No abstract

Fisheries scientists need to understand the relationships between river temperature, discharge and production of juvenile salmonids to inform evidence‐based management and regulation of rivers and to understand the potential effects of climate change. These relationships can be determined by characterising interannual variability in abundance and environmental conditions from long‐term monitoring data and assessing their inter‐relationships. Two major challenges are (1) the requirement to separate the relative effects of stock level and environment which both affect interannual variability in abundance and (2) obtaining long‐term environmental time‐series that do not suffer from temporal biases. This study built on recent advances in hydrological, river‐temperature and juvenile salmonid modelling to investigate the influence of temperature and discharge on interannual variability in Atlantic salmon (Salmo salar) fry (age: 0 years) and parr (age: 1 year) production. The study used a unique long‐term dataset (>50 years) with detailed age‐differentiated census data collected for multiple life‐stages. The study shows that most of the interannual variability in recruitment was explained by stock level. Discharge had a comparatively small effect on fry recruitment, but a greater effect than artificial stocking. Discharge had no discernible effect on parr recruitment. Temperature had no effect on recruitment of either life‐stage. This study suggests that salmon are well adapted to current environmental variability in natural upland rivers in Scotland, but reductions in discharge during spawning and emergence could negatively affect fry recruitment with consequences for regulation of river flows. The study highlights the importance of high‐quality census data for accurately determining the effects of environmental variability on recruitment.

Conservation stocking is frequently used by fishery managers to stabilize or increase production of depleted fish stocks. However, in the case of salmonids the benefits are increasingly questioned and generally poorly quantified. We investigated the effects of ova stocking on freshwater emigrant production in a declining Scottish population of “spring” Atlantic Salmon Salmo salar. The stocking program was designed to minimize risks, maximize expected benefits, and bypass population bottlenecks between the ova and fry stages. Long‐term data (33 cohorts over 42 years, including 8 years of stocking) on numbers of ova and juvenile emigrant production were used to investigate whether the ova–emigrant stock–recruitment relationship differed between conditions of natural spawning and stocking. We considered Ricker and Beverton–Holt models that included terms for the effects of stocking, intercohort competition, and changes in trap efficiency on emigrant production. The “best model” was considered to be that with the lowest corrected Akaike information criterion (AICc). The strength of evidence for competing models was provided by smooth AICc weights. There was strong support for intercohort competition whereby large cohorts reduced survival rates of subsequent cohorts. There was little support for a stocking effect, and the best model did not include stocking. Changes in juvenile emigrant production could have been detected with 80% power if those changes had increased by an average of around 24% over the eight stocked years, indicating relatively high power to detect stocking effects. Although focused on a particular river system and set of ova stocking protocols, this study suggests that stocking fails to increase Atlantic Salmon production where wild fish populations and suitable habitat remain.