Mitigating connectivity disruption is among the most critical initiatives required to conserve fish communities in heavily regulated river systems. The efficacy of using fishways to facilitate fish passage around instream barriers is largely subject to biology of the target species and local environmental conditions. Hence, evaluation of fish swimming ability to provide biological guidelines for suitable fishway hydraulics is becoming increasingly common. While reviewing studies from 1990 to 2023 on fish swimming performance, significant effect of testing method on swim speed estimates, fish volitional selection of swimming mode based on trade-off between swim speed and endurance, and variable responses to various environmental factors across taxa, indicated that conventional assessment practices may not be sufficient to inform fishway engineering. We proposed the need for more fishway-specific volitional performance assessments, which were subsequently implemented in a pilot evaluation of an experimental siphon fishway. This fishway utilises a siphon flow that remains constant regardless of discharge, driven by a state of reduced pressure in the passage. We used a novel experimental procedure for assessing fish motivation to negotiate hypobaric pressure gradients in a pipe, in which surface-acclimated silver perch (Bidyanus bidyanus) negotiated pressure reductions up to 2 m-elevation (~20% pressure reduction below ambient atmospheric pressure), with a passage probability as high as 80% within one hour. This assessment method enabled evaluation of fish responses specific to hydraulics associated with volitional passage through siphon-based fishways. Further trials with a wider range of configurations can provide insights into potential of the siphon fishway to ameliorate river connectivity due to low-head barriers.