Do fishways stress fish? Assessment of physiological and hydraulic parameters of rainbow trout navigating a novel W-weir fishway

Experiments were performed in a novel pool & weir fishway to study the relationship between hydrodynamic and physiological parameters using rainbow trout (Oncorhynchus mykiss; mean TL = 15 cm). Fish were tested randomly using three fishway slopes (4, 7 and 10%) and three spacing ratios of W shaped weirs. Effect on fish hematology, immunology and stress were examined. Results showed that changing fishway hydraulic conditions altered fish stress levels. Increasing fishway slope and decreasing weir spacing, resulted in increased number of red blood cells and hematocrit, and decreased white blood cell count and serum lysosomes. In addition, levels of the most important stress indicator, cortisol, were elevated significantly compared to the control group, as slope, and consequently turbulence levels increased. Results showed that maximum turbulent kinetic energy (TKE) for a 10% fishway slope was increased to 0.22 m2/s2 while for a 4% slope and larger weir spacing was reduced to 0.05 m2/s2 which is the best condition for fish to rest in the pools created between the weirs. Observations on turbulence metrics showed that a weir spacing ratio of 6 offered the lowest gradients of energy changes. Furthermore, increasing the spacing ratio from 2 to 6 reduced cortisol levels and generally improved the physiological status of fish in the fishway. Considering turbulence levels at different depths of flow, the study concluded that physiological conditions for the passage of rainbow trout were best for a spacing ratio of 6 and a fishway slope of 4%. The evaluation of physiological metrics proved to be a vital and reliable process to integrate with hydrodynamics so fishway designs may be optimized to avoid stressing fish, and to enhance survival opportunities for migrating populations.

Automated summary

Experiments in a novel pool & weir fishway showed that altering hydraulic conditions affected fish stress levels and physiological parameters. Increasing fishway slope and decreasing weir spacing led to changes in blood cell counts and stress indicators. Optimal conditions for fish included reduced cortisol levels with a weir spacing ratio of 6 and a fishway slope of 4%.