Ralston and Tagami: Exploitable biomass of Heterocarpus laevigatas in Hawaiian Is , Part 1 



501 



99 



cTcT 



UnT 14-25, 19Be N - Z.448 



July 11.1 



■ 16 20 26 30 36 40 46 60 65 15 20 26 30 35 40 46 60 65 80 



Carapace Length (mm) 



Figure 7 



Length-frequency distributions of female and male Heterocar- 

 pus laevigatus at the Kaulakahi Channel study site. 



o 



fl 



4) 



cr 



■•J 

 a 



0) 



u 

 u 

 u 



a, 



12.0 

 10.0 

 B.O 

 6.0 

 4.0 

 2.0 

 0.0 



$9 N- 16.970 

 Cfcf N-Z4.0B4 



FuU 

 Recruitment 



16 20 26 30 36 40 46 60 66 60 

 Carapace Length (nun) 



Figure 8 



Composite length-frequency distributions for all female and 

 male Heterocarpus lamgatiis sampled. 



distributions, all the length data were pooled (Fig. 8). 

 It is evident from the figure that female shrimp reach 

 substantially larger sizes than do males, in agreement 

 with previously published work (Dailey and Ralston 

 1986, Moffitt and Polovina 1987). Superimposed on the 

 combined length-frequency distributions of males and 

 females is the estimated size of//, lamngatus when fully 

 vulnerable to the traps (i.e., CLco = 30mmCL). The 

 carapace width of shrimp this size is equal to 1.27 cm 

 (0.5"), the minimum mesh size of the traps. 



We applied the Wetherall et al. (1987) regression 

 method to these length-frequency data and estimated 

 that 0, = 1.01 ± 0.052 and ©o- = 0.74 ± 0.075. For fe- 

 males we estimated that CL^ = 58 mm (SE 0.37) and 

 for males CL^ = 50mm (SE 0.44), corresponding to 

 W^ = 80g for females and 55g for males. 



The smallest shrimp we captured in the pyramid 

 traps was 16mmCL. Below this size, selectivity of the 

 gear was zero. By equating carapace width and mesh 

 size, we determined that CLeo = 30mm. Thus, our 

 estimate of the size at 50% recruitment to the fishery 

 (CL, ) is 23 mm. Given this result, and sex-specific 

 estimates of and W^ , we calculate that for females 

 Fo.i/M = 0.75 and for males Fo.i/M = 0.86 (Fig. 9). 



Discussion 



In this assessment, projections of exploitable biomass 

 depend greatly on the estimate of catchability obtained 

 from the Leslie depletion experiment. Due to its major 

 role in the calculations, sources of bias in its estima- 

 tion must be considered carefully. 



