Modelling dynamic fish-waterfall interactions with digital image field data: From rescaling to weir removal for migratory freshwater fish

Abstract

Major declines in river connectivity due to human infrastructure increase river fragmentation. Acting as barriers, weirs impair or disrupt the ecological process of fish migrations. Economic or cultural interests may prevent the removal of such barriers, the common approach being the use of standard guidelines to retrofit a fish passage system. In this study, we explore a novel approach of rescaling the dimensions of a weir in height and reach of the waterfall to match the swimming and leaping capacities of the fish. Hydraulic and hydrological scenarios at seasonal and multi-annual scales are considered quantitatively. The novel technique is based on field data obtained from digital image techniques. The fish-hydraulics interaction is addressed by means of a calibrated and validated biophysical model. A sensitivity analysis explored the degrees of weir downscaling to removal across a range of population viability objectives. The results show that it is possible to reach the efficiency goal of maintaining viable biological populations of barbels and, at the same time, retain the weir infrastructure for human water uses. Biophysical modelling of field image data and infrastructure rescaling proved to be successful quantitative tools to inform management decisions.