Development of fishways for longitudinal connectivity of Chilean rivers threatened by small-scale hydropower plant.

Abstract

Freshwater ecosystems have become severely fragmented by artificial instream structures. Significant efforts have been made to reconnect freshwater systems to support fish movement through the design and installation of dedicated fish passage structures, or fishways. However, fishway design is based on a traditionally unique focus on salmonids or a small number of species often from northern regions. As such, designs may not be effective in other parts of the world, such as the Southern Hemisphere. Particular examples include Australia, New Zealand and South America, where native species consist mainly of small-bodied fish, a term that refers to adult fish with a body length of up to 15 cm. The present research is motivated by the need to develop novel and innovative approaches to advancing fishway mitigation technologies for small-bodied fish and small juveniles exposed to instream structures. The situation of Chilean native fish species is used as the basis of a case study for the development of fishways for small-bodied fish worldwide. Three fundamental research questions are raised. (1) Which is the target group for the development of fishways in Chile? (2) What is the swimming performance of target group species? (3) Is there a fishway configuration suitable for passage of the target group, and can that suitability be quantified? Trichomycterus, Diplomystes and Percilia (the TDP group) were the most prevalent genera within the assemblages present at planned hydropower project sites in South Central Chile (present at 40% of the sites; Laborde et al., 2020). The TDP group is composed of two small-bodied fish species (T. areolatus and P. irwini) and a species that coexists alongside them in its juvenile stage (D. nahuelbutaensis, juveniles up to 15 cm; Arratia et al., 1983). All three are benthofagous and resident species. Consequently, the TDP group was identified as a target group for the facilitation of fish passage. Swimming capacities of TDP group species were estimated based on information obtained from the literature concerning applicability of proposed dimensionless variables and a proposed correlation of swimming capacities with fish species characteristics. Results showed that the dimensionless swimming capacities of small-bodied and large-bodied fish species are not comparable, even for the same taxonomic order, suggesting scale effects. Results also showed that the distance design curve proposed by Peake (1997) and applied to Percilia suggests fish passage velocity of up to 41 cm/s -1 along a 100-metre-long fishway. Based on a review of the literature, fishway baffle designs were identified as the most suitable for the TDP group. Consequently, three different fishway configurations were proposed and evaluated over a 4-metre-long prototype fishway: lateral baffles (treatment 1); baffle array (treatment 2); and alternating lateral baffles (treatment 3). Passage of individual fish and interspecific fish groups was assessed. Performance was quantified as passage efficiency, percentage of attempts, and percentage of impingements. In general, results showed that baffles increased passage efficiency and decreased the percentage of impingements. The lateral baffles configuration (treatment 1) showed the highest passage efficiency for T. areolatus (efficiency of 70%, 30% and 0% for treatments 1, 2 and 3, respectively) and P. irwini (efficiency of 43%, 0% and 20% for treatments 1, 2 and 3, respectively). In individual and interspecific group experiments, fish passage performance metrics in the lateral baffles configuration were not significantly different for T. areolatus, P. irwini and D. nahuelbutaensis. As such, lateral baffles constitute the optimum multi-species fishway design for small-bodied fish. Group passage efficiency in the lateral baffles configuration was 37%.