The Efficiency of a Seine Net

Abstract

We present a method to predict the capture efficiency of a 25-m, 5-mm mesh seine net as a function of fish size and taxon from a diverse fish community. This allows true abundance and size distribution to be estimated from observed catches. Predicted capture efficiency from an empirical model of field calibrations from the Amazon River floodplain was a positively skewed, unimodal function of fish length, whose magnitude depended on method of seine operation and fish taxonomic group. Capture efficiency is the product of efficiency of encirclement as the net is laid (which decreases with increasing fish size) and efficiency of retention as the net is hauled (which increases with increasing fish size). Retention was determined by modeling mark–recapture data. Dividing observed capture efficiency by this retention yielded empirical encirclement efficiency, which was then compared with encirclement efficiency determined from a simulation model of fishes' evasive behavior. The simulation accounts for the fishes' swimming speed relative to the speed of deployment of the seine, threshold distance (how close the disturbance from laying the net must be to initiate evasion), appraisal time (how long a fish continues evasive behavior when it moves outside the threshold distance), and the directionality of evasive movements. Simulated results of encirclement efficiency corresponded to empirically based predictions within plausible ranges of the simulation variables above, although for fish of length exceeding about 50 cm there is a high coefficient of variation in captured biomass due to small numbers and low catchability. We conclude that the method can be used for a wide range of conditions to convert seine capture data to unbiased estimates of abundance and size distribution, but that empirical determinations will still be needed for different net specifications and sampling conditions.