By Eero Niemela, Markku Julkunen, Jaakko Erkinaro
Fisheries Research, Volume 48, Issue 1, August 2000, Pages 15-22, ISSN 0165-7836, DOI: 10.1016/S0165-7836(00)00113-2.

Yearly variations in catchability were analysed in a long-term electrofishing monitoring programme aimed at determining juvenile salmon densities. The mean catchability for both fry (age 0+) and parr (age >=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 surround nets were used. Some changes in the sampling technique were introduced between the periods, which impacted on the catchability. The catchability decreased with parr density, but the predictability of the regression between the density and the catchability was weak. The mean catchability over the years and sampling sites was as high as over 0.4 for fry and over 0.5 for 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 possible.
Keywords: Atlantic salmon; Fry; Parr; Density; Monitoring; Surround nets; Electrofishing

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