Rainbow trout (Onchorhynchus mykiss) are an important aquaculture species raised for food and sport release, yet little is known about their response to stresses. There are few tools available to diagnose and monitor stress response in these and other species of fish. However, routine diagnostic hematology and clinical chemistry analyses are not used extensively in fish medicine due to the lack of reference intervals for various fish species. Also, external factors such as water quality, diet, culture conditions, and age or sex of the fish can affect some blood values.