Managed thermal refuge case study: Housatonic River, CT, USA

The upper Housatonic River in western CT, USA supports the angling of various colder-water fish species including trout (primarily brown and rainbow trout).Although some wild reproducing salmonid species exist in the river (e.g. brook and brown trout), the fishery is dominated by stocked species of which a portion are not expected to survive the summer season due to unsuitable warm river water temperatures. However, confluences with small cold-water tributaries provide discrete thermal refuges for stocked and native species on a yearly basis, as the main river channel can exceed lethal tolerances in many areas. During times of thermal stress, fish tend to concentrate within small (several m2) confluence mixing zones, rendering the animals vulnerable to predation and angling. There are two State-designated ‘Trout Management Areas’ along the Housatonic River in CT, and within those areas nine cold water tributary confluences have signs posted to prevent summer angling within a distance of 100 feet from the signs. Enforcement is opportunistic by State Environmental Conservation Officers. The state Fisheries Agency has partnered with angler groups to actively enhance a subset of the refuges. This has involved arranging rocks, logs, and cut brush to increase the size of cold-water patches by inhibiting mixing with main channel water and providing overhead cover to deter avian and human predation.

Several recent years of data collection have shown the managed refuges are essential for allowing some Brown Trout to hold over multiple years and grow large in the river. Large holdover Brown Trout are highly valued and are key to the rare quality of the Housatonic trout fishery. This unique example of actively managed thermal refuges (primarily) for stocked species increasing angler interest in the river, positively impacting outdoor recreation and the local economy. Also, thermal refuges along the Housatonic River were a key element to the 2005 relicensing of upstream hydropower dams, which went from ‘pond-and-release’ to ‘run-of-river’ operations. Under pond-and-release, surges of warm river water on hot days caused trout kills at some key refuges by compromising the natural cold-water patches. Stable summer flows under current operations prevent sudden flushing of cold-water pockets with lethally warm river water.

It is not uncommon for these natural confluence zones the refuges can be 'maxed out' in fish capacity on the warmest days with fish grouped so tightly one cannot see the river bottom, and such crowding can lead to stress and exchange of disease. A better physical understanding of how the confluence mixing zones could be physically manipulated to house more fish during warm flow times could enhance management success. Therefore, there is current collaboration between the State of CT and the University of Connecticut to collect data and detail hydrodynamic thermal mixing processes at the sites. Also, these nine managed refuges are chosen based on the observed presence of salmonoids during times of unsuitably warm main channel temperature. Future monitoring of these sites is ongoing, as if a refuge location no longer seems to hold trout in summer it can be dropped from the active management list and the no fishing signs removed. This has occurred in the past for some smaller tributary confluence sites.

We are thankful to Mike Beauchene, Michael Humphrey, and Christopher Sullivan for contributing to this information.
Additional links:
https://portal.ct.gov/DEEP/Fishing/Fisheries-Management/Trout-Management-Areas
https://onlinelibrary.wiley.com/doi/full/10.1002/eco.2295