Wilson and Seki: Biology and population characteristics of Squalus mitsukurii 



857 



60 cm in length (i.e. minimum observed size at 

 maturity; Kolmogorov- Smirnov test, P=0.069). 



Estimates of fecundity for female S. mitsukurii 

 were similar regardless of the reproductive stage 

 used. Gravid females had similar mean numbers 

 of large ovarian eggs (3.8, SE=0.10, n-95), candled 

 embryos (3.7, SE=0.15, «=40), and uterine em- 

 bryos (3.6, SE=0.15, «=57); numbers for any de- 

 velopmental stage ranged from 1 to 6. Fecundity 

 increased significantly with the size of the female 

 (Table 3, Spearman's r=0.44, 0.40, 0.46; P<0.05). 



The size range of uterine embryos (65 female, 

 51 male, and 73 unsexed) for S. mitsukurii was 

 6.6-25.9 cm TL (L=17.8 cm, SD=5.57). A very 

 large (30.5 cm TL) male embryo from winter 1988 

 was excluded because its length could not be veri- 

 fied. Bimodal size-frequency distributions of the 

 uterine embryos were evident for samples from 

 all periods (Fig. 6). All gravid females had one of 

 two size classes of uterine embryos. Further, the 

 total numbers of uterine embryos within all but 

 fall 1988 samples were apportioned approximately 

 equally between the "small" and "large" size 

 classes (x 2 ,P>0.05). 



No well-defined parturition season was detected 

 either from a large increase in catches of small 

 (i.e. 21-26 cm), free-living fish (Fig. 3) or by the 

 absence of large, near-term embryos in females 

 (Fig. 6). However, the smallest uterine embryos 

 were found in fall and winter and were absent in 

 spring and summer (Fig. 6), which is indicative of 

 a reasonably well-defined seasonal production 

 cycle for young. Specimens as small as 21 cm were 

 captured from longline gear in summer 1986. This 

 confirms that 21-26 cm long embryos would be 

 close to parturition. The only significant correla- 

 tion between numbers of "large," "small," or total 

 uterine embryos, candled embryos, or large ova- 

 rian eggs was a positive association between num- 

 bers of uterine embryos and large ovarian eggs 

 (Pearson's r=0.62, P<0.05). 



Size at sexual maturity for male S. mitsukurii 

 was estimated indirectly from 812 specimens. The 

 estimated size at 50% sexual maturity was 48 cm 

 (Fig. 5). This corresponds to a tentative age of 

 about 4 years based on the von Bertalanffy growth 

 equation. 



Feeding 



Overall, 101 of the 293 (34.5%) stomachs exam- 

 ined contained prey. Stomach contents averaged 

 2.0 prey items (SD=1.4) and weighed 4.5 g (SD=9.3 

 g). Fishes, cephalopods, and crustaceans were the 



Age (years) 



Figure 4 



Fitted von Bertalanffy growth curves and mean length- 

 at-spine increment counts for female (dashed line, filled 

 circles) and male (solid line, open circles) Squalus 

 mitsukurii. Vertical lines represent 95% confidence inter- 

 vals. The von Bertalanffy growth parameters are defined 

 in the text. 



1 .0 

 2? 0.8 



O 



2 0.6 



c 



o 



T 0.4 

 o 



Q 

 O 



£ 0.2 

 0.0 



7r„ o o 1 .,-*+-'- 1 

 /. • 



30 40 50 



60 



70 80 90 



Total Length (cm) 



Figure 5 



Fitted logistic function and proportion mature at each size- 

 class for female (filled circles) and male (open circles) 

 Squalus mitsukurii. Size class intervals are 2 cm. Points 

 representing the three largest size classes for males ap- 

 pear as outliers but were not eliminated because few ob- 

 servations comprised these points ( ;? =6, 1 , 1 ). Thus we were 

 uncertain whether they accurately represent the relation- 

 ship between clasper and total length for large fish (which 

 we had insufficient data to define) or whether the points 

 were a result of measurement error. Regardless of whether 

 these "extreme" values were removed or left in the data 

 set. they had relatively little influence in fitting the func- 

 tion (see text). 



