Biological Opinions - Bureau of Reclamation

The variability of early life history behavior of coho salmon observed by Chesney et al. (2007)
and by the Yurok and Karuk tribes mentioned in the sections above is not unprecedented; coho
salmon have been shown to spend up to two years in freshwater (Bell and Duffy 2007), migrate
to estuaries within a week of emerging from the gravels (Tschaplinski 1988), enter the ocean at
less than one year of age at a length of 60 to 70 mm (Godfrey et al. 1975), and redistribute into
riverine ponds following fall rains (Peterson 1982; Soto et al. 2008; Hillemeier et al. 2009).
Taken together, the research by the Yurok and Karuk tribes, plus the research from outside the
Klamath Basin, indicate that coho salmon in the Klamath River exhibit a diversity of early life
history strategies, utilizing the mainstem Klamath River throughout various parts of the year as
both a migration corridor and a rearing zone.
12.3.2 Risk of Extinction of Klamath Populations
While the Status of the Species section discussed the viability of the SONCC coho salmon ESU,
this section provides a more in-depth discussion of the extinction risk of the Klamath River basin
populations affected by the proposed action, which consist of the Upper Klamath, Middle
Klamath, Shasta, Scott, and Salmon River populations.
Within the California portion of the SONCC coho salmon ESU, estimating the risk of extinction
of a given coho salmon population is difficult since longstanding monitoring and abundance
trends are largely unavailable. Williams et al. (2008) proposed biological viability criteria,
including population abundance thresholds. The viability criteria developed by Williams et al.
(2008) address and incorporate the underlying viability concepts (i.e., abundance, productivity,
diversity and spatial structure) outlined within McElhany et al. (2000), and are intended to
provide a means by which population and ESU viability can be evaluated in the future when
more population data become available. Comparing population estimates against population
viability thresholds proposed by Williams et al. (2008) allow NMFS to make conservative
assumptions concerning the current risk of extinction of Klamath River mainstem and tributary
populations.
Generally speaking, none of the five populations of coho salmon affected by the proposed action
are considered viable. Even the most optimistic estimates from Ackerman et al. (2006) indicate
each population falls well short of abundance thresholds for the proposed viability criteria that, if
met, would suggest that the populations were at low risk of extinction for this specific criterion.
In some years, populations have fallen below the high risk abundance threshold, such as the
Shasta River population. A population is considered at low risk of extinction if all criteria are
met, therefore failure to meet any one specific criterion would result in the population being at an
elevated risk of extinction (i.e., not viable). The annual adult run size estimate between 2009 and
2012 has been fewer than 116, with a low of nine adults for the Shasta River, all of which were
males. Similarly, the Scott River coho salmon population fell well below the high risk
abundance threshold in three of the most recent four years (Table 12.3). For both of these
populations, abundance is low and they are likely experiencing depensation pressures. With
regard to spatial structure and diversity, Williams et al. (2008) abundance thresholds were based
upon estimated historical distribution and abundance of spawning coho salmon, and thus capture
the essence of these two viability parameters. By not meeting the low risk annual abundance
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