FISHERY BULLETIN: VOL. 85, NO. 3 



data exists or can be collected to determine rea- 

 sonable estimates of coefficient of variation. 



SUMMARY 



Use of three ships provides excellent physical 

 coverage of the eastern tropical Pacific dolphin 

 area. Coverage using two ships appears adequate 

 while use of one ship yields very sparse cover- 

 age. 



Assuming alpha and beta levels of 0.05, use of 

 two ships for each of 5 years will only allow us to 

 detect a 13% annual decrease in spotted dolphin 

 abundance. This means that the population could 

 decline by 50^% during the survey period before it 

 could be detected. If three ships are used for 9 

 years, a 5% decrease per year could be detected. 



Use of two ships instead of three only decreases 

 our ability to detect specific trends by about 1 

 year. For alpha and beta levels of 0.05, use of two 

 ships will allow detection of a 5% annual decrease 

 in 10 years, instead of 9 with three ships. 



The sampling period may be shortened if larger 

 alpha and beta levels and larger annual decreases 

 are acceptable. For alpha and beta levels of 0.10, 

 use of two ships will allow detection of a 10% 

 annual decrease after 5 years during which a 41% 

 decrease in the population could occur. 



ACKNOWLEDGMENTS 



We thank J. Barlow, J. Michalski, W. Parks, 

 S. Reilly, G. Sakagawa, S. Sexton, and T. Smith 

 for their constructive reviews of the manuscript. 

 The experimental design benefited greatly from 

 discussions with members of the SWFC PreSOPS 



panel members including G. Broadhead, K. Burn- 

 ham, D. Chapman, D. Goodman, and P. Patter- 

 son. J. Joy, S. Sexton, and K. Wallace provided 

 computer support, and B. Whalen contributed 

 statistical support. 



LITERATURE CITED 



Barlow, J., and R S Holt. 



1986. Proportions of species of dolphins in the eastern 

 tropical Pacific. U.S. Dep. Commer., NOAA-TM- 

 NMFS-SWFC-56, 44 p. 

 BuRNHAM, K . D. Anderson, and J. Laake. 



1980. Estimation of density from line transect sampling 

 of biological populations. Wildl. Monogr. No. 72, 202 p. 



BUTTERWORTH, D 



1982. On the functional form used for g(y) for Minke 

 whale sightings, and bias in its estimation due to mea- 

 surement inaccuracies. Rep. Int. Whaling Comm. 

 32:883-888. 



Gerrodette, T. 



In press. A power analysis for detecting trends. Ecology. 

 Hammond, P. S. 



1984. An investigation into the effects of different tech- 

 niques of smearing the IWC/IDCR Minke whale sight- 

 ings data and of the use of different models to estimate 

 density of schools. Rep. Int. Whaling Comm. 34:301- 

 308. 

 Hammond, P S., and K Tsai 



1983. Dolphin mortality incidental to purse-seining for 

 tunas in the eastern Pacific Ocean, 1979-1981. Rep. Int. 

 Whaling Comm. 33:589-597. 



Holt, R S , and J E. Powers 



1982. Abundance estimation of dolphin stocks in the east- 

 ern tropical Pacific yellowfin tuna fishery determined 

 from aerial and ship surveys to 1979. U.S. Dep. Com- 

 mer., NOAA-TM-NMFS-SWFC-23, 95 p. 

 Miller, R. G 



1974. The jackknife-a review. Biometrika 61:1-15. 

 Seber, G. a. F. 



1973. The estimation of animal abundance. Hafner, 

 N.Y., 506 p. 



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