The impact of a changing winter climate on the hatch phenology of one of North America's largest Atlantic salmon populations.

In northern temperate regions, some of the most dramatic effects of climate change are expected during the winter. Understanding how changing winter climates influence the seasonal timing of key life events is critical for implementing effective conservation strategies, especially for poikilotherms, whose physiology and development are particularly sensitive to changes in thermal environment. Four mathematical models are available to predict the timing of hatch and emergence in Atlantic salmon (Salmo salar); however, such models are only useful if the effect of temperature is both repeatable within and among maternal families, and predictable across variable temperature regimes. Using a split-brood experiment, we found the timing of hatch to be repeatable and predictable in Atlantic salmon from the Exploits River, one of the largest remaining wild populations in North America. Three of the available mathematical models under-estimated the timing of hatch by an average of 21-26 accumulated thermal units (ATU); however, we identified one model that provided reasonable estimates of hatch timing (average under-estimate 7 ATU) under the three incubation temperature regimes we tested. We applied this model to daily water temperature profiles from 2006-18 at four sites within the Exploits River watershed. Across all years and sites, the predicted dates at 50% hatch ranged between 8 March and 23 May, while predicted dates of 50% emergence ranged from 11 May to 13 June. By identifying the seasonal timing of these particularly vulnerable early life stages, this model can aid the implementation of conservation efforts for this ecologically and economically important population.