AWWF releases 2025 Coldwater Fishery Habitat Temperature Monitoring summary

The Allagash Wilderness Waterway Foundation’s Coldwater Fisheries Habitat Temperature Monitoring project continues to record vital baseline thermal conditions in the tributaries and along the mainstem of the Allagash Wilderness Waterway.

Ambient water temperatures are a leading habitat factor driving the distribution and persistence of brook trout. By identifying critical thermal refuge habitats, fisheries biologists can prioritize tributaries based on their current ability to sustain trout populations and provide the opportunity for resilience as future temperatures increase.

Summary of Recent Findings (2022–2025)

Overseen by AWWF and authored by Frank Frost, field work from 2022 to 2025 has documented a warming trend in the Allagash River compared to historical data from the late 1970s. For instance, the mean July river water temperature has increased by 2.40 C (4.30 F) since the 1975–1980 period.

Key 2025 Report Highlights:

• Main-stem Stress: Water temperatures in the main stem Allagash River frequently exceed 230 C (73.40 F), reaching levels considered unsuitable or lethal for brook trout if they are exposed for more than seven days.
• Critical Refuges: Monitoring has identified 15 tributaries that may provide essential thermal refuge. Ramsay Brook remains the coolest tributary studied, never exceeding the 180 C (64.40 F) thermal optimum during any month of the four-year study. Other consistently cool refuges include Savage, Shepherd, Harding and Whittaker brooks.
• Vulnerable Tributaries: Conversely, Lower Allagash Stream and Upper Ellis Brook were documented as the warmest habitats, with maximum daily temperatures exceeding the upper thermal tolerance threshold of 200 C, (680 F) for 88% and 78% of days from June through August, respectively.
• Trout Observations: During a survey in August 2025, a population of approximately 30 adult brook trout was observed holding in the cool-water influence from an unnamed tributary to the river, which recorded the coolest tributary mouth temperature (140 C,  (57.20 F) during that trip.
• Habitat Quality: New tributary “pool surveys” in 2025 found a high incidence of high-quality habitat in Five Finger Brook, which contained four “Class 1” pools (large, deep and with good cover).

Looking Ahead

The project will conclude its fifth and final year of baseline data collection in 2026. This ongoing monitoring is essential for understanding how brook trout display various life history strategies to persist in a large and variable watershed despite a warming climate.

To read the full March 2026 Report covering the 2025 season, click here: