Climate change is causing significant changes in streamflow patterns, water temperature, and the frequency and intensity of rainfall in the watersheds of the Northeast Pacific Coastal Temperate Rainforest. Stretching from southern Alaska to northern California, this region is experiencing an increase in intense rainfall events, resulting in large flood events during the critical period when salmon eggs are in the gravel. The probability of embryo mortality rises due to these floods, particularly when heavy rain combines with existing snowpack.
Bellmore conducted a study to assess the sensitivity of coho salmon populations in modeled streams fed by glaciers, snow, and rain to increasing intensities of winter floods, summer droughts, and summer heat. They created four distinct scenarios for winter floods by augmenting the empirical discharge observed in each stream from January 13-19th by increments of 25%, 50%, 75%, or 100%. While a 100% increase in winter floods may not be realistic in some present-day watersheds, this scenario was justified for two reasons: (1) it served as a threshold to understand the impact of dramatic hydrologic shifts on salmon populations and (2) it represented a plausible scenario for certain watersheds.
The response of coho salmon populations to the changing streamflow and thermal regimes varied among the modeled rain-fed, snow-fed, and glacier-fed streams. Across all scenarios, rain-fed streams exhibited the most diverse and pronounced population responses, ranging from minimal changes in salmon abundance relative to the baseline to complete loss. In contrast, the population responses in snow-fed and glacier-fed streams were less dramatic, with some cases even showing an increase in abundance.
“Fig. 5. Adult population response to flow and thermal regime modifications from base scenarios for rain (brown), snow (green), and glacier (blue) runoff watersheds. “Max. flow” and “Min. flow” refer to maximum and minimum instantaneous flows during the calendar year. Points on each line represent the median, ends of the thick lines represent 25th/75th percentiles, and ends of thin lines represent 5th/95th percentiles”.
Fig. 5 demonstrates that higher magnitude winter floods, which erode the streambed and cause mortality among incubating salmon embryos, are linked to population declines in both rain-fed and snow-fed streams. In rain-fed streams, a 50% increase in winter flood intensity led to a median decrease of 8% in salmon abundance. The abundance decreased significantly further with 75% and 100% increases in winter floods, resulting in median decreases of 96% and 100% in abundance, respectively. In snow-fed streams, the decreases in salmon abundance were comparatively less severe, ranging from 1% to 6% across all four winter flood scenarios. On the other hand, the modeled salmon abundance in the glacier-fed stream exhibited limited sensitivity to increased winter floods, with the median abundance slightly higher when winter floods were increased by 25% and 50%. However, a 100% increase in flood magnitude caused 72% and 81% mortality in the egg life stage of the glacier-fed and snow-fed streams, respectively. The reduced number of eggs resulted in lower juvenile density, leading to improved growth and survival of juveniles due to reduced density-dependence
“Fig. 6. Life cycle model mechanisms driving Coho salmon population response to shifting environmental conditions. Symbols indicate whether each component increased (+) or decreased (−). As you follow the arrows, read “therefore,” but against the arrows, read “because”. In the winter high flow panel, fewer eggs may result in increased or decreased returning adult abundance depending on the magnitude of egg mortality and the ability for improved juvenile growth and survival to mitigate for egg losses (represented by the curved arrows)”.
Climate change is causing significant changes in streamflow patterns, water temperature, and the frequency and intensity of rainfall in the watersheds of the Northeast Pacific Coastal Temperate Rainforest. Stretching from southern Alaska to northern California, this region is experiencing an increase in intense rainfall events, resulting in large flood events during the critical period when salmon eggs are in the gravel. The probability of embryo mortality rises due to these floods, particularly when heavy rain combines with existing snowpack. Bellmore conducted a study to assess the sensitivity of coho salmon populations in modeled streams fed by glaciers, snow, and rain to increasing intensities of winter floods, summer droughts, and summer heat. They created four distinct scenarios for winter floods by augmenting the empirical discharge observed in each stream from January 13-19th by increments of 25%, 50%, 75%, or 100%. While a 100% increase in winter floods may not be realistic in some present-day watersheds, this scenario was justified for two reasons: (1) it served as a threshold to understand the impact of dramatic hydrologic shifts on salmon populations and (2) it represented a plausible scenario for certain watersheds. The response of coho salmon populations to the changing streamflow and thermal regimes varied among the modeled rain-fed, snow-fed, and glacier-fed streams. Across all scenarios, rain-fed streams exhibited the most diverse and pronounced population responses, ranging from minimal changes in salmon abundance relative to the baseline to complete loss. In contrast, the population responses in snow-fed and glacier-fed streams were less dramatic, with some cases even showing an increase in abundance.
“Fig. 5. Adult population response to flow and thermal regime modifications from base scenarios for rain (brown), snow (green), and glacier (blue) runoff watersheds. “Max. flow” and “Min. flow” refer to maximum and minimum instantaneous flows during the calendar year. Points on each line represent the median, ends of the thick lines represent 25th/75th percentiles, and ends of thin lines represent 5th/95th percentiles”.
Fig. 5 demonstrates that higher magnitude winter floods, which erode the streambed and cause mortality among incubating salmon embryos, are linked to population declines in both rain-fed and snow-fed streams. In rain-fed streams, a 50% increase in winter flood intensity led to a median decrease of 8% in salmon abundance. The abundance decreased significantly further with 75% and 100% increases in winter floods, resulting in median decreases of 96% and 100% in abundance, respectively. In snow-fed streams, the decreases in salmon abundance were comparatively less severe, ranging from 1% to 6% across all four winter flood scenarios. On the other hand, the modeled salmon abundance in the glacier-fed stream exhibited limited sensitivity to increased winter floods, with the median abundance slightly higher when winter floods were increased by 25% and 50%. However, a 100% increase in flood magnitude caused 72% and 81% mortality in the egg life stage of the glacier-fed and snow-fed streams, respectively. The reduced number of eggs resulted in lower juvenile density, leading to improved growth and survival of juveniles due to reduced density-dependence
“Fig. 6. Life cycle model mechanisms driving Coho salmon population response to shifting environmental conditions. Symbols indicate whether each component increased (+) or decreased (−). As you follow the arrows, read “therefore,” but against the arrows, read “because”. In the winter high flow panel, fewer eggs may result in increased or decreased returning adult abundance depending on the magnitude of egg mortality and the ability for improved juvenile growth and survival to mitigate for egg losses (represented by the curved arrows)”.