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In The Field

Thermal refuge

Conservationists are enhancing cold-water habitat for Atlantic salmon and trout in the Miramichi River watershed, providing first aid for fish in the face of climate change and deforestation.

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Warm water causes stress and can be lethal for Atlantic salmon and brook trout. Erratic flows and high temperature events occur each summer in the Southwest Miramichi, pictured here. Photo Nathan Wilbur/ASF

Bridging the gap between science and real world application is difficult when practicality, economy, politics, industry, social values, and funding are factored in. One of ASF’s strategic goals is to restore freshwater habitat for wild Atlantic salmon, which means addressing root causes. This includes the issue of warming rivers, and in New Brunswick, the Miramichi is ground zero. There’s an impressive set of studies on the issue, but it’s not matched by real projects that tangibly help salmon. With key partners, we are bridging the gap.

In 2009 I started a deep dive into cold water. That year I joined a team at the University of New Brunswick doing research on water temperatures in the Miramichi. Since then, I’ve learned cold water is critical to the survival of salmon and trout in North America, and have been focused on helping translate the science into conservation action.

Atlantic salmon thrive in temperatures between 15 and 20 C (60-70 F), any higher and they become thermally stressed. When water temperatures exceed 25 C (77 F) it can be lethal. Salmon avoid these warm conditions and seek refuge in cold water created by brooks and springs entering rivers. These small water sources are better shaded and mostly fed by groundwater, providing a continuous supply of cold water to rivers throughout the summer.

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Sea run brook trout and one large Atlantic salmon aggregate in a cold water refuge on the Miramichi. Nathan Wilbur/ASF


A complex combination of climate, land cover, and geology form the basis of temperature regimes in rivers. Disrupting one of these variables can lead to changes in temperature and flow. Climate change is warming oceans, lakes, and rivers worldwide, and in New Brunswick this is exacerbated by extensive clearcutting within watersheds. When large swaths of land are cleared by forestry operations or other development, there is less shade and greater runoff after a heavy rain or during snowmelt. The result is warmer water, flashier flows, and more sediment movement in rivers.

My masters project with UNB was aimed at identifying cold water refugia and characterizing how Atlantic salmon and brook trout use the habitat. This work was mostly in the Cains River, a tributary of the Southwest Miramichi. I reasoned that determining which cold water sources provide the best refuge, and understanding what makes them the best, would help enhancement projects emulate nature and create a greater number of more effective cold water sanctuaries.

We mounted a thermal infrared camera on a helicopter and flew various sections of the Miramichi river system. In a nutshell, we found there were abundant cold water sources entering the river; however, very few of them had the physical conditions necessary to serve as refugia for fish.

Usually, the limiting factor was depth. Particularly for large fish, the water needs to be deep enough to provide a level of protection from overhead avian predators, of which there are plenty. This means fish prefer deep and cold water, or cold water next to a deep pocket to escape into should danger appear. We recently published a peer-reviewed scientific article on this work that appeared in the journal River Research and Applications.

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Abstract of recently published article on cold water research on the Miramichi. The article is not open source yet and not freely available to the public; please contact us if you would like more information.


Five years ago, the Miramichi Salmon Association began a concerted campaign of cold water enhancement, completing nine projects to date. The work is more complicated than digging out a pool at the mouth of a cold brook. Such a hole would quickly fill in because of gravel movement in the river.

Instead, successful cold water enhancement projects work with nature by installing subtle rock structures that divert river flow into focused areas. The energy of the river then scours out pools and maintains depth where cold water flows into the river.

Using principles of fluvial geomorphology, the designs are sustainable and blend into the riverscape. Before joining ASF, I worked for an engineering firm and was fortunate to bring my knowledge to bear on some of these projects, helping bridge gap between research and application.

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Otter Brook on the Little Southwest Miramichi before enhancement. Photo Nathan Wilbur/ASF
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Otter Brook after the Miramichi Salmon Association's 1-day cold water enhancement project carried out in 2015. Photo Amber Yates
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The Donnelly Brook cold water enhancement project was completed by the Miramichi Salmon Association in 2014. Photo Nathan Wilbur/ASF
Each time a sanctuary is created, Fisheries and Oceans Canada adds it to the list of approximately 30 cold water pools on the Miramichi that are closed to angling when temperatures get too warm. This move to protect salmon is called the “warm water protocol” and is widely accepted by the angling community.

A new phase of this work is about to begin. A partnership has been struck between ASF, the North Shore Micmac District Council, MSA, and UNB. There will be substantial investments over the next four years to aggressively expand cold water enhancement work throughout the Miramichi watershed with more details coming soon.


Helping valuable cold water reach holding pools for salmon through physical enhancement is treating a symptom of a bigger problem. Rivers like the Miramichi are suffering from an overall warming temperature regime resulting from a mixture of climate change and land-use impacts.

While the world grapples with solutions to global warming, there are local remedies for land-use. If we expect the landscape to sufficiently regulate flows and temperatures to sustain wild Atlantic salmon, we need more healthy, intact, diverse, and mature forests.

Whether traditionally deemed sustainable, or not, the way we practice forestry throughout the province must change if we want our freshwater ecosystems to be resilient in the face of climate change.

To answer this need, the province of New Brunswick recently announced plans to increase protected areas from 4.6 to 10 per cent of the province, a contribution to national biodiversity efforts under the Canada Target 1 initiative. In real terms, this means New Brunswick will add 381,655 hectares (~950,000 acres) of protected area, about two-thirds the size of Prince Edward Island, by the end of this year.

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Map of the Miramichi watershed showing headwater areas of each major river where we hope to gain significant new land protections under New Brunswick's drive to protect 10 per cent of the province.


Cold water refugia enhancement projects on their own are not enough to improve the thermal regime – and save salmon – on warming rivers. That’s why ASF and other groups are working with the team at New Brunswick’s Department of Natural Resources and Energy Development to identify critical forests worthy of new protection. Forests and soils are like a refrigerator for our rivers.

New Brunswick’s drive to protect more of the province’s forests combined with the work of the salmon community is a major step forward. We’re optimistic that results will manifest in the short and medium term – within months and years, not decades.

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Examples of Cains River thermal infrared images identifying cold-water sources during high-temperature events (>21 C) in 2010. The black lines represent sampling transects and the blue arrows indicate flow direction. From Wilbur et al, 2020


The research by Nathan Wilbur and others has been published in the journal River Research and Applications. Link:

Wilbur, NM, O’Sullivan, AM, MacQuarrie, KTB, Linnansaari, T, Curry, RA. Characterizing physical habitat preferences and thermal refuge occupancy of brook trout (Salvelinus fontinalis) and Atlantic salmon (Salmo salar) at high river temperatures. River Res Applic. 2020; 1– 15.