Do obstacles in fishways enhance upstream passage for non-sport fishes? ¿Los obstáculos en pasos de peces mejoran el paso aguas arriba en peces no-deportivos?

Abstract

Chilean ichthyofauna has a high degree of endemism due to its evolution on a territory isolated by natural barriers. This scenario makes its protection essential to maintain native biodiversity. Moreover, Chile possesses a high hydroelectric potential which has generated the development of numerous hydropower plants. Each hydropower plant represents a physical barrier that fragments the river and disconnects fish’s habitat. Fishways are the hydraulic structure designed to produce sustainable energy by reducing river’s fragmentation but, in order to achieve an efficient fish passage, the turbulent flow produced should be favorable to the specific species, making imperative to understand how Chilean fishes relate with turbulence. Research’s objective is to describe and analyze the behavior of two endemic non-sport Chilean fish species (Cheirodon galusdae and Basilichthys microlepidotus juveniles) on an obstacle’s wake, generated with a circular vertical cylinder in an open channel flow. Wake vortices of vertical axis, were periodically shed as in the flow fields typically occurring in real nature-like fishways. Three experimental series with a total of seventy-two experiments were performed. Six individuals of each species were tested in free flow and in five different diameter cylinders. Two-dimensional Particle Image Velocimetry, PIV, was used to measure the flow at an horizontal plane. Wake vortices were described by their length-scale and frequency. Digital video of fish motion were analyzed to quantify tail beat amplitudes and frequencies. Results show that both species interact differently with the wake. C. galusdae swam upstream escaping from vortices while B. microlepidotus kept its longitudinal position swimming on the wake. Tail beat amplitude, relative to fish length, increased with length-scale of the vortices and decreased with their frequency. Tail frequencies of C. galusdae increased with vortices length-scale and decreased with frequency. Tail frequencies of B. microlepidotus were constant in all experiments. Fish Strouhal number, correlated with vortex length-scale and fish length ratio for both species. It is concluded that wakes might serve as stimuli for upstream passage of C. galusdae. Additionally, B. microlepidotus would not favorable use cylinder wakes in a fishway and, compared to the undisturbed flow condition, it might exhibit higher energy consumption.