Vulnerability-size effects of electric fishing on population estimate, size distribution and mean weight of a sea trout, Salmo trutta L., stock

A dynamic mark-recapture electric fishing study on sea trout, Salmo trutta L., was carried out from July 1989 to January 1990. There was a significant difference in the recapture probability ( P = R / M ) between small (24–39 cm) and large (40–84 cm) sea trout with P- values of 0.31 and 0.62, respectively (χ² = 16.52, P < 0.001). The vulnerability-size relationship resulted in a slight overestimation of population size (2.5%) using the Petersen method by adding the two size classes together in the computations. Separate computations for the two size classes had no significant effect upon the estimate of the seasonal length distributions but a significant effect on the estimates of total length distribution ( P < 0.001) and the mean weight of the mature stock ( P < 0.001). The present data emphasises that size-selectivity should be taken into account in electric fishing studies on adult sea trout.