680 



Fishery Bulletin 101(3) 



A Townsville 



0.3 0.4 0.5 0.6 0.7 0.8 



500 

 490 

 480 

 470 

 460 

 450 

 440 

 430 



C Storm Cay 



21-133 



21-132 



0.3 0.35 0.4 0.45 



K (per year) 



0.5 



Rgure 6 



95% confidence regions for L. miiiiatus for 

 the von Bertalanffy growth function (VBGF) 

 parameters K and L, for four individual reefs 

 within each of three regions of the Great Bar- 

 rier Reef: (Al Townsville, (B) Mackay, and 

 (C) Storm Cay. The VBGF parameter tg was 

 constrained to zero in all cases. 



liams and Russ''). Under the assumptions that demo- 

 graphic parameters are density dependent, and densities 

 are high enough for density-dependent effects to operate, 

 the spatial pattern in densities is consistent with the ob- 

 served patterns in age structures and mortality. However, 

 the observed patterns in growth are inconsistent with the 

 expected pattern, if density dependence was the dominant 

 influencing mechanism. We would not expect L. miniatus 

 to reach a larger size in the southern regions, where densi- 

 ties are higher, compared with the Townsville region, where 

 densities are lower, unless conditions were more favourable 

 in the southern regions. It would be necessary to invoke 

 another mechanism, such as regional differences in food 

 availability, acting in combination with density depen- 



dence, to explain the overall pattern of growth, mortality, 

 and age structure. 



The likelihood that the observed regional variation 

 is due to a response to regional differences in historic 

 (1960s-1980s) fishing pressure bears consideration. L. 

 miniatus are vulnerable to standard line fishing gear from 

 approximately two years of age and are fully recruited to 

 the line fishing gear by six years of age. The reefs in this 

 study had been closed to fishing for seven years prior to 

 sampling. Consequently, all cohorts older than 9 years of 

 age could have been fished prior to the reef closures in 

 1988, and before the availability of spatially referenced 

 catch and effort data for the commercial fishery. Anecdotal 

 evidence on the development of the fishery and its opera- 

 tion suggests that it developed from the southern ports 

 of Gladstone and Mackay (Fig. 1), which have remained 

 the dominant commercial line fishing ports in the fishery 

 (Mapstone et al.'). It seems plausible, therefore, that po- 

 tentially higher historic fishing effort in the southern two 

 regions could have modified the population structure of L. 

 miniatus sufficiently to produce significant differences in 

 demography. Given the longevity of the species, it is also 

 plausible that current differences in the populations are 

 the result of lagged recovery following the closure of the 

 reefs to fishing. 



Brown and Sumpton (1998) found small differences in 

 growth rates, and significantly different total mortality 

 rates, between populations of L. miniatus in the Swain 

 Reefs and those in the Capricorn-Bunker regions of the 

 southern GBR (separated by -1° latitude). They attributed 

 the difference in growth estimates to the selectivity of the 

 gear used to obtain samples and to the different mortality 

 rates to differences in fishing pressure between the two 

 regions. They dismissed the possibility that these differ- 

 ences were a result of separate populations with different 

 dynamics. However it is difficult to separate the confound- 

 ing effects of regional differences in fishing pressure and 

 gear selectivity in sampling; thus variation in growth and 

 mortality between the Swain Reefs and Capricorn Bunker 

 region cannot be dismissed. 



The consistency in demographic parameters among reefs 

 within each region suggests that populations of L. minia- 

 tus may be well mixed at the spatial scale of reef clusters 

 and that any influence of environment on demographics is 

 relatively uniform among reefs within regions. This uni- 

 formity may be facilitated by movement of adults among 

 reefs, or by recruitment and postsettlement processes that 

 are relatively uniform within regions. With significant 

 movement of adults among reefs, the benefits of protection 

 from individual reef closures may be limited, depending on 

 the rate of exchange between open and closed reefs, and 

 the level of fishing mortality on open reefs (Russ et al., 

 1992; DeMartini, 1993; Walters and Bonfil, 1999). In such 

 a scenario, any historical effects of differences in fishing 

 effort among regions would be perpetuated, even though 

 individual reefs were closed to fishing. 



In this study whole otoliths of L. miniatus provided 

 similar age estimates to those from sectioned otoliths and 

 thus offered a much faster alternative for age estimation 

 for this species, without any apparent loss of precision or 



