Williams et al ; Scales of spatial variation in demography of a large coral-reef fish 



681 



400 

 300 

 200 

 100 

 



A Townsville 



^hh«*^ 



a 



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O 600 

 U. 



500 

 400 

 300 

 200 

 100 

 



C Storm Cay 



^ 



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500 

 400 

 300 

 200 

 100 

 



490 

 480 

 470 

 460 

 450 



B Mackay 



D 95% confidence regions 



Mackay 



5 10 15 20 



Age (years) 



34 39 44 49 54 



K (per year) 



Figure 7 



Von Bertalanffy growth function { VBGF ) fitted to size-at-age data for L. miniatus 

 from three regions of the Great Barrier Reef (A) Townsville, (B) Mackay, and 

 (C) Storm Cay. The VBGF parameter Jq is constrained to zero in all cases. Dashed 

 lines represent predicted growth between and 2 years of age. (D) 95% confidence 

 regions for the VBGF parameters K and L^ for fit to pooled data from four reefs 

 within each of the three regions. 



accuracy in relation to those estimated from sectioned oto- 

 liths. In contrast, Brown and Sumpton (1998) concluded 

 that whole otoliths from larger and presumably older L. 

 miniatus underestimated age by up to 40% with respect to 

 sectioned otoliths. The discrepancy between studies may 

 be due to differences in the techniques used to count incre- 

 ments in whole otoliths. It was noted early in the present 

 study that otoliths from older fish needed to be rotated to 

 reveal a number of increments close to the otolith margin. 

 By not using this technique Brown and Sumpton (1998) 

 may have underestimated ages from whole otoliths of 

 older fish. Readings from whole otoliths have been shown 

 to consistently underestimate the age of a number of reef 

 fish species (e.g. Ferreira and Russ, 1994; Newman et al., 

 2000) resulting in biased estimates of mortality and sub- 

 sequent yield estimates (Newman et al., 2000). The results 

 from this study suggest that whole otoliths are adequate 

 for estimating the age of L. miniatus and that estimates of 

 demographic parameters presented in the present study 

 were not biased by underestimates of age. 



The spatial patterns in the demography of L. miniatus de- 

 scribed in the present study are based on data collected from 

 a single survey in one year, thus leaving the temporal stabil- 

 ity of the patterns open to question. Continued monitoring 

 of the populations will be required to determine the stability 

 of the patterns, and focussed stock structure studies are 

 required to determine the most likely causal mechanism(s) 

 of the patterns. Notwithstanding the need for this work, the 

 significant regional differences in demographic parameters 



found in the present study suggest different levels of produc- 

 tivity of L. miniatus populations in each region. Consequent- 

 ly, there is the potential for less productive populations to 

 be overfished, even where the fishing effort for the stock 

 as a whole is managed at sustainable levels (Caddy, 1975; 

 Sheperd and Brown, 1993). This argues for assessments and 

 management of L. miniatus stocks to explicitly consider the 

 regional structure in demography in order to meet both 

 sustainable use and conservation objectives for the Great 

 Barrier Reef World Heritage Area overall and on a regional 

 basis. Furthermore, this study highlights a more general 

 need for the use of multiscale sampling and analyses offish 

 populations to understand the relative importance of the 

 processes affecting demographic parameters, and the scales 

 at which these processes operate. 



Acknowledgments 



We acknowledge financial support from the Cooperative 

 Research Centre for the Great Barrier Reef World Heritage 

 Area, the Fisheries Research and Development Corpora- 

 tion, the Great Barrier Reef Marine Park Authority, and 

 the CRC Reef Research Augmentative Grant Scheme. The 

 VBGF ConfRegion program developed by J. Kritzer, CRC 

 Reef Research Centre, was used to estimate the VBGF 95% 

 confidence regions. We would like to thank Robin Stewart, 

 Mary Petersen, and the ELF field team for assistance in 

 the collection and processing of the fish from which this 



