638 



Fishery Bulletin 93(4), 1995 



Discussion 



There are several important assump- 

 tions in a comparison of the effects of 

 fishing on populations from areas that 

 are open with those that are closed to 

 fishing. The first assumption is that 

 protection is enforced so as to guaran- 

 tee effective fishing closure. In the 

 Great Barrier Reef Marine Park, aerial 

 surveillance is conducted on a regular 

 basis and fines are levied on those who 

 fish illegally. Although violations still 

 occur, the fishing pressure is likely to 

 be considerably lower on the closed 

 reefs. The second assumption is that the 

 effects offish movements across closed 

 and open boundaries do not mask the 

 effects of protection from fishing on the 

 population structure. The minimum dis- 

 tance between two study reefs was of 

 1.6 km, and depth between reefs of the 

 order of 40-60 m. Tagging studies have 

 shown that reef fishes are highly site- 

 attached, and most studies on move- 

 ments of serranids have not shown sig- 

 nificant movements across distances 

 and depths such as those existing on the 

 present study (PDT, 1990). Davies 6 con- 

 ducted extensive tagging studies on 

 leopard coralgrouper and showed that 

 fish exhibited extremely limited inter- 

 reef movement in a study of six reefs in 

 the Central Great Barrier Reef. 



Expected effects of fishing are a re- 

 duction in the size and age range and 

 average size and age of the population 

 (Russ, 1991). In addition, line fishing 

 might select for the larger and older 

 individuals in a population (Ricker, 

 1969; Miranda et al., 1987), which 

 would exacerbate this effect. Significant 

 differences between size and age struc- 

 tures on closed and open reefs, however, 

 will depend largely on the duration of 

 closure in relation to species longevity 

 and fishing mortality. Therefore, a third 

 assumption is that the duration of ef- 

 fective closure is great enough (in rela- 

 tion to the longevity of the target species 



c 



3 



o- 

 <u 



I— 

 P. 



CLOSED 



Glow 



iS 



BLu- 



1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 



Age (years) 

 Glow 



30-35 35-40 40-45 45-50 50-55 55-60 



Fork Length (cm) 

 Yankee 



1 2 3 4 5 6 7 8 9 10 U 12 13 14 



Age (years) 

 Yankee 



30-35 35-40 40-45 45-50 50-55 55-60 



Fork Length (cm) 



OPEN 



Grub 



1 2 3 4 5 6 7 8 9 10 11 12 13 14 



Age (years) 

 Grub 



30-35 35-40 40-45 45-50 50-55 55-60 



Fork Length (cm) 

 Hopkinson 



w 



30" 



20" 



 



I 2 3 4 5 6 7 8 9 10 11 12 13 14 



Age (years) 

 Hopkinson 



30-35 35-40 40-45 45-50 50-55 55-60 



Fork Length (cm) 



r — I immature rzi mature mm transitional era young 

 LJ female M female  ™ male 



mature 

 male 



Figure 7 



Distribution of developmental stages of leopard coralgrouper, Plectropomus 

 leopardus, at each reef by age (years) and size (fork length). 



6 Davies, C. R. 1995. Patterns of movement of three species of 

 coral reef fish on the Great Barrier Reef. Ph.D. diss., Depart- 

 ment of Marine Biology, James Cook University of North 

 Queensland, Australia, 170 p. Unpubl. data. 



concerned) for an effect of closure to be detected. In 

 this study, given the short period of time for which 

 the reefs had been closed (3-4 years) in relation to 

 the longevity of the leopard coralgrouper ( 14+ years), 



