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.