Graduation date: 2006The goal of this research was to characterize thermal habitat requirements for juvenile redband steelhead trout (Oncorhynchus mykiss gairdneri) in the South Fork John Day River (SFJD), Oregon using physiological indicators of fish condition. Physiological indices of fish condition measured were whole body lipid content and heat shock proteins, specifically hsp70. Hsp70 is a family of highly conserved molecular chaperones proteins that protect cellular function by preventing irreversible protein damage. Hsp70 levels were measured in liver, white muscle, and fin tissue. This thesis includes a study of passive integrated transponder (PIT) tag handling stress in juvenile rainbow trout. PIT tags are used to identify individual fish. The objectives of the PIT tag study were to determine if there is a stress response, measured as a difference in plasma cortisol levels, between PIT tagged and non-PIT tagged rainbow trout (O. mykiss), to determine if PIT tag-related stress induces hepatic hsp70 synthesis in juvenile rainbow trout, and to examine the relation between cortisol and hsp70 levels. There was a difference in plasma cortisol six hours after tagging, with both treatment and tank effects. There were no differences detected after the 24h sampling period. Plasma cortisol levels were highly variable, but changes in plasma cortisol did not alter hepatic hsp70 levels. A laboratory experiment demonstrated that hsp70 levels increase significantly between 19 and 22ºC in both fin and liver tissue. The finding that hsp70 can be quantified in fin tissue is significant because it provides a non-lethal technique for assessing thermal stress in rare or endangered fish. The response of hsp70 in relation to temperature was sigmoid. During the summers of 2004 and 2005, a field study in the SFJD was conducted. The objectives were to: (1) determine if SFJD redband rainbow trout experience thermal stress, (2) develop a non-lethal technique for measuring cellular hsp70 levels, (3) and characterize the relation between whole body lipids and hsp70 for fish in the SFJD. Maximum hsp70 levels in liver and white muscle tissue in field collected fish occurred when mean weekly maximum temperatures (MWMT) exceeded 22-23ºC. Short and long term stream temperature averages of 15.6-18.2ºC and temperature maximums of 18.8-21.6ºC significantly increased hsp70 levels in liver tissue. Both the hsp72 and hsp73 isoforms were significantly elevated in white muscle tissue when long and short term average stream temperatures were 16-18.5ºC and 19.6-23ºC for temperature maximums. Lipid levels began to decrease when MWMT exceeded 23ºC. Results of this research suggest increased cellular hsp70 levels in juvenile redband rainbow trout in the SFJD River is symptomatic of cellular stress related to thermal conditions, as are decreasing lipid levels. Measurement of cellular hsp70 levels provides another tool that can be used to characterize physiological suitability of thermal habitat and potentially to define thermal limits, as differences of 1 or 2°C are sufficient to change expression of thermal stress proteins detected using these techniques. When using hsp70 as an index of thermal stress in different O. mykiss subpopulations, differences in hsp70 expression between tissues should be considered.