Potential Impacts of Low Flows on Fish Foodscapes and Production in a Braided River

ABSTRACT Changes in river discharge affect the physical composition and connectivity of habitats which, in turn, may shape the spatial distribution of fish food abundance, accessibility and quality—the ‘foodscape’—of river ecosystems. However, the influence of river flows on fish foodscapes has received very little attention from scientists. We studied how low flows affect the foodscapes and production potential of invertivorous fishes within reaches of a braided river. We hypothesised that low flow may affect fish foodscapes and production within reaches via two broad mechanisms: (1) reducing total standing crop biomass of benthic invertebrates through shrinkage of reach wetted area; and (2) changing the depth and velocity composition of river reaches, which in turn may change the invertebrate composition of reaches. Using a hybrid ecosystem model we show that both these mechanisms are likely. Low flows reduced reach wetted area, hence space for benthic prey production, but also changed the taxon composition of the prey community to one that is less profitable to the fishes we studied. This compositional shift in the prey community was a result of the hydraulic conditions at low flows—lower velocities, shallower depths—favouring prey that are both smaller and less accessible to the fishes we studied. Our study shows that low flows may limit fish carrying capacity by affecting both the quantity and quality of food. We discuss the need for pragmatic, cost‐effective foodscape models to advance our basic understanding of how hydrogeomorphic change affects fish carrying capacity, and to strengthen environmental flow assessments.