FIEDLER: PRECISION OF SIMULATED TRANSECT SURVEYS 



acoustic survey are as follows (Instituto del Mar 

 del Peru 1974; Cram and Hampton 1976; P. E. 

 Smith pers. commun.): 



1) Failure to discriminate between anchovy 

 schools and other acoustic targets. 



2) Unschooled fish and small schools not detected. 



3) Vessel avoidance. 



4) Inability to survey in shallow inshore waters. 



5) Movement of school groups relative to the sur- 

 vey grid. 



6) Fish in the top surface layer missed by the 

 acoustic beam. 



7 ) Errors in the factor for conversion of the acous- 

 tic signal information to a biomass estimate. 



8) Effect of varying hydrographic conditions on 

 the acoustic signal. 



9) Blocking of signal to and from fish far from the 

 ship by fish nearer to the ship. 



The magnitude of the error caused by these fac- 

 tors can now only be roughly estimated. They may 

 affect either or both the precision and accuracy of a 

 population estimate. Corrections to reduce the 

 biases are conceivable. The present study has de- 

 monstrated the magnitude of the error associated 

 with the patchiness of the anchovy population. 

 Although the model population distributions may 

 be crude approximations to the real distribution, 

 the general conclusions reached here are not 

 likely to be changed by adding further levels of 

 complexity to the model. The sampling error due to 

 patchiness can be reduced by properly designing a 

 survey, but never eliminated. Temporal and spa- 

 tial differences in population estimates must be 

 interpreted with an awareness that the error 

 exists. 



ACKNOWLEDGMENTS 



I thank Paul E. Smith for his guidance in com- 

 pleting this project, as well as Alec McCall, Keith 



Parker, Jim Zweifel, and Roger Hewitt for 

 valuable discussions. Ian Hampton and two 

 anonymous reviewers provided helpful comments 

 on earlier drafts of the manuscript. This work was 

 carried out under a State of California Marine 

 Research Corpmittee contract at the Southwest 

 Fisheries Center, La Jolla, Calif. 



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