Kritzer: Sex-specific growth and mortality, spawning season, and female maturation of Lutjanus carponotatus 



97 



ence to sex. Sex ratios at each island group were compared 

 with an expected ratio of 1:1 by x 2 goodness-of-fit tests by 

 using all specimens (i.e. immature and mature) whose sex 

 could be determined. 



Lifetime growth parameters were estimated for males 

 and females from each island group by fitting the von Ber- 

 talanffy growth function (VBGF), 



2»=A.( 1 " 



exp( 



-Kit 



■'„>)). 



where L t = FL at age t\ 



L^= the mean asymptotic FL; 



A' = the Brody growth coefficient; and 



t = the age at which fish have theoretical FL of 0. 



Growth functions were fitted by nonlinear least-squares 

 regression of FL on age by using samples for which sex was 

 determined. Because VBGF parameter estimates can be 

 sensitive to the range of ages and sizes used (see Ferreira 

 and Russ. 1994, for an empirical example), a common t 

 equivalent to the .v-intercept of the early growth estimates 

 was used in all models (see "Results" section). Although 

 the sex-specific sample sizes at the Lizard Island group 

 were smaller (n=65 for females; n=62 for males), VBGF 

 parameter estimates achieved high precision at sample 

 sizes between 50 and 100 (Kritzer et al., 2001); therefore 

 the Lizard Island group data were included in the analy- 

 sis. Growth parameters were compared by plotting 959c 

 confidence regions of the parameters K and L x (Kimura, 

 1980) for each sex from each location and assessing the 

 degree of overlap. 



Sex-specific total mortality rates, Z, were estimated by 

 using the age-based catch curve of Ricker (1975) as the 

 slope of a linear regression of natural log-transformed fre- 

 quency on age class. Everhart and Youngs ( 1981 ) proposed 

 that catch curve analysis should exclude age classes with 

 n<5 and Murphy ( 1997) proposed that age structures used 

 in catch curves should be truncated at the first age class 

 with n<5. Alternatively, Kritzer et al. (2001) proposed that 

 a sample should contain an average of at least ten fish per 

 age class irrespective of age class-specific sample sizes. 

 Therefore catch curves were fitted by two different methods 

 for each sex at the Palm Island group. The first catch curve 

 began at the modal age class and stopped before the first 

 age class with n < 5. The second catch curve likewise began 

 at the modal age class but included all age classes that 

 were thereafter represented in the data set. Sex-specific 

 sample sizes for the Lizard Island group were too small by 

 any of these criteria and this location was excluded. Mor- 

 tality estimates for Palm Island group fish were compared 

 between the fitting methods within each sex as well as 

 between sexes by ANCOVA. 



Reproductive biology 



Maturation schedules of female fish were estimated for 

 each island group by fitting a logistic model, 



P, = l/(l + exp(a-W)), 



where P- = the proportion of mature fish in age or 20-mm 

 size class i; 

 a adjusts the position of the curve along the 



abscissa; and 

 r determines its steepness. 



Age- and size-specific maturity functions were used to 

 estimate the mean age, r 50 , and size, L 50 , at which 50% of 

 females are mature at each island group. 



Monthly mean LSI and GSI values of mature Palm 

 Island group fish were plotted separately for males and 

 females to determine seasonal patterns of energy storage 

 and the peak spawning period of L. carponotatus. The pro- 

 portion of specimens at each mature female reproductive 

 stage in each month was also plotted to examine ovarian 

 development patterns throughout the year and the degree 

 of spawning activity occurring outside of peak months. 



To examine whether relative reproductive output in- 

 creases with body size, GW and GSI for stage-IV ovaries 

 collected during peak spawning months were regressed 

 against TW. Residual plots were used to assess deviation 

 from a linear relationship and to identify three outliers, 

 which were removed from the regression analysis. Regres- 

 sion slopes were compared between the two island groups 

 by ANCOVA. Also, mean GSI values and the proportion of 

 Palm Island group females with stage-IV ovaries during 

 spawning months were compared between females <230 

 mm FL and those >230 mm FL to examine whether the 

 duration of spawning varies between size classes (nota 

 bene: 230 mm FL is approximately the mean size of mature 

 Palm Island group females and splits each month's sample 

 approximately in half). 



Results 



Ovarian staging 



Five female reproductive stages were identified through 

 histological analysis (Table 1) and were based largely 

 on the scheme of Samoilys and Roelofs (2000). Ovarian 

 stages I (immature) and II (resting mature) have similar 

 oocyte stages. These can be distinguished by the presence 

 of brown bodies or atretic oocytes, which are typically prod- 

 ucts of prior spawning (e.g. Ha and Kinzie, 1996; Adams 

 et al., 2000) and are usually absent from stage-I ovaries. 

 However, these structures will not necessarily persist in 

 ovaries that have spawned, and in fact were rare among 

 the samples; therefore identification of immature females 

 was based primarily on structural organization of the 

 ovary. Stage-I ovaries typically have a thin ovarian wall 

 and more compacted oocytes, whereas ovaries that have 

 previously spawned tend to have a thicker ovarian wall 

 and a more disorganized arrangement of oocytes (Table 1). 

 Also, there were distinct size differences between stage-I 

 ovaries and other stages. The mean GW of stage-I ovaries 

 was approximately one-third that of stage-II ovaries, and 

 mean GSI was approximately one-half of that at stage II 

 (Table 1), and the distribution of body sizes offish at stage 

 I had much lower minimum, maximum, and modal size 



