276 



Fishery Bulletin 100(2) 



250 



200 - 



150 



100 



50 



SW New Zealand 

 (Francis and Stevens, 2000) 



Tag-recapture 

 (GROTAG) 



Australia 

 (Francis and Stevens. 2000) 



. Known age samples (n=6) 

 Length-frequency mode (n=1) 

 O OTC iniected samples (n=2) 



10 15 



Age (years) 



20 



25 



Figure 8 



Von Bertalanffy growth cur^-es generated from vertebral data, GROTAG, and MULTIFAN 

 seasonally oscillating parameters, as compared with the length-frequency mode, OTC 

 recaptures, and known-age recaptures I validated i. Included for comparison are the von 

 Bertalanffy gi'owth cur\'es of Aasen 1 19631 and Francis and Stevens (2000). 



well suited to slow growing species because the narrowness 

 of the bands at the margin makes it difficult to objectively 

 determine marginal growth. 



In the present study, we validated annual band-pair pe- 

 riodicity up to age 11 using recaptures of both OTC-inject- 

 ed and known-age porbeagles. Our data clearly indicated 

 that the vertebral band pairs are deposited annually and 

 that the translucent zone is deposited between November 

 and April. 



Validation of an annual frequency of band-pair forma- 

 tion confirms Aasen's (1963) interpretation of the verte- 

 bral growth zones in the porbeagle. Our size at birth gen- 

 erated from the vertebral bands (95 cm FL), however, was 

 unrealistically high. This may have been due to our sam- 

 ple of age-0 fish being biased towards the faster growing, 

 larger fish that were recruited first into the fishery. The 

 early portion of our growth cui've, corresponding to ages 

 and 1, may therefore have been overestimated (Fig. 8). 



Francis (1988b) suggested that growth cui-ves derived 

 from age-length and length-increment data were not di- 

 rectly comparable and that the comparison of growth rates 

 at length was more appropriate. The growth rates at Lj^,, 

 were similar for all methods, verifying the growth rate at 

 this size. However, the overall growth curves from the dif- 

 ferent methods were also similar (Fig. 8). The tag-recap- 

 ture curve shows a more reasonable early growth than the 

 vertebral curve but levels off well below the obsei-ved ma.x- 

 imum size. The lower L and higher A' for the tag-recap- 

 ture method was expected because of the different deriva- 

 tion of the parameters and the absence of recaptured old 



sharks (Francis. 1988b). The K and ?y parameters derived 

 from MULTIFAN are close to those obtained by using ver- 

 tebral ages, and the scatter in the age readings overlaps 

 the MULTIFAN von Bertalanffy curve (Figs. 5 and 8). Al- 

 though the MULTIFAN L . value was slightly higher than 

 that of the vertebral value (Table 2), a difference at the 

 upper end of these curves was not unexpected because 

 length-frequency models are generally considered unreli- 

 able for the older age classes where the modes are not 

 easily defined (Francis and Francis, 1992; Francis, 1997; 

 Francis et al., 1999). 



Maturity occurs in the porbeagle at 8 and 13 years of 

 age (174 and 218 cm FL, males and females, respectively; 

 Jensen et al.^i. Growth for both sexes is similar up to the 

 size of male maturity, whereupon, the male growth rate 

 is reduced. Females continue to grow rapidly until the on- 

 set of maturity, at which point their growth slows as well 

 (Fig. 5). Owing to this change in growth rate, males reach 

 a smaller maxiinum size than females; however, the over- 

 all growth rate for both sexes is not substantially differ- 

 ent. Additionally, the vertebral L generated for females is 

 higher than what is being observed in the fishery, suggest- 

 ing that it has been overestimated and that the combined 

 curve is more appropriate. Aasen" (1963) also found no dif- 

 ference in the growth rate between the sexes although the 

 basis for his conclusion is questionable. 



Aasen, O. 1961. Some observations on the biology of the por- 

 beagle shark iLamna nasus, L). ICES, CM. Copenhagen 1961, 

 Near Northern Seas Committee (109):l-7. 



