Country-wide analysis of large wood as a driver of fish abundance in Swedish streams: Which species benefit and where?

Donadi, Serena and Sandin, Leonard and Tamario, Carl and Degerman, Erik (2019). Country-wide analysis of large wood as a driver of fish abundance in Swedish streams: Which species benefit and where? Aquatic Conservation: Marine and Freshwater Ecosystems. 29 :5 , 706-716
[Research article]

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Abstract

1. Rivers are heavily affected by anthropogenic impacts that threaten many fish species. Among restoration measures, the addition of large wood (LW) in streams has been showed to increase fish abundance. However, what species benefit from LW, to what extent relative to other drivers, and what factors influence LW quantity is not clear, which limits our ability to use LW as an effective restoration measure.
2. Here, time series (from 1993 to 2016) of electrofishing data including 3641 streams across Sweden were used to investigate 1) beneficial effects of LW on the abundance of juvenile brown trout Salmo trutta, juvenile Atlantic salmon S. salar, and juvenile and adult sculpins Cottus gobio and C. poecilopus, while accounting for other abiotic and biotic factors, and 2) the drivers of LW abundance at country-wide scale.
3. LW benefitted brown trout, and the effects were larger with decreasing shaded stream surface. LW effects were comparable in magnitude to the positive effects of average annual air temperature and the negative effects of stream depth and predator abundance, factors whose influence was second only to the negative effects of stream width. LW did not benefit salmon abundance, which correlated positively with stream width and negatively with altitude, nor did it benefit sculpin abundances, which mainly decreased with annual average air temperature and altitude.
4. The quantity of LW strongly diminished with stream width, and, to a lesser extent, with stream depth, altitude, annual average air temperature and forest age, while it increased with stream velocity, slope and forest cover.
5. The results suggest that LW can be used as an effective restoration tool for brown trout in shallow and narrow streams, especially in areas with little shade. Here, the addition of large wood could help alleviate the impacts of forest clearance and climate change.