Ocean thermal conditions in the post-smolt nursery of North American Atlantic salmon

The effect of climate on the post-smolt survival of North American Atlantic salmon is obscure owing to the difficulty in interpreting the only relationships thus far observed between the abundance of these stocks and climate, which focuses on winter conditions. Placing significance on winter post-smolt survival is contrary to conventional thinking that the spring period is more important, because that is when the post-smolts migrate to sea and transition to ocean life takes place. The pre-fishery abundance for North American stocks was compared to thermal conditions in potential post-smolt nursery areas during the period 1982–1999. Pre-fishery abundance was modeled as a reconstruction of one-sea-winter (1SW) and two-sea-winter (2SW) age salmon populations. Cohort abundance was compared to mean temperature and thermal habitat (sea surface area within a given temperature range) in five index areas. Stock size was negatively correlated with mean sea surface temperature during June. Correlations were comparatively stronger between stock abundance and thermal habitat, further asserting that June conditions – the first month at sea for most stocks in the region – may be pivotal to survival. These correlations suggest that post-smolt survival is negatively affected by the early arrival of warm ocean conditions in the nursery area. Hypotheses related to post-smolt migration, predation, and the availability of suitable prey are discussed.