River restoration in Bognelv, Northern Norway : salmonid and macroinvertebrate responses to restoration measures

This is the fourth study of the response of salmonids to river restoration in Bognelv in Finnmark County, Norway. Bognelv was channelized, erosion secured and flood protected during the late 1930s to early 1990s period, and as a consequence of this, salmonid densities declined dramatically. The first restoration measures were conducted in 2006, and the last restoration measure to date were conducted autumn 2014. Following previous surveys’ sampling design, we sampled salmonid juveniles using electrofishing and benthic invertebrates were sampled by means of kick-sampling. Brown trout (Salmo trutta) responded quickly to restoration measures, already in 2008 with relatively much higher densities compared to before 2006, and continued to their highest overall mean production of juveniles in 2015. Atlantic salmon (Salmo salar) densities remained low through 2013, but then increased substantially in 2015, but not on the same level as brown trout. According to our data, Arctic charr (Salvenius alpinus) have barely responded to the restoration measures, being absent from all samples in 2013 and 2015, and was therefore excluded from our analyses. Macroinvertebrates were classified to the level of species for the first time in 2015 to better investigate their response to the restoration measures. The most important environmental variables influencing density and length for Atlantic salmon and brown trout juveniles were; depth, duration of growth period, temperature during growth period, moss cover, 1+ density, gravel and distance from E6 (the estuary). An ordination analysis was carried out to reveal environmental and restoration effects on the macroinvertebrate community. The most important environmental variables that affected diversity of macroinvertebrates were; distance from E6 and water velocity. Macroinvertebrate diversity increased with increasing distance from E6 and increasing water velocity. Type of restoration measure had different effects on brown trout and Atlantic salmon densities. Both species had highest 0+ densities in areas with weirs and riparian modifications, while brown trout and Atlantic salmon 0+ density was lower in side channels and tributaries than in unrestored stations. 0+ Atlantic salmon density increased with increasing time since restoration. 0+ brown trout length was greater in areas with weirs and riparian modifications, while length was lower in side channels and tributaries. 0+ brown trout length was greater in restored stations than in unrestored stations. 0 + brown trout length decreased with increasing time since first restoration measure (2006). 0+ Atlantic salmon length was greater in side channels and tributaries, while weirs and riparian modifications had little effect on their length. The highest diversity of macroinvertebrates was found in areas where riparian modifications and opening of side channels/tributaries was conducted. Areas with weirs had a similar effect as unrestored areas on diversity. The restoration process appears to have started, with promising responses for brown trout production, but further monitoring and broader scaled sampling is needed to better test the effects of different restoration measures on all three salmonids and macroinvertebrates in Bognelv