676 



Fishery Bulletin 101(3) 



made from the nucleus to the proximal surface, along the 

 dorsal margin of the sulcus acousticus. 



The precision of age estimates from whole and sectioned 

 otoliths was calculated by using the index of average per- 

 cent error (Beamish and Fournier 1981). The estimates of 

 age from whole and sectioned otoliths were compared by 

 a paired ?-test. Difference in bias between the two reading 

 methods was observed by plotting the difference between 

 the two readings (sectioned age minus whole age) against 

 sectioned age, based on the assumption that sectioned age 

 provided the best estimate of true age (Beamish 1979). The 

 results from this comparison indicated no significant dif- 

 ference between whole and sectioned otolith readings and 

 there was no discernible difference in bias in the plot. As a 

 result, all remaining otoliths were read whole for greater 

 efficiency. Age estimates from whole otoliths were accepted 

 and used in subsequent analyses when counts from the 

 first two readings agreed. If the counts differed, otoliths 

 were read a third time. The otolith was excluded from sub- 

 sequent analyses if no two counts agreed, but included if 

 any two counts agreed. 



Comparison of demographic parameters 



The central objective of this study was to estimate the vari- 

 ation in demographic parameters of L. miniatus, specifi- 

 cally otolith and somatic growth rates, age structure, and 

 mortality, at different spatial scales. In the first instance, 

 parameters were compared among the four reefs within 

 each region to estimate the magnitude of variation at the 

 inter-reef scale. Data were then pooled from individual 

 reefs within each region to generate regional parameter 

 estimates, which were used to estimate the magnitude of 

 variation at the regional spatial scale. 



The relationship between otolith weight and age (rep- 

 resenting otolith growth) was examined for each reef by 

 least-squares regression analysis, with otolith weight as 

 the dependent variable. The relationship was compared 

 among reefs within each region and among regions by us- 

 ing analysis of covariance (ANCOVA). 



Reef-specific age-frequency distributions were construct- 

 ed for all reefs. Multidimensional contingency tables were 

 used to compare age frequencies among reefs within re- 

 gions and among regions. Age classes 4 years and younger 

 and age classes 10 years and older were pooled into 4 and 

 10* age classes, respectively, because of low frequencies in 

 the tails of the age distributions. As a result, the analyses 

 included a total of seven age classes. 



Age-based catch curves (Ricker, 1975) were used to esti- 

 mate the instantaneous rate of total mortality (Z) at each 

 reef expres.sed on an annual basis. The number offish in 

 each age class was regressed against the corresponding 

 age, and the descending slope provided an estimate of Z. 

 Regressions were fitted from the first age class that was 

 fully selected by the sampling gear through to the oldest 

 age class that was preceded by no more than two consecu- 

 tive zero frequencies. As a result, the age range used to 

 estimate mortality varied slightly among reefs. Mortal- 

 ity rates were compared among reefs within regions and 

 among regions by using ANCOVA. 



The von Bertalanffy growth function (VBGF) provided 

 the best fit to length-at-age data for most reefs according 

 to the parameter estimates of the Schnute (1981) growth 

 function. For consistency, and to enable spatial compari- 

 sons of growth, the VBGF was used to estimate growth 

 parameters for each reef and region: 



L, = L^[1-^ 



-KH-lr,)] 



where L, = the fork length at age t; 



L^ = the mean asymptotic fork length; 

 K = the rate at which L^ is approached; and 

 ^Q = the age at which fish have a theoretical length 

 of zero. 



It was difficult to obtain a reliable estimate of initial growth 

 because the youngest fish collected was 2 years old. There 

 are also no published size-at-age data for larval or juve- 

 nile L. miniatus, or any other lethrinid. We constrained the 

 VBGF parameter t^ to zero to provide a better description 

 of the likely early growth of L. miniatus. This procedure 

 also allowed growth curves to be compared among reefs 

 within regions and among regions by using 959'f confidence 

 regions of the VBGF parameters L^ and K described by 

 Kimura(1980). 



Results 



Comparison of otolith reading methods 



Age estimates from whole and sectioned otoliths did not 

 vary significantly over the range of ages between 2 and 21 

 years (^00.5 2 3.54-'^-'^^' ^=0.73). That is, for each age class 

 estimated from sectioned otoliths, the average difference 

 between whole and sectioned otolith readings did not 

 differ significantly from zero (Fig. 2). The index of average 

 percent error was very low for whole ( 1.6^* ) and sectioned 

 ( 1.4%) otolith readings, indicating that otolith readings for 

 both methods were highly repeatable. This low index was 

 reflected in the agreement of at least two age estimates 

 for all whole otoliths, and hence no otoliths were excluded 

 from analyses. 



Otolith growth 



There was a significant positive linear relationship between 

 otolith weight and age for all reefs, with regression coef- 

 ficients ranging from 0.64 to 0.90. ANCOVA revealed that 

 the slope of this relationship was not significantly different 

 among reefs within each region (Townsville: F., .j^g=1.91, 

 P=0.13; Mackay: F.^.^^^=1.02, P 0.38; Storm Cay: Fl~i^=l.?>h. 

 P=0.20). Thus, otolith weight and age data were pooled for 

 each region to compare the region-specific relationships 

 between otolith weight and age (Fig. 3). The slopes of the 

 region-specific relationships differed significantly among 

 all regions (F.^y4g=28.9, P<0.001). The average growth in 

 otolith weight was greater in the Mackay region (26.91 

 mg/yr) than in the Storm Cay region (24.48 mg/yr), and 

 was least in the Townsville region (19.50 mg/yr). 



