||An individual’s physiological and behavioural response to its environment can have fitness implications. To address hypotheses about the roles of physiology and behaviour on spawning success in sockeye salmon (Oncorhynchus nerka), I conducted experiments in an artificial spawning channel during three spawning seasons. Experiments involved biopsy sampling and behavioural observations; physiology and behaviour were then related to reproductive longevity and egg retention of spawning females.
Females living longer on the spawning grounds retained a lower proportion of eggs, indicating that females were running out of time to complete spawning. However, several long-lived females (> 7 d) failed to complete spawning before death, indicating that time limitation may not have been a factor for these females.
Physiological changes associated with rapid senescence were characterized for both sexes. Salmon exhibited three major physiological trends during senescence that were independent of sex or reproductive maturity – a large increase in plasma indicators of stress and exercise (i.e., lactate and cortisol), a decrease in major plasma ions (i.e., Cl- and Na+) and osmolality, and a decrease in gross somatic energy reserves. Females exhibited a greater magnitude of change than males for gross somatic energy, plasma ions, and reproductive hormones.
Females that arrived at spawning grounds with high plasma lactate and low plasma Cl- concentrations not only died sooner after arrival, but also retained more eggs at death. Premature mortality was also linked with three other indices of stress and osmoregulatory dysfunction (i.e., elevated plasma glucose concentrations, reduced plasma osmolality, and Na+ concentrations), suggesting that these fish were stressed and / or senescing prematurely. Levels of reproductive hormones (i.e., testosterone, 17β-estradiol, 17,20β-progesterone) decline as females become reproductively mature and approach senescence. Females lived longer if they arrived with higher reproductive hormone levels, indicating that females that were more reproductively advanced were more likely to die prematurely. My data did not support an energy limitation hypothesis as short-lived females died with greater energy reserves than longer-lived females, indicating something other than energy exhaustion was responsible for mortality.