FISHERY BULLETIN: VOL. 85, NO. 2 



Table 6.— Equilibrium yield of Heterocar- 

 pus laevigatas and relative spawning stock 

 biomass as a function of fishing mortality 

 (F). 



mates the recommended yield from the yield-per- 

 recruit derived yield equation as the yield which 

 corresponds to that level of effort where an increase 

 in one unit of effort will increase the catch by 0.1 

 of the amount caught by the very first unit of ef- 

 fort (Gulland 1983, 1984). This effort is denoted as 

 Fq 1 and the corresponding yield as Yf, i- The value 

 of Fq 1 for H. laevigatus in the Marianas is 0.8 and 

 Fo.i is 192 t annually (Table 6) from areas within 

 the depth range of 500-825 m. 



A second approach to estimate recommended yield 

 uses a computation of the spawning stock biomass. 

 With an age estimate of the onset of sexual matur- 

 ity, the spawning stock biomass for a level of F 

 relative to the spawning stock biomass, in the ab- 

 sence of fishing, can be computed from the Bever- 

 ton and Holt yield-per-recruit equation (Polovina and 

 Ralston 1986). This relative spawning stock biomass 

 can be used to determine the maximum value of F 

 before a substantial decline in recruitment occurs. 

 The recommended yield can then be estimated as 

 the yield from the constant recruitment yield curve 

 which corresponds to that maximum value of F. This 

 is a conservative approach because it does not in- 

 corporate any density dependent compensation, i.e., 

 size at onset of sexual maturity does not decrease 

 as density decreases. The relationship between the 

 relative spawning stock biomass and recruitment is 

 not known for H. laevigatus, but it has been sug- 

 gested that as a lower bound the relative spawning 

 stock biomass should not be reduced below 20% of 

 the unexploited level if a substantial reduction in 

 recruitment is to be avoided (Beddington and Cooke 

 1983). When F is 0.5, the relative spawning stock 

 biomass is estimated to be 20% of the unexploited 

 level, and the equilibrium yield at this level of fishing 

 is estimated at 162 1 annually (Table 6) for the depth 

 range of 500-825 m. To be conservative, the lower 

 yield estimate of 162 t annually from the Mariana 



Archipelago will be used. Given the habitat area 

 from 500 to 825 m, this yield is equivalent to 0.20 

 t/nmi^. An approximate variance for this yield, and 

 hence an approximate confidence interval, can be 

 computed from a Taylor series expansion of the yield 

 estimator if it is assumed that the variance of the 

 yield estimate is due primarily to variances in bank 

 CPUE and catchability. The yield at each bank is 

 computed as 



Yield = (CPUE/g)(Area)(y/B). 



Thus the variance of the yield (y(Yield)) can be ex- 

 pressed as 



y (Yield) = (Area)2(y/£)2y(CPUE/g), 



= (AreajHYmy 



r y(CPUE) (CPUE)^ Viq) 



X O 1 A 



Estimates of F(CPUE) were obtained from the 

 repeat sampling at each bank and ranged from 0.02 

 at Guam to 0.64 at Sarigan, while V{q), estimated 

 at 5.5 X 10"", was obtained from the intensive 

 trapping work. The variance of the total yield was 

 estimated as the sum of the individual bank vari- 

 ance. The 95% confidence interval, derived from the 

 estimate ± 1.96 times the standard deviation of the 

 estimate of total yield, resulted in a targeted yield 

 range of 102 to 218 t (0.12 to 0.27 t/nmi^) per year. 

 About 85% of this yield would come from the south- 

 ern islands and banks, 13% from the northern 

 islands, and about 2% from the western seamounts 

 (Table 7). 



DISCUSSION 



Although trap design, depth fished, and species 

 present undoubtedly affect catch rate, catch rates 

 reported in other studies using differing trap designs 

 in various areas fall within a fairly tight range of 

 1.2 to 6.6 kg/trap-night (Table 8). This indicates that 

 the productivity of deepwater pandalids is relative- 

 ly uniform throughout the tropical central and 

 western Pacific, and the first estimate of recom- 

 mended yield of 0.2 t/nmi^ obtained from the Mari- 

 ana Archipelago can be applied to other Pacific 

 islands, though the relative importance of the vari- 

 ous species may differ greatly from area to area. 



In our study H. ensifer, H. laevigatus, and H. 



346 



