Ralston and Tagami: Exploitable biomass of Heterocarpus laevigatas in Hawaiian Is , Part 



503 



(1987), who estimated 0g = 1.9 and ©o- = 2.1 for H. lae- 

 vigatus in the Mariana Is. Additionally, use of the 

 "mode" criterion to establish the minimum size at full 

 vulnerability results in substantially greater sensitiv- 

 ity of to input estimates of CLeo- For example, if 

 CLfo is 30 mm as we suggest (Fig. 8), ±2 mm pertur- 

 bations in CLco result in - 9% and -i- 12% changes in 

 estimates of Q„. However, if the mode of the size- 

 frequency distribution is used instead (CLeo = 44 mm), 

 the same perturbations alter estimates of Q„ by 

 - 16% and -i- 46%. Similar sensitivity was observed in 

 estimates of 0, . In summary, it is our belief that in- 

 dependent estimates of CL^o are superior to those ob- 

 tained from the size data analyzed, particularly when 

 there is no reason to suspect that agonistic interactions 

 affect the catch size-structure. 



Lower values of indicate a reduction in the instan- 

 taneous mortality rate (Z), an increase in the von Ber- 

 talanffy grovvi;h coefficient (K), or both. We favor the 

 first hypothesis, largely because of the cold (4-6°C), 

 trophically-impoverished habitat in which H. laeviga- 

 tus reside. In many respects these shrimp represent 

 a crustacean analog to Pacific ocean perch Sebastes 

 alutus, which grow very slowly, exhibit extreme 

 longevity, and display low rates of natural mortality 

 (Leaman and Beamish 1984). Consequences of this life- 

 history pattern are that (1) under pristine conditions, 

 individuals accumulate in the largest size-categories 

 (Fig. 8), (2) the ratio of production to biomass is low, 

 and (3) stocks are very susceptible to overfishing. 



Acknowledgments 



We would like to thank the scientists and crew of the 

 NOAA RV Townsend Cromwell for their assistance in 

 helping us complete this study. This paper benefited 

 from reviews by G.W. Boehlert, A.D. MacCall, R.B. 

 Moffitt, and J.J. Polovina. 



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