Historic logging contributes to water temperature increases for salmon, study finds

A collaborative study between researchers at Simon Fraser University and Fisheries and Oceans Canada (DFO) has found that high logging intensity in the Interior watersheds of British Columbia is associated with warmer stream temperatures in salmon—bearing streams, potentially contributing to increased heat stress in salmon.

Researchers examined forestry activities, watershed characteristics, and stream habitat in the watersheds of 28 tributaries of the North Thompson River to understand how forestry influences salmon habitat in streams.

They found that while stream slope explained most of the variation in physical stream habitat, the intensity of logging in a watershed accounted for higher summer stream temperatures. The relationship between logging and stream temperatures was even stronger when the researchers looked at logging within the riparian area, the stretch of trees and vegetation that buffers streambanks and shades streams from sunlight. Their findings are published in the Canadian Journal of Fisheries and Aquatic Sciences.

High stream temperatures have a negative impact on salmon. Warmer water holds less oxygen, making it more difficult for salmon to breathe and increases their metabolism, which causes them to burn energy at a faster rate. This can make it challenging for juvenile salmon to grow while also making them more vulnerable to disease. High temperatures can also result in fewer adult salmon reaching their spawning grounds and reduced spawning success in the fish that do arrive.

Water temperatures are already rising due to climate change. The study authors note that their results can help forestry and fisheries resource managers protect salmon by avoiding further logging near salmon-bearing streams. The shade trees provide over streams helps to keep water temperatures cool enough for the fish to thrive.

Managing the heat

The study found that streams where 35 per cent of the riparian (riverbank) area was harvested between 1970 and 2019 had average maximum daily summer temperatures 3.7 °C higher than streams where five per cent of the riparian area was harvested.

“This is a really big difference for fish like salmon that are adapted to cool water. At higher water temperatures fish will stop growing, struggle to get enough oxygen, and even die,” says the studies’ lead author, Dylan Cunningham, who carried out this research for his masters as a student in SFU’s School of Resource and Environmental Management (REM) in the Salmon Watershed Lab. Cunningham was also a co-op biologist for DFO, while collaborating with other DFO researchers based out of REM in the Cooperative Research Management Institute.

These streams on the high (35 per cent) end of riparian harvests saw average daily maximum summer temperatures of 17.2°C, which is high enough to cause stress to juvenile coho.

“Harvesting riparian habitats in headwater streams leads to warmer water temperatures with potential downstream consequences for fish," says study co-author and research scientist with DFO, Doug Braun.

The study authors suggest that maintaining and or increasing forest cover, including in the currently unprotected riparian areas of headwater streams, could buffer predicted temperature increases.

Additionally, managing forestry and logging practices at a finer spatial scale could allow for more control over forestry impacts to reduce the negative impacts on salmon. The authors note that the proportion of each watershed logged ranged from one to fifty-nine percent. Cunningham notes that fifty-nine is a large and shocking amount of logging for a single watershed.