Welsford and Lyle: Estimates of growth of Notolabrus fucicola from length- and age-based models 



709 



Parameterization of seasonal growth significantly 

 improved the fit of the GROTAG models, indicating 

 that seasonal variability in growth is significant for 

 N. fucicola. The estimates of seasonal growth from our 

 study constitute the first for this species. The LRTs 

 indicated significant differences in the timing of maxi- 

 mum growth (h>) between sites and between sexes at 

 Lord's Bluff. This result was repeated in the randomiza- 

 tion tests based on the outputs of bootstrapping. Peak 

 growth in N. fucicola at both sites is estimated to oc- 

 cur over the austral spring-summer, during maximum 

 water temperatures and increased productivity off the 

 coast of Tasmania (e.g., Halpern, et al. 4 ), and peak 

 growth occurs significantly later in the season at Lord's 

 Bluff than at Point Bailey. The mechanism affecting the 

 timing of seasonal growth at this reef-by-reef scale is 

 worthy of further investigation but is likely to include 

 variability in seasonal cycles of oceanography, in avail- 

 ability of food (Denny and Schiel, 2001; Shepherd and 

 Clarkson, 2001) and in temperature effects on metabo- 

 lism, controlling the amount and timing of resources for 

 allocation to growth throughout the year. 



The estimate of the size of the difference in w be- 

 tween the sexes at Lord's Bluff had very broad CIs, 

 and it is difficult to propose a hypothesis that could 

 result in seasonal growth varying between the sexes 

 by as much as five months, although resource allocation 

 for reproduction could be involved. It may be that the 

 particularly small size of the female data set at this 

 site limited our ability to estimate seasonal growth ac- 

 curately with GROTAG, and further study is required 

 to more precisely determine how important seasonal 

 growth differences between the sexes are in temperate 

 reef fishes such as N. fucicola. 



Sex-specific GROTAG analyses indicated a significant 

 difference in measurement errors; females were under 

 measured by a mean of 3 mm, compared to less than 

 1 mm for males at Lord's Bluff. Greater measurement 

 errors for females have been detected in other studies 

 with GROTAG (e.g., Simpendorfer, 20001, but a reason 

 for greater difficulty in measuring females is difficult 

 to determine. A possible explanation from our study is 

 the high individual growth variability and small sample 

 sizes. Both of these factors have been shown to affect 

 accurate estimation of measurement error in GROTAG 

 (Francis and Mulligan, 1998), and therefore the high 

 estimate of m in our study may be an artifact of the 

 data set. 



4 Halpern, D., V. Zlotnicki, P. M. Woicheshyn, O. B. Brown, 

 G. C. Feldman, M. H. Freilich, F. J. Wentz, and C. 

 Gentemann. 2000. An atlas of monthly mean distribu- 

 tions of SSMI surface wind speed, AVHRR sea surface tem- 

 perature, TMI sea surface temperature, AMI surface wind 

 velocity, SeaWIFS chlorophyll-a, and TOPEX/POSEIDON sea 

 surface topography during 1998. Jet Propulsion Labora- 

 tory Publication 00-08, 102 p. National Aeronautics and 

 Space Administration, Jet Propulsion Laboratory, California 

 Institute of Technology, 4800 Oak Grove Drive, Pasadena, 

 CA 91109. 



A significant difference in growth between the sexes 

 at Lord's Bluff indicates that under conditions of rapid 

 growth, females may grow significantly faster than 

 males. As discussed above, the current minimum legal 

 size limit is effectively protecting the reproductive out- 

 put of the prerecruit population of N. fucicola. However, 

 any significant lowering of the legal minimum size is 

 contraindicated where, in prerecruitment size classes, 

 females grow more rapidly than males, because lower- 

 ing the legal size may result in differences in sex-spe- 

 cific fishing mortality. 



As demonstrated in the present study, the choice of 

 growth model and the methods used to compare pa- 

 rameter estimates are critical to ensuring that growth 

 is adequately described, differences in growth are de- 

 tected, and if detected, are interpretable. In combina- 

 tion, the tests we employed are shown to be generally 

 robust, even in situations where data sets are limited 

 in sample size or by coverage across the full range of 

 age and length classes. We recommend the use of a 

 combination of approaches, including growth models 

 with biologically interpretable parameters, statistical 

 tests such as LRTs, plots of bootstrap parameters, and 

 nonparametric randomization tests, to provide insight 

 into the growth dynamics of fish species. 



Acknowledgments 



We wish to thank Malcolm Haddon, John Hoenig, Craig 

 Johnson, Paul Burch, and Philippe Ziegler for their con- 

 structive suggestions for the manuscript. Alan Jordan 

 and Graeme Ewing made invaluable contributions to the 

 field and laboratory analyses. This study was conducted 

 as a part of a Ph.D. program by the primary author, 

 through the Faculty of Science and Engineering at the 

 University of Tasmania. 



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