860 



Fishery Bulletin 92|4). 1994 



area. In the case of SE Hancock Seamount, for ex- 

 ample, the summit diameter is only about 2.4 km. 

 Assuming recruitment is entirely dependent on the 

 seamount population (i.e. little or no immigration 

 from elsewhere), relatively little fishing effort is re- 

 quired to seriously deplete the stock. 



It is not possible to verify whether the assump- 

 tions that are required by the Leslie model were met 

 in the present study. The model assumes that 

 changes in population abundance (i.e. CPUE) are due 

 to fishing removals whereas other losses such as 

 emigration and natural mortality are balanced by 

 additions such as immigration and recruitment of 

 young to the exploited population. The model has 

 generally been applied over shorter time periods in 

 other studies where the assumptions are more readily 

 satisfied (Polovina, 1986; Somerton and Kikkawa, 

 1992). The fact that our model data were taken over 

 a period of about four years may have introduced 

 some noise into the results. Nevertheless, studies 

 have indicated that members of the genus Squalus 

 do exhibit low levels of natural mortality (Wood et 

 al., 1979) and fecundity (see Reproduction below). 

 Whether S. mitsukurii move among seamounts is 

 unknown. Unfortunately, results from tagging stud- 

 ies withS. acanthias (McFarlane and Beamish, 1986 

 and references therein) are of limited usefulness to 

 our work since they were not conducted over rela- 

 tively isolated seamounts found in deep oceanic wa- 

 ters. However, tagging work on S. acanthias within 

 the Strait of Georgia suggests that most recoveries 

 were made within the areas of release although some 

 long distance movements were also recorded 

 (McFarlane and Beamish, 1986). Thus, while our 

 application of the Leslie model to data taken over a 

 period of several years may be somewhat unusual, 

 there is no existing evidence that invalidates our as- 

 sumption that the losses and additions to the sea- 

 mount shark population were generally in balance 

 over this time period. 



Bottom longlines rather than trawls were the pri- 

 mary sampling gear used during the NMFS surveys. 

 The few numbers of S. mitsukurii taken earlier in 

 the commercial trawl fishery (Sasaki 5 ) may have been 

 the result of the different gear types. Other studies 

 have reported the reduced effectiveness of trawls, 

 relative to bottom longline gear, in catching S. 

 mitsukurii ( Litvinov, 1990). 



The decreasing catch rates of S. mitsukurii at SE 

 Hancock Seamount suggest a decline in the popula- 

 tion size. However, a concomitant decrease in mean 

 length was not observed for either sex (Fig. 3). This 



5 Sasaki, T. National Research Institute of Far Seas Fisheries. 

 Shimizu, Japan. Personal commun.. 1992. 



may have occurred because there were large remov- 

 als from the population with little or no recruitment 

 from smaller (younger) size classes. It is interesting 

 that no size class(es) appeared to progress through 

 the population during the period of study. 



Little has been reported concerning the bathymet- 

 ric distribution patterns of S. mitsukurii from other 

 regions. Although S. mitsukurii at the seamount and 

 in other regions have a similar maximum depth of 

 occurrence (Compagno, 1984), it is unknown whether 

 S. mitsukurii females and males from other popula- 

 tions exhibit depth distributions similar to those at 

 SE Hancock Seamount ( Table 2 ). Sexes of the closely 

 related species S. acanthias generally show the op- 

 posite trend; males were found at shallower depths 

 than were females (Compagno, 1984). 



The largest specimens of S. mitsukurii from our 

 study attained sizes similar to the maximum gener- 

 ally reported for other areas. Off South Africa, maxi- 

 mum lengths for S. mitsukurii. of 81 cm for males 

 and 95 cm for females were reported by Bass et al. 

 ( 1976). The maximum size we report agrees with that 

 from an earlier study (Taniuchi et al., 1993) at SE 

 Hancock (88-92 cm; n=72 specimens) but not that 

 from another locality in the western North Pacific 

 (112-116 cm). 



Age and growth 



Unvalidated estimates of age based on the second 

 dorsal spine increment counts from 63 fish have been 

 reported for S. mitsukurii from the SE Pacific Ocean 

 (Litvinov, 1990). In that study, the maximum age for 

 males was 14 years and for females was 16 years. In 

 the present study maximum ages were somewhat 

 higher: 18 years for males and 27 years for females. 

 Although tentative, our estimates of age based on 

 spine increment counts suggest that S. mitsukurii, 

 like S. acanthias (McFarlane and Beamish, 1987), is 

 long-lived. However, maximum ages for S. mitsukurii 

 were generally less than those for S. acanthias. Maxi- 

 mum ages for S. acanthias are quite variable. In the 

 NE Pacific Ocean, ages may exceed 80 years (McFar- 

 lane and Beamish, 1987), in the Atlantic Ocean, ages 

 to 40 years have been reported (Nammack et al., 

 1985), and in the Black Sea, maximum ages may be 

 only 20 years (Kirnosova, 1989). Whether S. 

 mitsukurii is a shorter-lived species than S. acanthias 

 will require the evaluation of additional samples. 

 Fishing on S. mitsukurii at SE Hancock Seamount 

 may have removed much of the older segment of the 

 population before we collected our age sample in the 

 summer of 1986. Thus, the largest fish we aged was 

 only 80 cm, whereas the largest fish caught was 91 

 cm. This suggests thatS. mitsukurii may live longer. 



