Use of Frequency and Duration Analysis for the Determination of Thermal Habitat Thresholds: Application for the Conservation of Alasmidonta heterodon in the Delaware River

A novel approach for setting thermal habitat recommendations for an endangered aquatic species will be proposed in this paper. The method, uniform continuous above-threshold (UCAT) analysis, evaluates the duration and frequency of continuous events in which the temperature is higher than a specified value and identifies temperature levels that, because of their rare occurrence in the past, can be considered stressor thresholds. The UCAT analysis was applied to set thermal habitat recommendations for Alasmidonta heterodon, an endangered mussel species in the Upper Delaware River. It was found that a maximum daily water temperature of 26.5°C lasting for more than 7 days is a rare event in the A. heterodon population centers in Upper Delaware River and consequently this condition should be avoided by adjusting already regulated cold-water releases from upstream reservoirs. Knowledge of temperature thresholds and their relationship with persistent low-flow periods provided further decision support for the development of flow management recommendations for species protection. The case study of A. heterodon in the Upper Delaware River demonstrates that in the absence of laboratory research on physiological temperature tolerance of a species, UCAT analysis provides an effective way to approximate habitat conditions that fall within thermal tolerances. Because of quick and easy preparation and analysis, this method may also have a broader application to habitat studies of other animal groups.