Abstract
Acid and metalliferous drainage (AMD) reduces fish diversity with effects extending downstream. Discharges may also create chemical barriers for migratory species into otherwise unimpacted headwaters. We investigated kōaro ( Galaxias brevipinnis ) response to remediation of an AMD‐impacted tributary in a 5 th ‐order West Coast, New Zealand river. Kōaro communities were surveyed once pre‐remediation and on seven subsequent occasions between 2005 and 2024, at six sites within the Ngākawau catchment and two reference streams, using backpack electric‐fishing. Aluminium, pH and turbidity measurements were analysed for temporal patterns and exceedances of threshold values of fish sensitivity, derived for naturally acidic West Coast streams. Significant differences in kōaro demographics were observed between pre‐ and post‐remediation surveys. 2 years post‐remediation, kōaro abundance had increased, dominated by juveniles 100 mm. Current abundance and size distribution, plus interannual demographic constancy, suggest the population is stable and not recruitment limited.