Development of Historical Population Structure and Framework for Assessing Viability of ESA Listed Pacific Salmonids in California

In California, there are four recovery domains of Pacific salmon and steelhead. For each of these recovery domains, a Technical Recovery Team (TRT) was formed to develop the technical foundation on which recovery planning would be built. The Viable Salmon Populations (VSP) approach provided a consistent conceptual base for TRTs throughout the West Coast. In California, the TRTs used an approach developed by Allendrof et al. (1997) as a framework to assess viability; this framework provided consistency, but allowed for the development of slightly different approaches as a result of the differences in environmental conditions, species, and data availability throughout these recovery domains.

For each TRT, development of biological viability was a two-step process. The first step was to define the historical population structure within each ESU/DPS. In particular, the TRTs identified populations that were considered to be independent populations, defined as any collection of one or more local breeding units whose populations dynamics or extinction risk over a 100-year time period are not substantially altered by exchanges of individual with other populations. In addition, TRTs identified groups of populations that span the diversity and distribution that historically existed within the ESU/DPS into diversity strata or biogeographic groups. These groups were largely based on the geographical arrangement of the populations and basin-scale environmental and ecological characteristics.

The second step of the process was the development of population and ESU/DPS viability criteria. The TRTs extended a general approach used for assessing extinction risk developed by the IUCN and later modified by Allendrof et al. (1997) specifically to prioritize conservation actions for Pacific salmonids. The TRT’s took this path based on the widespread lack of data and recognition that appropriate data sets for performing more sophisticated analyses would not be available for many years. The framework provides general criteria thresholds and rules are such that, if met, we believe would indicate that the ESU/DPS would have a high likelihood of persisting for 100 years or more. The criteria address the Viable Salmonid Populations (VSP) characteristics of abundance, productivity, diversity, and spatial structure that were the foundation of all TRTs, and they include criteria for both individual populations and diversity groups that make up each ESU/DPS. These criteria are intended to maintain diversity throughout the ESU/DPS, ensure connectivity among populations to maintain long-term demographic and genetic processes, and provide a buffer against potential catastrophic risks.