Development of an accurate model to predict the phenology of Atlantic salmon smolt spring migration

Changes in migration timing, resulting from the alteration in river continuity or the effect of climate change, can have major consequences on the population dynamics of diadromous fish. Forecasting the phenology of fish migration is thus critically important to implement management actions aimed at protecting fish during their migration. In this study, an 11-year monitoring survey of Atlantic salmon smolts (Salmo salar) from the Ourthe River, Belgium, was analysed within a European Special Area of Conservation to improve the understanding of environment-induced spring migration. A logistic model was fitted to forecast smolt migration and to calculate phenological indicators for management, i.e. the onset, end, and duration of migration, while accounting for the influence of photoperiod, water temperature, and hydrological conditions. The results indicated that the photo-thermal units accumulated by smolts above a 7 degrees C temperature threshold was a relevant proxy to reflect the synergistic effect between temperature and photoperiod on smolt migration. After integrating the effect of river flow pulses, the model accurately explained the inter-annual changes in migration timing (R-2= 0.95). The model predictions provide decisive management information to identify sensitive periods during which mitigation measures (e.g. hydropower turbine shutdown, river discharge management) should be conducted to promote smolt survival. The model was used to predict phenological characteristics under future scenarios of climate change. The results suggest a joint effect of hydrological alterations and water warming. Temperature increases of 1-4 degrees C were associated with earlier initiation of migration, 6-51 days earlier, and spring flood events greatly influenced the duration of the migration period. Accordingly, the combined effects of human-induced modifications of the hydrological regimes and increasing temperatures could result in a mismatch between the smolt and favourable survival conditions in the marine environment.