Brown trout 29-year dataset - Glenariffe Stream, New Zealand

Globally, many large rivers are modified to meet human needs, often with adverse impacts on fish populations. In New Zealand, such rivers support important recreational brown trout (Salmo trutta) fisheries, but understanding of river alterations on trout is limited. This study utilized New Zealand’s most extensive brown trout dataset from Glenariffe Stream (1965–1993). Annual brown trout spawning numbers varied eight-fold; larger runs had more small, first-time spawners, while smaller runs were sustained by return spawners. Spawning timing differed by sex, with larger fish arriving two months later than initial smaller spawners. Juvenile outmigration was driven by water level, time of year, and lunar phase. Tagged fish data highlighted the significance of longitudinal connectivity for post-spawning adults, particularly females, which travelled over 100 km downstream to estuarine habitats to rapidly regain condition. Collectively, our findings quantify the inherent annual variability in brown trout spawning runs and the catchment-wide scale that brown trout population dynamics occur over. Managers need to understand these upstream-downstream linkages when making river-modification decisions.