Cumulative Impacts of Water Abstraction on a Recreational Salmon Fishery on the Rangitata River, New Zealand

This study investigates factors contributing to the unexpected loss of a recreational Chinook salmon fishery in a reach of the Rangitata River in Canterbury, New Zealand. The Rangitata is one of only 14 rivers nationwide in which flows are protected by water conservation orders to provide for instream values in the face of competition from extractive uses. However, the river has experienced significant water abstraction or takes for irrigation and hydroelectricity generation, even recently. In this study, local fishers determined flow requirements and conditions for successful fishing based on historical catch data and gauged flows. Relationships between preferred and historical flows were examined using scenario analyses to identify the cumulative impacts of water takes on the conditions required for fishing. An irrigation take of $19.6{\mathrm{m}}^3/\mathrm{s}$ beginning in 2013 was found to have severely reduced the duration of flows in a $30{\mathrm{m}}^3/\mathrm{s}$ flow range that previously supported fishing success. A recently proposed further take of $10{\mathrm{m}}^3/\mathrm{s}$ will completely remove any remaining flows from this preferred flow range. These results show that cumulative takes of water from river systems may have profound and unintended consequences when the flow regimes that support recreational uses are not well understood and retained or protected. Impact assessments that address both cumulative takes and flow-specific values are needed to reliably detect and manage adverse effects. In this case, additional controls are required to maintain the values specifically recognized by the water conservation order.