663 Influences of Sex and Activity Level on Physiological Changes in Individual Adult Sockeye Salmon during Rapid Senescence * Corresponding author; e-mail: hruskak@interchange.ubc.ca. Physiological and Biochemical Zoology 83(4):663–676. 2010.  2010 by The University of Chicago. All rights reserved. 1522-2152/2010/8304-9139$15.00 DOI: 10.1086/652411 Kimberly A. Hruska 1, * Scott G. Hinch 1,2 Michael C. Healey 2 David A. Patterson 3 Stefan Larsson 4 Anthony P. Farrell 5 1 Centre for Applied Conservation Research and Department of Forest Sciences, University of British Columbia, Vancouver V6T 1Z4, British Columbia; 2 Institute for Environment and Sustainability, University of British Columbia, Vancouver V6T 1Z4, British Columbia; 3 Cooperative Resource Management Institute, Fisheries and Oceans Canada, Simon Fraser University, Burnaby V5A 1S6, British Columbia; 4 Umea ˚ Marine Science Centre, Umea ˚ University, SE-910 20 Ho ¨rnefors, Sweden; 5 Faculty of Land and Food Systems and Department of Zoology, University of British Columbia, Vancouver V6T 1Z4, British Columbia Accepted 2/28/2010; Electronically Published 5/19/2010 ABSTRACT A noninvasive biopsy protocol was used to sample plasma and gill tissue in individual sockeye salmon (Oncorhynchus nerka) during the critical life stage associated with spawning—arrival at a spawning channel through senescence to death several days later. Our main objective was to characterize the physiological changes associated with rapid senescence in terms of the phys- iological stress/cortisol hypersecretion model and the energy exhaustion model. Salmon lived an average of 5 d in the spawn- ing channel, during which time there were three major phys- iological trends that were independent of sexual status: a large increase in plasma indicators of stress and exercise (i.e., lactate and cortisol), a decrease in the major plasma ions (i.e., Cl - and Na + ) and osmolality, and a decrease in gross somatic energy reserves. Contrary to a generalized stress response, plasma glu- cose decreased in approximately of the fish after arrival, as 2/3 opposed to increasing. Furthermore, plasma cortisol levels at spawning-ground arrival were not correlated with the degree of ionoregulatory changes during rapid senescence. One mech- anism of mortality in some fish may involve the exhaustion of energy reserves, resulting in the inability to mobilize plasma glucose. Sex had a significant modulating effect on the degree of physiological change. Females exhibited a greater magnitude of change for gross somatic energy, osmolality, and plasma concentrations of Cl - , Na + , cortisol, testosterone, 11-ketotes- tosterone, 17,20b-progesterone, and estradiol. The activity level of an individual on the spawning grounds appeared to influence the degree of some physiological changes during senescence. For example, males that received a greater frequency of attacks exhibited larger net decreases in plasma 11-ketotestosterone while on the spawning grounds. These results suggest that rapid senescence on spawning grounds is influenced by multiple physiological processes and perhaps behavior. This study pro- vides some of the first data to look at sex differences in se- nescence in Pacific salmon. Introduction Semelparity, wherein an organism dies shortly after breeding once, is a life-history characteristic found in taxonomic groups as divergent as insects, fishes, and dasyurid marsupials (Dick- hoff 1989; Finch 1990). Shortly after mating, semelparous an- imals undergo a period of rapid deteriorative change involving a loss of homeostasis, decreased ability to respond to stressors, and increased risk of disease. These physiological changes, a process often referred to as senescence, are believed to be re- sponsible for organism mortality (Finch 1990). While all se- melparous individuals die quickly after spawning, the length of the period of senescence can have important life-history con- sequences. For example, female Pacific salmon that live longer on the spawning grounds not only have lower levels of egg retention at death (K. Hruska unpublished) but also are able to guard their redds longer in order to prevent superimposition by later-arriving females (Morbey and Ydenberg 2003; Hendry et al. 2004). Therefore, there is considerable interest in the mechanisms that cause and prolong the senescence process. This study examined individual sockeye salmon (Oncorhynchus nerka) during residence on the spawning grounds in order to characterize the physiological changes associated with senescence. Through the use of physiological telemetry, adult Fraser River sockeye salmon have become one of the best-studied fishes in terms of understanding the behavioral physiology of migrations (Cooke et al. 2008). Reproductive hormone and osmoregula- tory indexes suggest that fish are preparing for entry into fresh- water and spawning 1700 km from the Fraser River mouth (Crossin et al. 2009). Symptoms of immunosuppression and disease are also becoming evident at this phase of their migra-