Disentangling local from macroenvironmental effects: Quantifying the effect of human encroachments based on historical river catches of anadromous salmonids
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In this paper, we use dynamic factor and intervention analysis to identify and quantify the effect of human encroachments on anadromous salmonid catches in 22 Norwegian rivers over a 128-year period. In particular, we address effects of hydropower development, gravel removal, and stock enhancement (fish ladder construction and stocking). The methods allow for quantification of perturbation effects in the river environment on the annual catch, after removing long-term temporal trends at the appropriate spatial scale. The estimated effects of the encroachments differed among rivers, indicating that the mechanisms differ among rivers even for the same type of encroachment. In three rivers where hydropower development and stock enhancement had occurred, a significant increase of 0.8–9.8 tonnes (t) (34%–44%) was estimated, whereas a significant reduction of 11.2 t (38%) was estimated for the river where gravel removal had taken place. For rivers with additional biological information available, we find support for our estimated effects. In general, removal of gravel significantly reduced catch and there were positive effects of fish ladder constructions and variable effects of hydropower development and stocking (both positive and negative coefficients).