Wilson and Seki: Biology and population characteristics of Squalus mitsukurii 



855 



differences in stomach fullness (defined 

 simply in terms of presence versus ab- 

 sence of food) between time blocks. Sun- 

 rise was at about 0545 h and sunset at 

 1930 h. The x 2 test was similarly used 

 to identify differences in stomach full- 

 ness between longline- and gillnet- 

 caught fish from a single time block 

 (1800-2400 h). 



Results 



Abundance 



A total of 1,392 female, 1,539 male, and 

 7 unsexed, hook-caught S. mitsukurii 

 were collected. CPUE was highest in 

 1985 and declined by more than 50% 

 by spring 1987 (Table 1). The CPUE 

 data plotted as a function of cumula- 

 tive catch (C) appeared generally lin- 

 ear with a negative slope (Fig. 2); coef- 

 ficients of the fitted model were CPUE 

 = -6.87 x 10" 5 x C + 0.2498 (r 2 =0.76). 

 The model estimate of the initial exploit- 

 able population was 3,641 fish (±1,954). 

 Based on this estimate, about 80% (±55%) 

 of the initial population of S. mitsukurii 

 had been removed by fishing. 



Both sexes of S. mitsukurii were 

 caught over the full depth range 

 sampled. The median depth of occur- 

 rence for males was usually greater 

 than that of females (Table 2). 



Size 



Female S. mitsukurii were on average longer and 

 reached larger maximum sizes than males (Fig. 

 3). Maximum lengths recorded were 91 cm for fe- 

 males and 82 cm for males. Differences were also 

 detected in length-weight data between female 

 and male S. mitsukurii (ANCOVA, P<0.05). Non- 

 linear fits of length-weight growth curves between 

 sexes diverged at about the size at 50% maturity 

 for male sharks (see Reproduction). Length-weight 

 parameter estimates were a = 1.7 18 x 10" 2 , 6=2.687 

 for males and a=3.773 x 10 3 , 6=3.089 for females. 

 The size of male sharks increased with depth. 

 For example, for each cruise and sex, fish were 

 divided into either a shallow or deep subgroup, 

 depending on whether they were caught above or 

 below the median depth of occurrence for that 

 group on that cruise. Estimates of the median 

 length for males from the deep group were 



0.30 

 0.25 

 0.20 

 0.15 

 0.10 

 0.05 

 0.00 



500 1000 1500 2000 2500 3000 

 Cumulative Catch (number) 



Figure 2 



Catch per unit of effort (CPUE) for each sampling period 

 (filled circles) and predicted CPUE based on the Leslie 

 model (solid line), and plotted as a function of cumulative 

 catch for Squalus mitsukurii from SE Hancock Seamount 

 during 1985-88. 



