FISHERY BULLETIN: VOL. 85, NO. 3 



In this paper, we investigate the annual 

 changes in the size of spotted dolphin populations 

 that can be detected given various levels of re- 

 search vessel survey effort within specified time 

 periods. We investigate how many research ves- 

 sels, assuming 120 days searching per vessel per 

 year, would be required to survey the physical 

 area inhabited by the major stocks. We also inves- 

 tigate how many vessels would be required to de- 

 tect various levels of population declines in spot- 

 ted dolphins during 5 years or, given fixed 

 number of vessels, how many years of survey ef- 

 fort it would take to detect various population 

 declines or, given fixed number of vessels for fixed 

 number of years, the probability of detecting a 

 decline (i.e., the power). We use historical data 

 and current abundance techniques to predict 

 variability of data which will be collected during 

 the sampling period. 



AREA INHABITED AND 

 DATA SOURCES 



For our analyses, the study area included the 

 area described by Au et al. (1979)^ as being inhab- 

 ited by the target species (Fig. 1). The area north 

 of lat. 20°N was excluded because spotted dol- 



phins do not usually occur there. We partitioned 

 the study area into four strata: the inside, middle, 

 and west strata, which are located north of lat. 

 1°S, and a south stratum. The three northern 

 strata were collectively termed the north area 

 and all strata were termed the total area. In addi- 

 tion, a calibration area was defined as including 

 part of the inside stratum (Fig. 1). 



Data used in our analyses were collected from 

 1977 through 1983 by scientific observers aboard 

 the NOAA ships David Starr Jordan and 

 Townsend Cromwell. Survey coverage from the 

 two ships for all years combined was thorough 

 (Fig. 1). Data collected for each school included 

 estimates of dolphin school size, species composi- 

 tion, and line transect observations, which we 

 used to calculate density estimates. 



SURVEY COVERAGE 



We investigated the physical coverage of the 

 area that is possible when using 1, 2, or 3 ships for 



■^Au, D., W. L. Perryman, and W. Perrin. 1979. Dolphin 

 distribution and the relationship to environmental features in 

 the eastern tropical Pacific. Southwest Fish. Cent. Adm. Rep. 

 No. U-79-43, 59 p. SWFC La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, P.O. Box 271, La Jolla, CA 

 92038. 



130 H 



120* H 110" 



LONGITUDE 



70' H 



Figure 1. — Research vessel tracklines in each stratum during 1977 through 1983. 



436 



