Summary and Discussion
This report analysed pre-existing information on the timing of migration and sizes of Atlantic salmon smolts leaving Scottish rivers and attempted to predict emigration time using spatial covariates readily obtained from a GIS. The final model included Elevation and Year as fixed effects and Site as a random effect. A single estimate of S25 (DoY 118; 95% CL 103-134) was obtained for the entire Scottish coast. This was broadly consistent with that reported previously (Otero et al., 2014). However, there was no evidence of an effect of Latitude, Longitude, Coast or Distance Around Coast that would indicate predictable spatial variability at the Scottish scale. This likely reflects the limited and patchy availability of data (over time and space), and considerable inter-site and inter-annual variability in emigration time. In common with Otero et al. (2014), this study also observed a significant long-term decline in S25 (i.e. earlier migration), although the estimated effect of ca. -1.5 days per decade was considerably lower than the -2.5 days reported by Otero et al. (2014) for the Atlantic region as a whole. This study also extended previous studies by modelling S75 (DoY 127; 95% CL 110-145), thereby characterising a "sensitive window" for development activity (103-145) where large numbers of smolts are likely to be migrating into coastal waters. However, this window provides no information on the amount of time that fish will spend in the coastal zone and thus further information on coastal behaviour and migration would be required to further qualify this window appropriately.
Unfortunately it was not possible to model spatio-temporal variability in smolt sizes. However, the preliminary analysis presented here identifies considerable inter-site and inter-annual variability in the percentage of fish that would exceed the investigated tagging threshold. Given the lack of a robust spatial model, it is suggested that the data presented here are used to guide future tagging studies, but that where new sites are to be used, then pilot work should be carried out to identify the likelihood of catching sufficient numbers of large smolts in advance of any large scale resource investment. Even where pilot data can be collected there is still a risk that sizes could be markedly different between years. This is likely to be especially true in smaller rivers, where returner numbers can vary markedly from year to year affecting competition, growth and subsequently smolt age.