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Fishery Bulletin 103(4) 



populations are expected to diverge over time with re- 

 spect to their genetic composition (Doherty et al., 1994). 

 Numerous studies have examined the genetic relation- 

 ships between populations of A. polyacanthus on the 

 GBR (Doherty et al., 1994, 1995; Planes and Doherty, 

 1997a, 1997b). Isozyme analyses of populations of dif- 

 ferent color morphs at various spatial scales have shown 

 significant genetic variation at both the regional ( 1000's 

 of km) and local (100's of m) level, which under normal 

 circumstances would suggest separate species for each 

 color morph (Doherty et al., 1994; Planes and Doherty, 

 1997a). However, differences in the growth rates of A. 

 polyacanthus across the continental shelf in this study 

 are unlikely to reflect genetic differences between the 

 populations sampled because all individuals collected 

 were of the same color morph and were from a rela- 

 tively small area (about 400 km 2 , cf. 450,000 km 2 for 

 the entire GBR). 



Environmental influences that can affect patterns of 

 growth include predation pressure, temperature, and 

 related effects on metabolism, variations in resources 

 (e.g., abundance of planktonic food), and variation in 

 water condition (e.g., turbidity). 



High rates of predation may "drive" faster growth 

 (Werner, 1984), or conversely, select for early matura- 

 tion and smaller adult size (Reznick et al., 1990; Hutch- 

 ings, 1997). It is unlikely that the cross-shelf patterns 

 in growth that we found were determined by differences 

 in mortality rates. Some data on serranid abundance 

 (Williams, 1982) and anecdotal accounts have indicted 

 that predator abundance is greatest on mid- and outer 

 reefs of the GBR (Gust et al., 2001). Our measures of 

 instantaneous mortality (Z) and age maximum, how- 

 ever, did not vary with distance from the mainland. 

 Furthermore, in contrast to the patterns that Gust et 



Inner shelf (n=109) 



Mid-shelf (n=21 9) 



4  



3 



y=-0.48x+6.04 

 r 2 =0.89 



10 



12 



5 1 



4 

 3 

 2 

 1 



Outer shelf ( n=266) 



y=-0.43x+5.49 

 c 2 =0.95 



4 6 



Age (years) 



10 



12 



Figure 7 



Age-based catch curve estimates of Acanthochro- 

 mis polyacanthus mortality rates for reefs pooled 

 by distance strata. 



al. (2001) found for scarids, L. y increased with distance 

 from the coast. Mortality rates have been shown to vary 

 among locations within reefs for several species of coral 

 reef fish (Aldenhoven. 1986; Eckert, 1987; Sale and 

 Ferrell, 1988; Beukers and Jones, 1997) including A. 

 polyacanthus juveniles (Connell, 1996), as well as over 

 larger spatial scales (Meekan et al., 2001; Gust et al., 

 2002). In contrast to these last two studies, particularly 

 that of Gust et al. (2002), mortality rates for A. poly- 

 acanthus were similar at all three cross-shelf strata. 

 We acknowledge, however, that no data were available 

 on mortality rates of fish from zero to two years of age. 

 It is possible that mortality rates do vary with distance 

 from shore over this age range. 



An increase in adult size may occur when individu- 

 als experience a decline in average temperature during 

 development (Atkinson, 1994). It is also well established 

 that metabolism and growth are increased at higher 

 ambient temperatures in ectotherms (Schmidt-Nielsen, 

 1990). Differences in temperature between the water 

 bodies spanning inner-, mid- and outer-shelf positions 

 in the central GBR do occur; relatively shallow near- 



