Initial observations on the effects of varying levels of deciduous bankside vegetation on salmonid stocks in Irish waters

Abstract

Abstract. The effects of varying levels of deciduous bankside vegetation on salmonid stocks in Irish rivers were investigated. In summertime, when marginal vegetation limited the extent of incident light reaching the river bed, a marked decline in both juvenile salmon, Salmo salar L., and juvenile and adult trout, Salmo trutta L., numbers were observed relative to stocks in adjacent areas with a less dense canopy. This appears to be a countrywide phenomenon in all available salmonid habitats. In the case of both juvenile salmon and all trout numbers a correlation is evident between the extent of shade, as measured in terms of the reduction in aquatic vegetation, and fish numbers, which fall as shade levels increase. Data also suggest that the length of tunnelled channel, upstream of tunnelled sites electrofished, seems to influence the standing crop of juvenile salmon with numbers of these fish falling with increasing tunnel length. This relationship is not evident in relation to trout numbers.