Puget Sound Chinook and Chum Viability Targets

Puget Sound Chinook and Hood Canal summer chum are both listed as threatened under the Endangered Species Act.  The Puget Sound Technical Recovery Team (TRT) was formed in 1999 to determine the population structure and viability criteria for these two listed ESUs.  The Chinook ESU was determined to consist of 22 extant populations and the chum ESU, of 2 populations, both consisting of a number of subpopulations.  Viability was based on the status of four parameters: abundance, productivity, spatial structure and diversity.  Viability may be determined at both the population level and the ESU level.  NOAA has defined, for the purposes of assessing recovery of listed salmon, viability to mean that the risk of extinction to the population or ESU is less than 5% within a hundred years.

The TRT used both population viability analysis (PVA) and habitat-based methods to develop abundance and productivity targets for individual populations.  The first PVA method was based on a simple density-independent exponential growth model with variance in growth rate estimated from recent spawning escapement observations.  A second PVA method, using the Viability and Risk Assessment Procedure Model (VRAP), was based on density-dependent productivity and the variance of observed to predicted returns.     One habitat-based approach related habitat quantity and condition to spawning capacity at historical or pristine conditions.  A second habitat-based approach used the Ecosystem Diagnosis and Treatment (EDT) model to estimate population performance under “properly functioning conditions” defined by NOAA to sustain  salmon.  While the PVA methods incorporate risk assessment, the habitat-based estimates provide a check on capacity of the salmon habitat.    The VRAP method has the additional advantage of being able to incorporate harvest as part of a recovered resource, providing various viability target levels for various desired harvest levels.   These four methods of assessing viability targets gave somewhat different results, as might be expected, and we compare those here.  The TRT used a modification of a flowchart originally developed for Columbia River recovery planning to combine these into viability ranges for each population. The recovery target ranges in the 2007 Puget Sound Salmon Recovery Plan are derived from this TRT analysis.