Quantitative electrofishing for juvenile salmon densities: assessment of the catchability during a long-term monitoring programme

Yearly variations in catchability were analysed in a long-term electrofishi ng monitoring programme aimed at determining juvenile salmon densities. The mean catchability for both fry (age 0+) and parr (age greater than or equa l to 1+) was significantly lower during the later nine years of the 18-year data set, without surround nets, than during the first nine years when sur round nets were used. Some changes in the sampling technique were introduce d between the periods, which impacted on the catchability. The catchability decreased with parr density, but the predictability of the regression betw een the density and the catchability was weak. The mean catchability over t he years and sampling sites was as high as over 0.4 for fry and over 0.5 fo r parr which fulfilled the general criteria of proper population estimation . A three-removal electrofishing technique is considered adequately precise for a long-term monitoring. The crew should consist of experienced members and the sampling procedure should be as standardised and consistent as pos sible. (C) 2000 Elsevier Science B.V. All rights reserved.