FISHERY BULLETIN: VOL. 83, NO. 4 



trusiveness after March 1981 is unlikely to have 

 eliminated entirely the observer effect because the 

 data collected by scientific observers after 1981 were 

 still used to monitor dolphin kills relative to annual 

 kill limits. Observers collecting data that could not 

 be used for monitoring kill limits would be even less 

 obtrusive than the scientific observers, and covert 

 observers would be, of course, completely unobtru- 

 sive 



Based on our analysis, we would expect that the 

 frequency of zero-kill sets would be lower on 

 unobserved vessels than on vessels with a scientific 

 observer. This lower frequency of zero-kill sets, cou- 

 pled with an increased frequency of sets with 1-9 

 dolphins killed, suggests that the average kill rate 

 on unobserved vessels would be higher. Estimates 

 of total kill, based on the average kill rates from the 

 scientific observers, would therefore be underesti- 

 mated. 



ACKNOWLEDGMENTS 



We gratefully acknowledge the assistance of M. 

 M. Groom in providing data and in early discussions 

 of the problem. We also acknowledge the assistance 

 of K. E. Wallace and J. S. Cole in providing data. 

 We are indebted to D. G. Ghapman and W. F. Perrin 

 for suggesting analytical methods. We also ap- 

 preciate the helpful reviews by F. G. Alverson, I. Bar- 

 rett, P. S. Hammond, R. S. Holt, N. G. H. Lo, N. A. 

 Mendes, J. M. Michalski, G. T. Sakagawa, and K.-T. 

 'Rai, as well as the reveiws by two anonymous 

 individuals. 



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