Mollet et aL: Reproductive biology of Isurus oxynnchus 



309 



about 3-4 months later and have 

 a UWI around Tv^ at that time. 

 The UWI decreased in postpartum 

 females over a 6 month period to 

 values around 2—^'7( (Fig. 3B, cui"ve 

 fitted by eye). With a 2-year cycle, 

 UWI showed no decreasing pat- 

 tern for postpartum females and 

 low UWI values between 2^c and 

 A'Jc extended over almost the entire 

 second year of the reproductive 

 cycle (Fig. 3C). This result would 

 suggest aseasonal parturition; how- 

 ever, this conclusion is in conflict 

 with the observed seasonal partu- 

 rition indicated by Figure 3A. 



The GSI was not as useful in de- 

 termining the length of the repro- 

 ductive cycle because it decreased 

 during pregnancy. Recently ovu- 

 lated females had a GSI of \.1'7( 

 and an early-term female had an 

 estimated GSI of about 5% (Fig. 

 3D). We suggest that the GSI 

 remains high until the embryos are about 40-50 cm 

 long, and then decreases (Fig. 3D, cui-ve fitted by 

 eye). Two females with large embryos (mean: 52.0 

 and 59.9 cm) had low GSIs of -O.m and 0.185%, 

 respectively, indicating that ovulation had ceased 

 (spent ovary). The GSI during at least the last quar- 

 ter of gestation (i.e. >4 months) was similar to that of 

 postpartum females (mean: 0.26% , SE=0.03%, ?2=38). 

 When we tried to fit the GSI data with a 2-year 

 cycle, we found that data for six postpartum females 

 caught in winter (five from South Africa) overlapped 

 with those for the two pregnant females with large 

 embryos. This would extend parturition into fall and 

 would conflict with convincing seasonal parturition 

 data (Fig. 3A). 



The remaining reproductive parameters were not 

 helpful in determining the reproductive cycle. No 

 temporal pattern was evident for MOD. Oviducal 

 gland diameters were available mostly for summer 

 captures only and possible temporal changes could 

 not be investigated. There was no clear temporal 

 pattern in the HSI because we had few data for pre- 

 ovulating and early-term pregnant females. 



Regional differences 



The scarcity of data precluded a meaningful statis- 

 tical comparison of many reproductive parameters 

 for female shortfin makos from different regions. 

 However, we were able to substantiate differences 

 for mass and length-at-maturity for females from 



30 

 Total length (m) 



Figure 4 



The relationships between maturity and length of female Isurus oxyrinchus from 

 the western North Atlantic and the Southern Hemisphere. A logistical model was 

 fitted to the binomial maturity data (0=immature, l=mature). Mean TL and mean 

 fraction mature of 10-cm-TL ranges (instead of individual data points) were plot- 

 ted for clarity. Estimated TLs when 50*7^ are mature iMTLlare included. • = west- 

 ern North Atlantic (n=61);0=Southern Hemisphere (;i=82l. 



the western North Atlantic and the Southern Hemi- 

 sphere (South Africa and Australia; no data were 

 available from Brazil). 



Western North Atlantic females were significantly 

 heavier by 19-66 kg or 16-19% than Southern Hemi- 

 sphere females (P<0.001 in TL range 2.5-3.5 m). The 

 latter data were combined because the power regres- 

 sions for South African (n =47 ) and Australian females 

 (?!=22) were not statistically different (P=0.164,F*<F 

 (0.95,2,65)). The back-transformed M-TL relation- 

 ships were 



M=7.299 rL3 224 

 M=6.824 rL3 137 



(n=63, 7-2=0.94, western North 

 Atlantic females 2.0-3.7 m TL); 

 (n=69, r2=0.90, Southern Hemi- 

 sphere females 2.0-3.4 m TL). 



The median TL-at-maturity (MTL) of 61 western 

 North Atlantic females ( 2.98 m, SE=0.045 ) was signif- 

 icantly larger by 25 cm (P<0.001,F*»F( 0.95,2, 147), 

 than that of 82 Southern Hemisphere females (2.73 

 m, SE=0.02 m) (Fig. 4). The data from South Africa 

 and Australia were combined because MTL in South 

 Africa was only 11 cm smaller than MTL in Aus- 

 tralia and the length of these fish was not mea- 

 sured consistently. The maturation size of western 

 North Atlantic sharks extended from 2.76 m, when 

 IC/f were mature, to 3.20 m when 90% were mature 

 and was 16-35 cm larger than that in the Southern 

 Hemisphere, where the range was 2.60-2.85 m. 

 The largest immature female from the western 



