Dammed Clutha Salmon: Recruitment Above Hydro Dams Increases the Resilience of an Anadromous Chinook Salmon Population in Regulated River Systems without Fish-Passage Structures

Metapopulation dynamics may play a critical role in the persistence and recovery of vulnerable salmonid populations. We investigated the natal origin of Chinook salmon in the Clutha River, New Zealand, where a hydropower dam has blocked access to traditional spawning areas since 1956. Despite an absence of fish passage facilities and high quality spawning areas below the dam, a small residual run of anadromous Chinook salmon has persisted. Analysis of otolith microchemistry (⁸⁷Sr:⁸⁶Sr, Sr:Ca and Ba:Ca ratios) assigned harvested adult salmon from the lower river to natal sources located both above (61%) and below (39%) the hydropower dam. Above-barrier recruitment contribution was dominated by progeny from one of three self-sustaining landlocked salmon populations, which on average represented 55% of all recruitment across the study period. Reconstructed length at ocean entry, based on otolith chemistry, revealed 82% had adopted a stream-type life history strategy, typically emigrating at a mean size of 138 mm, regardless of natal origin. Results demonstrate that progeny of landlocked populations can be important resources for the resilience, and possible recovery, of vulnerable anadromous populations. As such our findings may have implications for conservation plans aimed at restoring or enhancing other anadromous salmonid populations in other regulated catchments.