E. Niemela et al., Quantitative electrofishing for juvenile salmon densities: assessment of the catchability during a long-term monitoring programme, FISH RES, 48(1), 2000, pp. 15-22
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.