amphibolis spp. and by a variety of algal 

 species. 



The approximate area of reef III, the treatment 

 reef, is 0.104 ha, and that of reef V, the main con- 

 trol reef, is 0.103 ha. Reef I, the secondary control 

 reef, has an area of approximately 0.071 ha. Reef 

 I is located approximately 750 m south of reef V, 

 while reef III is located approximately 60 m direct- 

 ly west and offshore from reef V (Fig. 1). Adequate 

 separation of the treatment and control reefs from 

 each other for the purposes of the experiment was 

 assumed, based on maximum foraging ranges of up 

 to 50 m for P. cygnus in the Seven Mile Beach area 

 reported by Chittleborough (1974a). Water depths 

 are 2-3 m around reefs I and V and 3-5 m around 

 reef III. The tops of the reefs are nearly exposed 

 at low tide. 



Single Molt Increments, Annual 

 and Seasonal Growth 



The numbers of juvenile P. cygnus from each reef 

 which were sexed, measured, and marked or re- 



FlGURE 1.— Map of study area at Seven Mile Beach, Western Australia, showing the locations of reefs, I, III, and V (dark shaded) in 



relation to other reefs (light shaded). 



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