562 



Fishery Bulletin 103(4) 



ences in the demographic and life history features rep- 

 resented phenotypic plasticity. 



Aeanthochromis polyacanthus (Bleeker) is one of a 

 few species of fish that are found in abundance at all 

 distances across the Great Barrier Reef (Williams. 

 1982, 1983) and, therefore, was ideal for comparisons 

 of cross-shelf patterns of demographic characteristics. 

 Aeanthochromis polyacanthus is a polymorphic gono- 

 choristic pomacentrid and site-attached planktivore 

 that inhabits reefs of the Indo-Australian Archipelago 

 and adjacent regions (Allen, 1975). It is extremely wide 

 spread and abundant along (north-south) the GBR (Wil- 

 liams, 1982, 1983). It is unusual among marine reef 

 fishes and unique among damselfishes in that it lacks 

 a dispersive planktonic larval stage (Robertson, 1973). 

 Instead, adult A. polyacanthus lay demersal eggs and 

 after hatching, both parents defend a brood of larvae 

 and juveniles for several months (Robertson, 1973; Al- 

 len. 1975; Thresher, 1985a, 1995b; Kavanagh, 2000). 

 In contrast to other taxa, therefore, dispersal is likely 

 to be slow within and among reefs. Aeanthochromis 

 polyacanthus is one of the best studied coral reef fishes 

 on the GBR with respect to predation (Connell, 1996, 

 1998, 2000), genetics and evolution (Doherty et al., 

 1994, 1995; Planes and Doherty, 1997a, 1997b), be- 

 havior (Robertson, 1973; Allen, 1975; Thresher, 1985a. 

 1995b; Nakazono, 1993; Kavanagh, 1998), reproductive 

 success (Thresher, 1983). and early life history (Ka- 

 vanagh, 2000), but no data exist on age, growth, and 

 demographic parameters, such as mortality rates (but 

 see estimates of juvenile mortality while larvae and 

 juveniles are brooded by adults; Connell, 1996). 



The objective of this study was to compare the demo- 

 graphic characteristics of A. polyacanthus across the 

 continental shelf. Our approach was to sample replicate 

 reefs in the central region of the GBR at multiple dis- 

 tance strata from shore (inner-, mid- and outer-shelf 

 distances). In addition, we chose a section of the GBR 

 where A. polyacanthus exhibited the same color pattern 



(brown anterior and white posterior) and are known 

 to be genetically isolated (Planes and Doherty, 1997b). 

 Any variation in demographic parameters, therefore, 

 could be largely attributed to phenotypic plasticity. The 

 specific objectives of this study were the following: 1) to 

 validate the deposition of annual growth increments for 

 fish of a wide range of sizes and ages by using tetracy- 

 cline, 2) to describe patterns of growth of populations 

 of A. polyacanthus within and among distance strata; 3) 

 to describe the age and size structures of populations of 

 A. polyacanthus within and among distance strata, and; 

 4) to calculate the instantaneous mortality and survival 

 rates (Z) of populations of A. polyacanthus within and 

 among distance strata. 



Materials and methods 



Study sites and sampling design 



Spatial variation in demographics and structures of 

 cross-shelf populations of A. polyacanthus was deter- 

 mined by using a partially hierarchical sampling design. 

 Individuals of a wide range of sizes were collected from 

 three replicate reefs within each of three distance strata 

 (inner-, mid- and outer-shelf) spanning the width of the 

 continental shelf of the central Great Barrier Reef near 

 Townsville, Australia (Fig. 1, Table 1). At least 16 fish 

 were collected with hand spears from each of three sites 

 on each reef during September and October 2001. All 

 fish collected were the same brown and white morph 

 (Allen, 1975). 



Sample processing 



All fish were measured (standard length [SL] to the 

 nearest mm) and weighed (to the nearest 0.01 g). Sag- 

 ittal otoliths were extracted, cleaned in freshwater 

 to remove the sagittal membrane, and allowed to dry 



