This report attempts to establish guide-lines for electrofishing in population studies and is the result of literature studies and experience from electrofishing in Denmark, Finland, Norway and Sweden. Equipment, safety and training, sampling design and precision requirements for various types of investigations, population estimation and fishing practice are discussed. The results are put forward in the form of recommendations.Special attention is paid to the sampling design of surveys in streams of different types and for different purposes. Examples of the computation procedures are also included.

We review factors affecting the withinriver spawning migration of Atlantic salmon. With populations declining across the entire distribution range, it is important that spawners survive in the last phase of the spawning migration. Knowledge on the factors affecting migration is essential for the protection of populations, and to increase the success of reintroduction programmes. A number of studies have documented that the upstream migration may be delayed for many weeks at man-made obstacles such as power station outlets, residual flow stretches, dams, weirs and fishways. The fish may also be delayed at natural migration barriers. Often, the magnitude of delay is not predictable; fish may be considerably delayed at barriers that appear to humans to be easily passable, or they may quickly pass barriers that appear difficult. Stressful events like catch-and-release angling may affect upstream migration. Impacts of human activities may also cause altered migration patterns, affect the withinriver distribution of the spawning population, and severe barriers may result in displacement of the spawning population to other rivers. Factors documented to affect within-river migration include previous experience, water discharge, water temperature, water velocity, required jump heights, fish size, fish acclimatisation, light, water quality/pollution, time of the season, and catch and handling stress. How each of these factors affects the upstream migration is to a varying extent understood; however, the effects may differ among different river sections and sites. There are likely a number of additional important factors, and the relationship between different factors is complex. The understanding of general mechanisms stimulating fish within-river migration are still lacking, and it cannot be reliably predicted under which conditions a fish will pass a given migration barrier or which conditions are needed to stimulate migration at different sites. The strong focus on the effects of water discharge in past work may have hampered consideration of other factors. Exploration of the influence of these other factors in future studies could improve our understanding of what controls the upstream migration.

A hypothesis that thermal regime regulates the timing of spawning in Atlantic salmon (Salmo salar) was analysed by correlating time of commencement and peak of spawning in 16 Norwegian streams with temperature, latitude, and stream flow. Only temperature during incubation of the eggs proved to have any statistically significant effect. Since the duration of egg incubation is known to depend on temperature regime (i.e. on degree-days), a similar linking of spawning time to stream temperature allows spawning to occur at a time which will result in hatching of eggs at a specific and presumably optimal time for survival of fry.

No abstract

Daily ascents of adult Atlantic salmon (Salmo salar) in the Laksforsen waterfall in the River Vefsna were recorded over 3 years. Ascent was correlated to 12 physical and meteorological parameters by multiple regression analysis. Change in water temperature was included in the regression model in all 3 years, and change in river flow was also included in 2 of the 3 years. No other variable was included in the regression model at the 0.05 significance level.