THE INFLUENCE OF SEX AND ENVIROMENTAL TEMPERATURE ON THE ACUTE STRESS RESPONSE OF JUVENILE YELLOW PERCH Perca flavescens.

Yellow perch are an ecologically and economically important fish in the North America.  At its peak, the Great Lakes yellow perch fishery constituted the largest inland capture fishery in North America.  Even though wild populations have declined, consumer demand for yellow perch remains high, which has driven the interest in yellow perch aquaculture.  Although female yellow perch grow faster and larger than males, and methods exist for generating all-female yellow perch progeny, efforts to utilize all-female perch in a production setting may result in unforeseen issues.  Consequently, a greater understanding of the sex-specific physiological differences in how they respond to a stressor is important.  We hypothesized that there is a sexual dimorphism in the stress response of yellow perch that are held in constant temperatures 15°C, 18°C, 21°C or a 15-21°C diel fluctuating temperature regimen.  Groups of juvenile male and female were subjected to a 5-min handling disturbance, and  sub-groups of individuals (equal numbers of male and female) were sampled at 0 h (pre-stress), 0.5 h, 2 h and 6 h post-stressor exposure.  Individuals were assessed for physiological measures of stress response, including plasma cortisol, glucose, osmolality and hematocrit.  Temperatures significantly influenced all measured parameters (cortisol, glucose, osmolality and hematocrit), and sex had a significant influence on basal levels of plasma cortisol and glucose levels, with higher levels occurring in females.  Plasma cortisol levels peaked at 0.5 h post-stress in all thermal treatments and reached pre-stress levels by 6 h post-stress.  Plasma glucose followed a similar pattern, but a return to pre-stress levels was delayed in animals held at the 15°C constant temperature.  Plasma osmolality was also affected by temperature with higher levels occurring in animals held at higher temperatures.  Following the 5-min stressor, plasma osmolality levels decreased in all groups and only returned to pre-stress levels in the constant 15°C and 18°C groups.  Plasma hematocrit levels were influenced by holding temperature, with highest levels occurring in those animals held at the 21°C constant temperature.  These results show that a brief handling stressor and environmental temperature influence key markers of stress response (cortisol and glucose), as well as important hematological parameters (osmolality and hematocrit) in yellow perch, and that female and male yellow perch respond differently to handling stress.