Biological Opinions - Bureau of Reclamation

and genetic processes. Resiliency is the ability of populations to withstand natural and humancaused
stochastic events, and it depends on sufficient abundance and productivity. The
following attributes are necessary for the SONCC coho salmon ESU to demonstrate
representation, redundancy, connectivity, and resiliency: core populations must be viable and
well distributed; non-core populations must not have a risk of extinction; and dependent
populations must have functioning habitat for all life stages of coho salmon (Williams et al 2008,
NMFS 2012a).
In order to achieve viable core populations and low or moderate risk of extinction for non-core
populations, good quality habitat must be available to support SONCC coho salmon populations
(NMFS 2012a). The rationale for having good quality habitat is that NMFS expects that as
habitat is restored and key threats are abated, more coho salmon will survive and reproduce.
Good quality habitat for coho salmon includes sufficient invertebrate organisms for food; cool,
flowing waters; high dissolved oxygen concentrations in rearing and incubation habitats; water
with low suspended sediment during the growing season (for visual feeding); clean gravel
substrate for reproduction; and unimpeded migratory access to and from spawning and rearing
areas. Specific metrics for good quality habitat are defined in NMFS’s public draft recovery plan
for SONCC coho salmon ESU (NMFS 2012a) using the indicators of aquatic habitat suitability
listed in Kier Associates and NMFS (2008) and the disease infection rates summarized by True
(2011).
12.2.4 Extinction Risk Criteria
Williams et al. (2008) built on the population structure and the concepts of VSP (McElhany et al.
2000) to establish the extinction risk criteria at the population and ESU scales. The population
extinction risk criteria represent an extension of an approach developed by Allendorf et al.
(1997), and include metrics related to population abundance (effective population size),
population decline, catastrophic decline, spawner density, hatchery influence, and population
viability assessment. Populations that fail to satisfy several extinction risk metrics are likely at
greater risk than those that fail to satisfy a single metric. A viable population must have a low
extinction risk for all of the 6 population metrics (Table 12.1). For a population to be at
moderate risk of extinction, the population must meet the moderate risk description for each
criterion shown in Table 12.1.
Sharr et al. (2000) modeled the probability of extinction of most Oregon Coast Natural
populations and found that as spawner density dropped below 4 fish per mile (2.4 spawners/km),
the risk of extinction rises rapidly (Figure 12.3). When Chilcote (1999) tracked the collapse of
four coho salmon populations in the Lower Columbia River, they found the depensation
threshold was 2.4 spawners/km. Using spawner-recruit relationships from 14 populations of
coho salmon, Barrowman et al. (2003) found evidence of depensatory effects when spawner
densities are less than 1 adult female per km of river (2 spawners/km).
Wainwright et al. (2008) chose a value of 0.6 spawners/km as the density at which a population
of salmon would be very likely to have significant demographic risks. This was the lowest of
four bins the Wainwright et al. (2008) workgroup used to populate a decision support system.
Williams et al. (2008) essentially chose this value then divided it by 0.6, which is equivalent to
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