5.5 Emigrants (autumn parr and smolts)
There is increasing evidence that migrating juvenile salmon are leaving river systems earlier in the year (Malcolm et al., 2015). As with the younger age of smolts, this is likely related to an increase in stream temperature although the controls on emigration timing within years and season are hard to determine with confidence given available data (Buddendorf, 2018). The timing of migration is likely to be important to subsequent survival, matching arrival at sea with available food resources and potentially allowing migrants to avoid other pressures (Thorstad et al., 2012). The consequences of changing migration time from the Girnock are currently unknown, although the observed patterns are not unusual in the wider context (Otero et al., 2013).
The relationship between the production of 1-year old parr and emigrants is not obviously density dependent (Fig. 10c). However, emigrant production is also affected by density dependent growth of juveniles (further modified by upward trends in river temperatures), changes in age at emigration and thus mortality. Although there are clear downward temporal trends in emigrant numbers, there are also complex temporal trends in the age composition of emigrants, and timing of migration (autumn vs. spring). Ignoring these important changes (e.g monitoring only spring migrants) could provide an extremely biased and misleading impression of the status of populations. Understanding these complex processes and their interactions calls for sophisticated numerical lifecycle models such as those deployed at Girnock previously (Gurney et al., 2008). Indeed, such lifecycle models are increasingly proposed for assessing and informing management action for salmon as populations decline and there is a need for careful consideration of resource expenditure (Bull et al., 2022). The Girnock remains one of the very few locations globally where such models can be realistically parameterised through detailed observational data collected across all relevant life stages and stock levels.