The influence of water temperature on the ecology of aquatic organisms with optimal thermal ranges is quite substantial, as these regimes regulate activity, growth, survival, and spawning behaviour. In consequence, a deviation from normal temperature ranges often cause displacement, disease, or even mortality therefore water temperature constitutes an important limiting factor for the condition of fish habitat and freshwater ecosystems. The purpose of the fish thermal stress monitoring program at Kouchibouguac National Park is to determine the number of consecutive “hot” days within a particular time period and detect changes in the level of thermal stress imposed on fish due to warm temperatures. Atlantic salmon (Salmo salar) was selected as a focus species due to its lowest thermal tolerance compared to several other fish groups. This measure is based on scientific information with bioenergetic considerations that thermal stress can ensue after two successive 24-hour cycles where daily minimum water temperature is ≥ 20 degrees Celsius. The methods for this measure involve an annual sampling from late spring to early fall, where water temperatures are recorded every hour with data loggers positioned at permanent sites within riffles of five tributaries: Portage River, Black River, Rankin Brook, Tweedie Brook, and Kouchibouguacis River.