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

1. 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.
2. 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.
3. The results indicated that the photo-thermal units accumulated by smolts above a
7 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 (R2 = 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.
4. 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 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.