The biological significance of thermal refuges to juvenile Atlantic salmon (Salmo salar) in a changing climate

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University of New Brunswick


The frequency of extreme thermal events is expected to increase under current climate change scenarios. These events may have drastic consequences for many river species, particularly in temperate climates. In recent decades, increasing water temperatures have had an effect on the phenology and life history of Atlantic salmon (Salmo salar) within the Miramichi River System, located in New Brunswick, Canada. Juveniles in this river spend 2-4 years in freshwater prior to smoltification and are susceptible to the impacts of increased water temperatures. Juvenile Atlantic salmon are territorial but will abandon territories when water temperatures become unfavourable. To understand how juvenile salmon respond to increasing water temperatures, several studies were conducted using a variety of experimental techniques in both the field and the laboratory. In this dissertation, 1+ and 2+ parr are studied over a series of summer periods to examine the i) movement patterns over the course of a high temperature event; ii) implications of temperature events on broadscale juvenile abundance; iii) observed behavioural thermoregulation response in the Miramichi and compare to a second temperate, geographically distinct river; iv) physiological response of multi-day thermal stress over the course of a thermal event in a controlled setting. When seeking refuge, juvenile salmon largely relocated to cool water sources within river reach. However, migrations of several kilometres were detected throughout a particularly warm thermal event (£7.4 km; Tmax = 30.7˚C). In years where temperature events occurred, an increase in juvenile abundance was observed in reaches containing cool water refuges, with a corresponding decline in abundance in reaches where refuges were lacking. The temperature of the onset of behavioural thermoregulation varied between geographically distinct river systems (27.3 ˚C vs. 28.3 ˚C), indicative of a localized preconditioning response. Exposure to multi-day thermal stress resulted in a cumulative physiological response (HSP70 & ubiquitin) in juvenile salmon, irrelative of diel minimum temperature. Cool water refuges are important thermal moderators for many ectothermic species, including salmonids. Understanding how species exploit thermal heterogeneity in the landscape will help to inform managers on best practices for protecting and conserving both key species and the critical resources on which they rely.