Simultaneous assessment of two passage facilities for maintaining hydrological connectivity for subtropical coastal riverine fish

Abstract

Engineering solutions that aim to restore hydrological connectivity in river systems fragmented by dams and weirs frequently include the installation of fish passage facilities (i.e. fishways). Assessment of fish passage performance is often limited to single fishway designs and with a focus on species of commercial or conservation significance, meaning that conclusions regarding fishway success may not apply to all species that require access to fragmented habitats. We simultaneously compared the population structure and fish assemblage composition at two fishways (vertical slot and fish lock designs) at weirs in a subtropical coastal river system. We used hydrological and physico-chemical water quality data to explore if fishway performance was linked to particular environmental conditions. At both fishways, fish assemblage composition differed between fishway exit and downstream river samples, yet differences between exit and river samples were not associated with hydrological variables related to the size and frequency of weir drown-out events or water quality. Population structure of the most abundant species (sea mullet, Mugil cephalus; 61% of fish samples) also differed significantly among fishway exit and river habitats. A low-level structure spanning the river channel downstream of one weir was not associated with differences in fish assemblage composition indicating that access to the fishway entrance (and therefore fishway use) was not hindered by in-stream obstructions. Eight and seven species (predominantly small-bodied species that migrate within freshwater systems) were exclusively sampled in downstream river locations at the vertical slot and fish lock fishways, respectively, indicating that neither design was fully capable of facilitating the passage of the local fish fauna. Further assessment over a broader range of hydrological conditions will be necessary to determine if use of either fishway by fish is affected by low flows. Coupled with evidence produced from landscape-scale analysis of spatial and temporal variation in fish assemblage structure and dispersal patterns of diadromous species associated with river flow regime highlight that hydrological connectivity may be maintained by natural weir drown-out events. This study highlights that factors controlling hydrological connectivity for riverine biota need to be integrated among co-occurring anthropogenic impacts to identify and overcome constraints to effective conservation management in rivers subject to water resource development.