Estimating Changes in Juvenile Chinook Habitat and Rearing Capacity throughout the Puget Sound Region

Nearly one third of the populations of Pacific salmon (Oncorhynchus spp.) that once inhabited the Northwest have gone extinct during the past 200 years.  Many of the remaining populations have experienced dramatic declines since that time.  For example, half of the extant “evolutionarily significant units,” or ESUs, of Chinook salmon (O. tshawytscha) are currently listed under the US Endangered Species Act as threatened or endangered.  One of the listed ESUs covers the Puget Sound region of Washington State.  Cited in the listing as contributing to this ESU’s decline is a reduction in the quantity of habitat used by juveniles.  As part of the recovery effort, scientists and stakeholders have tried to quantify how much juvenile habitat has been lost in various basins and how these losses have altered juvenile carrying capacity in those basins.  However, we know of no attempts to quantify changes in juvenile habitat or capacity for the entire region.  This was the overarching goal of our analysis, which we divided into two main parts.  In the first part, we estimated the amount of rearing habitat available to juvenile Chinook salmon currently and historically throughout the Puget Sound region.  In the second part, we estimated the rearing capacity of juvenile Chinook in the region under current and historical conditions.  Results from the first part served as inputs into the second.  Each part of the analysis relied on geographic information systems (GIS), with the goal of using widely available and freely downloadable data covering as much of the Puget Sound region as possible.  Our results indicate that the current amount of rearing habitat is 45% less than the historical amount, when all estuarine and freshwater habitat types Sound-wide are aggregated.  Likewise, rearing capacity is estimated to have decreased over time by nearly 75%.  These results highlight the vulnerability of juvenile salmon to habitat loss and alteration, and the potential for competition between natural- and hatchery-origin juveniles where capacity is limited.  The results also underscore the need for monitoring outmigrant abundance, which would improve the ability to validate capacity estimates such as these.