Natanson et al : Age and growth of Lomnus nasus in the western North Atlantic 



271 



shark had one full band and 83'^'f growth of the next band, 

 based on the size of the last full band. This, along with the 

 YOY known-age individuals, confirmed annulus formation 

 and our band interpretation from birth to 11 years of age. 

 Sharks older than age 1 1 were assumed to have been aged 

 correctly due to a similar interpretation of the bands. 



Data analysis 



Comparisons of counts between the two readers indicated 

 no appreciable bias (Fig. 4). The coefficient of variation 

 for age 1+ sharks was approximately 157c. In the absence 

 of bias, this level of precision was considered acceptable; 

 thus the counts generated by one reader for the entire set 

 of vertebrae were used for the analyses. 



Length-at-age data showed that males and females grow 

 at similar rates until approximately 170 cm FL. at which 

 point the relative growth rate of the males declines (Fig. 

 5). The change in relative gi-owth between the sexes coin- 

 cides with the size and age of male maturity (Jensen et 

 al.^). Von Bertalanffy gi-owth functions fitted to the verte- 

 bral band-pair count data suggested that males attain a 

 smaller maximuin size than females (Table 2). The growth 

 rate of females also declined at size at maturity (approxi- 

 mately 218 cm FL; Jensen et al.'*). The considerable over- 

 lap in size at age between the sexes indicated that the 

 difference in growth rate is minor; therefore subsequent 

 comparisons were made for the sexes combined. 



Tag-Recapture analysis 



A total of 76 porbeagles was recaptured with sufficient 

 information for tag-recapture analysis. Time at liberty 



ranged from 0.02 to 6.0 years and size at tagging ranged 

 from 78 to 204 cm FL. Sharks were tagged and recaptured 

 in all months of the year. Tagging effort was fairly evenly 

 distributed throughout all months, whereas most recap- 

 tures were made between March and May (539'f ). For both 

 tagging and recapture, January and February were repre- 

 sented by the least data OVc and 1% for tagging and recap- 

 ture, respectively). More tags were released in December 

 than were recaptured ( 12% of the tags but only 2% of the 

 recaptures). Most tagged sharks were small (74% <150 cm 

 FL) because the majority were opportunistically tagged 

 onboard commercial fishing vessels. Data from 54 sharks 

 at liberty greater than 0.9 years were used in the Gulland 

 and Holt (1959) analysis, whereas all individuals were 

 used for GROTAG (Francis 1988a). 



The results of the likelihood ratio tests with GROTAG 

 (Francis, 1988a) demonstrated that the more complex non- 

 linear model with five of the six parameters included was 

 the best fit for these data (model 3, Table 3). The high value 

 of ,s suggests a lack of sufficient information for GROTAG 

 (Francis, 1988a) to distinguish between growth variabil- 

 ity and measurement variability (Francis and Mulligan, 

 1998). The mean annual growth rates at FL= 95 cm and 

 150 cm were 19.21 cm/yr and 9.52 cm/yr, respectively (Fig. 

 6). Von Bertalanffy estimates from the Gulland and Holt 

 (1959) and GROTAG (Francis, 1988a) methods produced 

 similar results (Tables 2 and 4). 



Length frequency 



Analysis of modal length-frequency progressions verified 

 the size at age and growth rate of age-0 and age-1 indi- 

 viduals (Fig. 7). Age-0 fish entered the fishery in July with 



