An empirical approach was used to determine the sample size of transects in
acoustic surveys to estimate the abundance of three pelagic species in
northern Chile. Relationships between the coefficient of variation of fish
density and modified degree of coverage were established, where the modified
degree of coverage is proportional to the distance sailed and the surface
occupation index (Co) of the species and inversely related to the square
root of the study area. From this relationship, an equation was obtained to
estimate the number of transects required in order to obtain a predefined
level of precision, given a known level of occupation. The surface
occupation index corrects the degree of coverage and explains to a large
degree the differences in the estimated coefficients of variation among the
different species. Results showed that sample size declined with an increase
in the surface occupation index of the species, and the magnitude of that
reduction was appreciably greater for higher levels of precision. The number
of transects must be limited to sample sizes with a minimum transect
separation in order to assure independence between transect densities. The
empirical procedure used for the estimation of the number of transects can
be applied to other species situations if the information is available from
previous surveys, since the approach only requires repeated echo integrator
surveys.