In-river behaviour and freshwater return rates of sea trout, Salmo trutta L., from two coastal river populations

The effective management of anadromous Salmo trutta resources is challenging because long-term data on life history, phenology and survival are sparse and most stocks across the range are highly diverse and data-limited. The current study employed acoustic telemetry to tag 448 sea trout across three life stages, to describe the phenology, spawning behaviour and return rates of smolts, finnock (0+ sea age) and adult (>= 1+ sea age) sea trout in two Irish river systems during 2018-2021. Tagged smolts (n = 206) exhibited river to sea transition rates of 78%-92% and a number of surviving smolts returned to their natal river as 0+ sea age finnock, exhibiting overall smolt to finnock return rates of between 6% and 17%. Short-term vagrancy occurred among smolts, and 14 individuals were detected in adjacent non-natal rivers. Finnock tagged during the late summer (n = 205) exhibited a range of behaviours with a minority (<30%) ascending upstream to spawning areas. Tagged adult sea trout (n = 37) ascended upstream to the spawning grounds and between 50% and 80% successfully returned to sea as kelts after spawning. Subsequent return rates of kelts back to the river in the following year ranged from 9% to 40%. The current study indicated that body size was an influential predictor of behaviour and survival across all three life stages. Increased body size was positively associated with marine transition success in smolts, long-term marine survival in kelts and spawning behaviour in finnock. This work further demonstrates the complexity of sea trout life-history dynamics and provides a comparative perspective across different age classes. An understanding of life-history variation, behaviour and survival is fundamental for the successful management and conservation of sea trout stocks.