Wilson and Seki: Biology and population characteristics of Squalus mitsukurn 



861 



The von Bertalanffy growth model was fit to age- 

 length data for S. mitsukurii . Increases in L with 

 age occurred for both sexes, although few larger, pre- 

 sumably older, specimens in the age sample may have 

 prevented a reliable evaluation of growth over the 

 entire age spectrum. Nonetheless, the growth model 

 seemed to represent observed patterns fairly well 

 over younger ages and smaller sizes for each sex. 

 Rather perplexing is the fact that observed and pre- 

 dicted L values at age (Fig. 4) were larger than 

 the observed size at parturition (see below). Possi- 

 bly the birth of age-0 fish occurs at times other than 

 when our age sample was taken (August-Septem- 

 ber). If this is the case, the formation of the first in- 

 crement may not have occurred although the fish had 

 grown in length. Alternatively, young-of-the-year fish 

 may not lay down an increment until their second 

 winter, or we may have simply missed the first in- 

 crement which is often poorly defined, as with S. 

 acanthias (Saunders 6 ). 



Reproduction 



Our results provide some insight into the gestation 

 period for S. mitsukurii. The bimodal uterine size 

 distributions that we observed during all seasons, 

 and which exhibited no clearly dominant mode, are 

 consistent with the two-year gestation period of the 

 closely related species S. acanthias (Compagno, 1984; 

 Ketchen, 1986; but see Kirnosova, 1989). To confirm 

 the gestation period, however, information is needed 

 on the growth rate of uterine embryos and the time 

 required for the smaller size class to replace the 

 larger size class. 



Slight differences in reproductive traits of S. 

 mitsukurii were detected between specimens col- 

 lected at SE Hancock Seamount and from other ar- 

 eas. The minimum size at sexual maturity was 65 

 cm for females and 55 cm for males off South Africa 

 (Bass et al., 1976), which was similar for females but 

 larger than that of males (<48 cm) in our study. Only 

 the female minimum size at maturity (85 cm) was 

 reported for the SE Pacific ( Litvinov, 1990). The value 

 exceeded that of our study. In the North Pacific, 

 Taniuchi et al. (1993) reported larger minimum 

 lengths at maturity for both sexes than those re- 

 ported here. Their reported minimum lengths at 

 maturity, which included data from 72 fish from SE 

 Hancock Seamount and which were expressed in 

 4-cm intervals, ranged from 68-72 cm to 96-99 cm 

 for females and 48-52 cm to 68-72 cm for males. 



6 Saunders, M. Pacific Biological Station, Nanaimo. B.C. Personal 

 commun., 1991. 



Length and age at maturity for male S. acanthias 

 are relatively high in the North Pacific (e.g. 72 cm, 

 14 years; Ketchen, 1975) compared with other ar- 

 eas, such as the Northwest Atlantic (e.g. 60 cm, 6 

 years; Nammack et al., 1985) and South Pacific (e.g. 

 58 cm, 6 years; Hanchet, 1986). For S. mitsukurii 

 males from the central North Pacific, however, our 

 estimated age at maturity is quite low (ca. four years). 

 Thus, unlike S. acanthias, it seems doubtful that 

 younger ages at maturity will be found in other re- 

 gions (e.g. South Pacific), assuming our preliminary 

 estimates of age are not seriously in error. 



There were no apparent geographical differences 

 in fecundity for S. mitsukurii. The reported number 

 of large ovarian eggs (two to five) for females in the 

 southeast Pacific (Litvinov, 1990) agrees with our 

 study. A mean of 6.4 embryos per gravid female 

 (range, 4-10) was reported off southeast Africa (Bass 

 et al., 1976) compared with 3.6 from our study. How- 

 ever, the data from South Africa were taken from 

 larger (76-95 cm) females. Likewise, litter sizes 

 ranged from 2 to 15 (present study 1-6) from rela- 

 tively large specimens collected in the western North 

 Pacific (Taniuchi et al., 1993). When compared over 

 sizes similar to those in the present study, however, 

 litter sizes were nearly identical (2-6 versus 1-6). 

 Thus, it is probable that the difference between our 

 results and the latter two studies can be attributed 

 to the positive relationship between parent length 

 and fecundity reported in our study (Table 3) and by 

 Taniuchi et al. (1993). 



The lengths of near-term embryos from the South 

 African specimens were at least 22 cm (Bass et al., 

 1976), and those from the southeast Pacific were 29- 

 30 cm (Litvinov, 1990). Although our estimated near- 

 term embryo lengths were similar to those from 

 South Africa, those from the southeast Pacific were 

 larger, possibly because of a positive relationship 

 between the length of the parent and those of the 

 near-term embryos. 



Feeding 



Squalus mitsukurii fed on a variety of benthic and 

 particularly mesopelagic fishes, crustaceans, and 

 cephalopods at SE Hancock Seamount. Other diet 

 studies have also concluded that S. mitsukurii con- 

 sume a variety of prey types. In the Indian Ocean, for 

 example, S. mitsukurii consumed teleosts ( 57% ), cepha- 

 lopods (33%), and crustaceans ( 10%) (Bass et al, 1976). 

 On seamounts in the Southeast Pacific, prey included 

 fishes, crustaceans, and cephalopods (Litvinov, 1990). 

 Along the west coast of South Africa, a species tenta- 

 tively described as S. mitsukurii fed predominately on 

 fishes and cephalopods (Ebert et al., 1992). 



