Mant et a\ Biological characteristics and mortality of Pentapodus vitta 



515 



Results 



Relationships of total length and weight to fork length 



The relationships between the FL and TL of females and 

 males, which covered essentially the full size range of P. 

 vitta. did not differ significantly (P>0.05) and therefore 

 the length data for the two sexes were pooled. The linear 

 relationship between FL and TL for both sexes of P. vitta 

 combined is described by the equation 



TL = -2.35 + 1.12 FL 



{n=344, r2=0.99). 



Moreover, because the relationship between weight and 

 fork length of western butterfish did not differ signifi- 

 cantly between males and females (P>0.05), these data 

 for the two sexes were pooled. The relationship between 

 weight and fork length for both sexes combined was 

 described by the equation 



logW = log 1.22.V lO-"^ + 3.06 log FL (n=1026, /-- = 0.99). 



Validation of aging method 



The mean monthly marginal increments on sectioned 

 otoliths of western butteriish with one, two, three, and 

 four-seven opaque zones reached their maxima in Octo- 

 ber, after which they declined markedly to their minima 

 in either November or December and then rose progres- 

 sively over the ensuing months (Fig. 1). The pronounced 

 decline and then progressive rise, exhibited by the mean 

 monthly marginal increments on the otoliths of western 

 butterfish, irrespective of the number of opaque zones, 

 demonstrates that a single opaque zone is formed in the 

 otoliths of P. vitta each year and that the number of such 

 zones can thus be used to age this species accurately. 



Growth 



The von Bertalanffy growth curve provided a good fit 

 to the fork lengths at age of the females and males of 

 P. vitta (Fig. 2). This good fit is reflected in the high 

 coefficients of determination (r^) of 0.942 for females 

 and 0.924 for males and by the values for f,, being close 

 to zero (Table 1). The von Bertalanffy growth curves of 

 the females and males of P. vitta in Shark Bay differed 

 significantly (P<0.05), and the difference in L^ between 

 females and males contributed more to the difference 

 than that of either /; or ?,,. 



Females were more prevalent than males in length 

 classes below 160 mm, whereas the opposite was true 

 above this length (Fig. 3A). However, age-frequency 

 distributions showed that females were more prevalent 

 in all age classes (Fig. 3B). The overall sex ratio was 

 1.4 females:l male. 



Reproductive biology 



Because virtually all P. vitta became mature at the end 

 of their first year of life, the GSIs for all fish al year 



0.8 r 

 0.6 

 0.4 

 0.2 

 L 



1.0 

 0.8 



1 opaque zone 



14 7 21 



2 opaque zones 



S L 



1 10r 



3 opaque zones 



I I I 



JJASONDJFIVIAM 

 Montti 



Figure 1 



Mean monthly marginal increments (±1 stan- 

 dard error) for sagittal otoliths of the western 

 butterfish (Pentapodus vitta), derived from 

 samples collected between 1999 and 2004. 

 Sample size is given for each month. The closed 

 rectangles represent the summer and winter 

 months and the open rectangles the autumn 

 and spring months. 



of age were calculated. The mean monthly GSIs for 

 female western butterfish remained low (<0.9) in winter 

 (June to August) and early spring (September), but then 

 increased sharply from September to reach a maximum 

 of 4.0 in late spring (November), before declining mark- 

 edly during mid-summer (January) (Fig. 4). Although 

 the pronounced seasonal trend displayed by the mean 

 monthly GSIs for males was very similar to that for 

 females, the maximum mean monthly GSI of males was 

 approximately one tenth of that for females. 



All female western butterfish that were al year old 

 and caught between February and August (apart from 

 a few in July) possessed ovaries at stage I or II (i.e., 

 they were virgin or immature; Fig. 5). Females with 



