Biological Opinions - Bureau of Reclamation

fisheries south of Cape Falcon, Oregon. Preliminary post-season estimates of ocean exploitation
rate for 2010 and 2011 are 2.2 and 3.8 percent, respectively (PFMC 2011, 2012). Because of the
generally limited Chinook salmon fishery since 2005, NMFS believes the commercial fishery
has been a small threat to the SONCC coho salmon ESU.
Figure 12.12. Rogue/Klamath (R/K) coho salmon ocean exploitation rate forecast for years 2000-2010 (PFMC
2010).
12.2.6.5 Small Population Size
SONCC coho salmon populations have declined significantly (e.g., Shasta River population) and
are facing an additional threat from the effects of small population size. Many populations, such
as the Shasta River population, are at a high risk of extinction because of their small population
size (e.g., only 44, 62, and 115 spawners returned to the Shasta River in the 2010, 2011 and 2012
spawning seasons, respectively). With a majority of SONCC coho salmon populations at low
abundance, random events become an increased and significant factor in the extinction process.
Small populations have a significantly increased risk of extinction (Shaffer 1981, McElhany et
al. 2000, Fagan and Holmes 2006). In fact, time-to-extinction decreases logarithmically with
decreasing population size (Lande 1993, Fagan and Holmes 2006). Population declines are
likely to cause further declines, especially for small populations because stochastic factors exert
more influence (Fagan and Holmes 2006). Small populations can be affected by different forms
of stochastic pressure, not all of which affect large populations (Lande 1993). The fact that small
populations can be affected by different forms of stochastic pressure results in extinction
probabilities substantially greater than the extinction probabilities that would occur from a single
form of stochasticity (Melbourne and Hastings 2008).
Small populations are likely largely influenced by random processes that affect population
dynamics and population persistence. If the rate of population growth varies from one
generation to the next, a series of generations in which there are successive declines in
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