Evaluating a fishway reconstruction amidst fluctuating abundances

Abstract

AbstractFishways have traditionally been designed according to the capabilities of salmonids, but other species may be unable to navigate such structures. To improve passage for the entire local fish fauna in a cost‐effective manner, pool‐and‐weir fishways can be reconstructed to a hybrid vertical slot design. Here, we contrast a typical short‐term before‐after monitoring approach with a long term data series to assess whether we could document an effect of the reconstruction on the numbers and composition of species ascending the Høyegga fishway in the Glomma River, Norway. We found a significant difference in the species composition ascending the fishway before (2014–2015) and after (2016–2017) reconstruction. Species diversity increased and the combined number of fish ascending the fishway approximately doubled, largely driven by greater numbers of European grayling and European whitefish. However, the variance of the total number of fish and their relative composition found in the long‐term monitoring data (1985–2013) exceeded the range of values found in the apparent short‐term change. We could therefore not reliably attribute the changes to the reconstruction, as they could perceivably result from other, unexplained factors. Although diversity increased, we did find evidence that the fishway remained species‐selective after reconstruction. We conclude that although there have been improvements to passage in the fishway, the potential fish migration has not been restored.