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Numerous laboratory and field studies have found that female Pacific salmon have higher mortality than males during their once-in-a-lifetime upriver spawning migration. However, the proximate cause(s) of this increased mortality are poorly understood. This study exposed sockeye salmon (Oncorhynchus nerka) to a mild capture and tagging stressor and evaluated physiological recovery and movement behaviour at 1 and 4 h postrelease. Female sockeye salmon did not expend more anaerobic energy in response to the stressor but did have higher plasma lactate levels 4 h after the stressor, indicating that females took longer to physiologically recover compared with males. In addition, female salmon had lower plasma glucose but higher plasma cortisol, plasma [K.sup.+], and cardiac lactate levels compared with males. Male and female salmon had markedly different postrelease behaviours within the first hour of release; males were more likely to hold position within the staging area. Two potential mechanisms leading to increased mortality in female salmon were identified in this study: (a) prolonged recovery duration (possibly mediated by elevated plasma cortisol levels) and (b) insufficient oxygen delivery to the heart. De nombreuses etudes en laboratoire et de terrain ont releve que les saumons du Pacifique femelles presentent des taux de mortalite plus eleves que les males durant leur unique montaison de frai. Les causes immediates de cette mortalite accrue demeurent toutefois mal comprises. Des saumons sockeyes (Oncorhynchus nerka) ont ete exposes a un faible facteur de stress associe a la capture et au marquage, et leur retablissement physiologique et leur comportement de deplacement ont ete evalues 1 et 4 h apres qu'ils aient ete relaches. Les sockeyes femelles ne depensaient pas plus d'energie anaerobie en reaction au facteur de stress, mais presentaient des concentrations de lactate plasmatique plus elevees apres 4 h, indiquant que leur retablissement physiologique etait plus long que celui des males. En outre, les saumons femelles avaient de plus faibles teneurs en glucose plasmatique, mais de plus fortes teneurs en cortisol plasmatique, en [K.sup.+] plasmatique et en lactate cardiaque que les males. Les saumons males et femelles presentaient des comportements nettement differents durant la premiere heure apres leur lacher; les males etaient plus susceptibles de rester en place dans l'aire de repos. Deux mecanismes possibles menant a une plus importante mortalite des femelles ont ete cernes dans l'etude, a savoir : (a) une duree prolongee du retablissement (possiblement modulee par des concentrations elevees de cortisol plasmatique) et (b) un apport insuffisant d'oxygene au coeur., Introduction Sex-biased adult mortality can strongly influence population dynamics and extinction risk (Boukal et al. 2008; Melbourne and Hastings 2008), yet sex is often not considered as a factor in [...]