RALSTON: FISHING EXPERIMENT FOR CARIDEAN SHRIMP 



tion was reduced an estimated 45.3% are persua- 

 sive elements here. Additionally, the size-frequency 

 data show no indication of a major alteration in 

 population structure. As long as the selective prop- 

 erties of the fishing gear remain unchanged, alter- 

 ations in the length composition of the catch are not 

 expected over short time intervals, at least due to 

 the direct effects of fishing. Further, no recruitment 

 of small shrimp is evident. That the mean size of H. 

 laevigatus seemed to increase as the experiment pro- 

 gressed might support the hypothesis that growth 

 of the stock was significant. An alternate explana- 

 tion, however, is that size structure varies with 

 depth of capture. Results from the Hawaiian Islands 

 (Gooding 1984; Dailey and Ralston 1986) have now 

 demonstrated this. The three samples presented in 

 Figure 4 are confounded by this variable; other 

 unknown factors may also have affected the shrimp 

 size-frequency data (e.g., sexual dimorphism, con- 

 tagious dispersion, sampling error, etc.). In addition, 

 the estimated growth rate from the data (3.9 mm 

 CL over 8 d = 0.49 mm/d) is biologically unten- 

 able. 



Other investigators, notably Schnute (1983) and 

 Crittenden (1983), have cautioned against the effects 

 of changing catchability and unequal variance on 

 Leslie model estimates. From the data gathered, 

 there is little statistical evidence to suggest that 

 these factors affected parameter estimates and I 

 therefore assume that 0.001945 trap-night -1 and 

 1,714 kg are reasonable estimates of standard trap 

 catchability and virgin population size, respec- 

 tively. 



Given that the virgin biomass of H. laevigatus in 

 the study area was 1,714 kg, the next question is: 

 How large an area was intensively fished? From 

 Figure 2 it is clear that there is no simple answer 

 to this question. A number of sets were located in 

 areas peripheral to the main trapping area. Desig- 

 nating the stipple bordered area as the effective area 

 fished is arbitrary, but provides a useful starting 

 point to allow calculation of shrimp densities. This 

 area was calculated to be 312 ha, corresponding to 

 a projected density of 5.5 kg of exploitable H. laevi- 

 gatus per hectare. Since individuals weighed 28 g 

 each, on average, this is equivalent to 1 exploitable 

 shrimp/51 m 2 of bottom, a remarkably low density. 

 Furthermore, a catchability coefficient of 0.001945 

 trap-night -1 indicates that one unit of standard 

 trap effort can reduce a 312-ha population of shrimp 

 by about 0.2%. This is certainly a significant impact. 

 The vulnerability to trapping that this species 

 demonstrates is cause for attention and careful 

 resource management. 



ACKNOWLEDGMENTS 



This paper is the result of the Resource Assess- 

 ment Investigation of the Mariana Archipelago at 

 the Southwest Fisheries Center Honolulu Labora- 

 tory, National Marine Fisheries Service. 



I would like to thank the crew of the Townsend 

 Cromwell for their help in completing this study and 

 Samuel G. Pooley, Victor A. Honda, Leigh Neil, and 

 Ahser Edward for their tireless efforts and good 

 spirits while at sea. This paper benefited greatly 

 from a review provided by C. D. Knechtel. 



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