A hydraulic turbine that includes a turbine runner that reduces fish mortaliy as the fish pass through hydroelectric power stations. The turbine runner includes at least two turbine runner blades included in a corkscrew configuration. Significantly, the corkscrew blades are exceptionally long in the overall direction of fluid flow through the turbine. In accordance with preferred embodiments, fluid flow to the runner from the inlet is mixed flow; the fluid flow at the outlet is directed substantially axially. In certain highly preferred embodiments of the turbine runner: the ratio of the sum of the leading edge lengths to the runner diameter is less than 1.0, the ratio of each blade running length to the runner diameter is greater than 1.0, the ratio of each blade running length to the respective blade width is greater than 3.0, the ratio of the axial length to the runner diameter is greater than 0.6, and the turbine runner (at its best efficiency point): is at least 80 % efficient and achieves a rate of pressure change less than 80 psi/second along substantially the entire length of the runner blades, and the minimum absolute pressure maintained along the main fluid flow path is 4 psia when the trailing edges are located at a tailwater elevation.