Climate Change and Northern Puget Sound Steelhead

Steelhead populations in the Puget Sound have undergone a steep decline since the mid 1980s, and were federally-listed as a threatened in 2007.  I analyzed steelhead return data for the Skagit River, Washington, to assess the influence of ocean temperature conditions and hydrological variability on population trends over this period.  The effects of ocean temperatures were assessed using monthly Pacific Decadal Oscillation (PDO) index values - a measure of surface sea temperatures in the North Pacific.  The influence of hydrological variability was assessed by calculating annual peak flow and summer baseflow values from USGS gaging station records.  Steelhead returns were found to be positively correlated with PDO values during their first year of ocean residency, indicating that warmer ocean surface temperatures improve early marine survival rates.   Skagit steelhead returns were negatively correlated with peak flows, but positively correlated with summer base flows.  The combined influence of ocean conditions and freshwater hydrology on Skagit steelhead returns was analyzed using a multiple-regression model with four independent variables: brood-year escapement, the peak flow during the first two years of juvenile freshwater residency, the summer baseflow during this same period, and the PDO index during the first year of marine life.  The resulting statistical model was highly significant (p < 0.001), and explained 80% of the variability in Skagit steelhead returns over the past three decades.  The four independent variables employed in this model were all found to have a significant effect (p < 0.01) on steelhead returns.  Skagit steelhead returns were found to be correlated with returns observed in other northern Puget Sound and southern British Columbia Rivers, suggesting that these effects are occurring on regional rather than local scale.  The model was then used to examine how steelhead would respond under climate changes predicted for the over the next 50 years.  Steelhead populations would be favored by increasing surface temperatures in the North Pacific, but would be negatively effected by the increasing frequency of high flow and low flow events in the freshwater environment.  Consequently, habitat protection, restoration and management actions that reduce the magnitude and frequency of peak flows while improving summer baseflows will be vital to steelhead recovery in the future.