FISHERY BULLETIN: VOL. 85, NO. 2 



decline in size (kilograms/individual) with increas- 

 ing depth for H. laevigatus in the NWHI. In all of 

 these studies, changes in sex ratio with depth were 

 not taken into account. As we have shown, sex ratio 

 does change with depth and the sexes do grow at 

 different rates. The observed changes in size with 

 depth may be due to changes in sex ratio rather than 

 size-specific stratification. Dailey and Ralston (1986) 

 examined the sexes separately and found that for 

 H. laevigatus in Hawaii the carapace length of males 

 and egg-bearing females displayed no apparent 

 change with depth, whereas that of nonegg-bearing 

 females showed a strong inverse relationship. In the 

 Marianas, significant changes in mean carapace 

 length with depth were not observed for either sex 

 of the three species of Heterocarpus, except for 

 female H. longirostris (Table 2). For this group, an 

 inverse relationship was observed much like that 

 found for nonegg-bearing female H. laevigatus in 

 Hawaii (Dailey and Ralston 1986). 



The estimates of growth parameters for H. laevi- 

 gatus obtained in this study correspond well with 

 those of other authors (Table 9). Using the regres- 

 sion method (Wetherall et al. in press), we estimated 

 L^ to be 51.3 mm CL for males, 55.4 mm CL for 

 females, and 55.2 mm CL for the pooled population 

 in the Marianas. Using the same method, Dailey and 

 Ralston (1986) obtained estimates of 57.9 mm CL 

 for males, 62.5 mm CL for females, and 61.7 mm 

 CL for the combined sexes in Hawaii. Apparently, 

 H. laevigatus grows about 7 mm larger in Hawaii 

 than in the Marianas. King (1983), using the Bever- 

 ton and Holt method, estimated L^ = 57 mm CL 

 for H. laevigatus in Fiji. 



Estimates of Z/K for H. laevigatus in the Mari- 

 anas were 2.1 for males, 1.9 for females, and 2.5 



Table 9. — Asymptotic length (Z-oo), instantaneous growth constant 

 (K), length at maturity {L,^), and age at maturity (T^) of Hetero- 

 carpus laevigatus. 



for the sexes combined. In Hawaii, ZIK estimates 

 of 4.3, 2.9, and 2.6 were obtained for the same 

 categories, respectively (Dailey and Ralston 1986). 

 The ZIK estimates for the combined sexes are nearly 

 identical in the two studies. In our study, Z/K esti- 

 mates for the two sexes were similar to each other 

 and lower than that of the sexes combined, whereas 

 in the Hawaii study they were very different from 

 each other and both larger than that of the combined 

 sexes. In our study, if we assume that instantaneous 

 growth of the two sexes is similar, then mortality 

 will also be similar and close to the 0.75 yr~^ value 

 estimated for the pooled sexes. In the Hawaii study, 

 ZIK estimates for males and females differed wide- 

 ly. Mortality estimates differed considerably as well, 

 Z = 1.51 yr"^ for males and 0.73 yr"^ for females. 

 The K parameter for H. laevigatus in the Mari- 

 anas was estimated as K = 0.30 yr"^ for the com- 

 bined sexes (Table 9). Estimates of K for the in- 

 dividual sexes were ambiguous and inconsistent. 

 King (1983) estimated K as 0.27 yr-i for H. laevi- 

 gatus in Fiji and Dailey and Ralston (1986) estimated 

 K as 0.35 yr " ^ for male and 0.25 yr" ^ for female H. 

 laevigatus in Hawaii. Because growth estimates for 

 H. laevigatus are similar for the various areas 

 studied, it is not surprising that estimates of age at 

 maturity are also similar (Table 9). King (1983) 

 reported the age at maturity for female H. laevi- 

 gatus in the South Pacific as 4.6 years (40.5 mm CL). 

 He further suggests that males mature at about 24 

 mm CL (age not calculated). Dailey and Ralston 

 (1986) found that females in Hawaii mature at 40 

 mm or about 4 years. In the Marianas female matur- 

 ity is estimated at 43 mm CL or about 4.5 years. 

 Males mature earlier at 35.7 mm CL or about 3.0 

 to 3.5 years. Male maturity estimates by King (1983) 

 and this study are based on the relative growth of 

 the appendix masculina on the second pleopod. King 

 appeared to have chosen 50% of the asymptotic 

 value as the point of maturity much as the point 

 where 50% of the female shrimp are bearing eggs 

 is used to define maturity for females. We feel that 

 90% of the asymptotic value is a better estimate of 

 male maturity and have used that point in our esti- 

 mate for the Marianas. 



ACKNOWLEDGMENTS 



This manuscript is a result of the Resource Assess- 

 ment Investigation of the Mariana Archipelago 

 (RAIOMA) program conducted by the National 

 Marine Fisheries Service, Southwest Fisheries 

 Center Honolulu Laboratory. We thank the facul- 

 ty, staff, and students of the University of Guam 



348 



