Heat

The Unspoken Fifth and Master Threat to Salmon Survival

Heat is a threat multiplier, essentially increasing the detrimental effects of each of the factors impacting salmon conservation known as the “Four H's:” habitat, hydropower, harvest, and hatcheries. As human-caused climate change accelerates, the impacts of the other H's are expected to intensify. Here are some of the ways climate change will also change the needs of fish species and the conservation challenges they face.

Warming temperatures and decreased snowpack: the new normal for the Pacific Northwest.

The impacts of climate change are becoming increasingly apparent across the region’s landscapes. Hotter summers, warmer winters, and decreased snowpack have hit waterways and native fish especially hard. Native fish face the double whammy of decreasing stream flows and a hotter climate. The warm winter of 2015 and the resulting drought conditions the following summer are a harbinger of the region’s not-to-distant future. For native fish, and the communities that care for and depend upon them, this grim reality necessitates that the fish and wildlife managers take proactive steps to adopt and implement policies that will ensure the resiliency of our native fish in the face of climate change. Average temperatures in Oregon are forecast to climb an additional 3-7 degrees Fahrenheit by 2050 if current global trends continue; even under the most optimistic scenarios, Oregon’s temperatures will climb an additional 2-5 degrees Fahrenheit by mid-century.

Warmer water temperatures are problematic for native fish.

Of all the factors that affect salmonid biology, temperature is probably the most important environmental influence (Brannon, 1993; USEPA, 2001).

Different salmonids have evolved to adapt to different temperature regimes based on local environmental conditions, and run-timing for anadromous salmonids is critically linked to water temperature in order to optimize survival strategies for migration, spawning, incubation, emergence, and rearing (USEPA, 2001).

Studies on the effects of temperature on salmonids are numerous and have identified that water temperature can affect critical biological processes including: “metabolism (Fry, 1971), cellular function (Prosser, 1991), protein structure (Somero and Hofmann, 1997), enzyme activity (Lehninger, 1982), and diffusion rates,” and water temperature has been found to impact nearly every phase of salmonid life histories including upstream and downstream migration, spawning, rearing, feeding ability, smoltification, swimming speed, result in direct and delayed mortality, cause increased disease, and alter the competitive dominance of other predators (USEPA, 2001; Carter, 2005; Kubicek, 1977; Elliot 1981). Moreover, the duration and severity of the time in which salmonids are exposed to thermal pressures can affect their long-term survival (Carter, 2005; Ligon et al., 1999), and as water temperatures rise, dissolved oxygen levels decrease, resulting in further physiological stress on fish.

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