Tackling Fish Passage Problems: Conservation Implications of Fish Ascending Hypobaric Pressure Gradients in a Pipe

Abstract

Instream barriers disrupt the connectivity of rivers worldwide, threatening fish communities by limiting access to essential habitats. Innovative conduit fishways, such as the siphon fishway with flow regulators, have potential for improving native fish passage in Australian rivers but necessitate fish to negotiate pressure reduction below ambient atmospheric pressure. We investigated the volitional passage of juvenile silver perch (Bidyanus bidyanus) against hypobaric pressure gradients to 1-m (~91.5 kPa) and 2-m (~81.8 kPa) elevations inside a pipe, with and without flow. Fish negotiated both elevations (~20% maximum pressure reduction relative to ambient atmospheric pressure) without adverse impacts, and the presence of flow resulted in faster passage across elevations. The probability for a proportion of the group of five fish passing within an hour ranged from 38% to 80% across four treatments, with a predicted 50% chance of passage of three out of five individuals occurring under 45 min for three treatment groups. A reliable comparison of passage performance between treatments, however, was complicated by the possible effect of fish conditioning. In demonstrating fish ability to negotiate hypobaric pressure gradients, we provided a foundation for using the siphon fishways as a cost-effective fish passage for threatened native species. These findings present significant conservation opportunities for improving connectivity at sites affected by low head barriers globally.