This part of ISO 23893 provides guidance on how to sample fish for determination of biochemical and physiological characteristics, such as the composition and enzyme activities of blood, liver, muscle and other tissues in order to asses the health of fish in the field as well as in the laboratory. The biochemical and physiological variables used for this purpose are often called biomarkers. This part of ISO 23893 includes recommendations and methods for: obtaining a site-specific sample of a representative number of fish; sampling fish tissues in the field and in the laboratory; and handling and preservation of samples prior to analysis of biochemical and physiological variables