Smolt outmigration timing in Norway

Aim - Accurate predictions about transition timing of salmon smolts
between freshwater and marine environments are key to effective
management. We aimed to use available data on Atlantic salmon smolt
migration to predict the emigration timing in rivers throughout Norway.
Location - In this study, we used data outmigration timing data of
Atlantic salmon (Salmo salar) smolts from 41 rivers collected from
1984-2018 to make a predictive model for the timing of out-migrating
salmon smolts along 12 degrees latitude. Methods - Smolt migration data
were collated from rivers where smolts are monitored with traps, video,
and tagging and matched to river-specific metadata. Using a mixed effect
generalized additive model, we tested for effects of spatial location, air
temperature, river flow, and counting methods on the timing of 25% smolt
emigration from rivers. Results - After accounting for spatial effects and
repeated measurements (across years and among rivers), air temperature and
counting method were significant drivers of the estimated timing of smolt
emigration. In-sample predictions yielded strong correlation with observed
values, as did 10-fold cross-validation. Out-of-sample predictions
suggested that the previous national estimates underestimated the
migration timing in southern populations (linear model intercept = 39.73
days). Conclusion - Model-derived estimates of run timing can be used to
more accurately predict the timing of outmigration in order to better
calibrate environmental flows and regulate management of marine industries
such as aquaculture that may affect migration success at this particularly
sensitive life stage.