Biological Opinions - Bureau of Reclamation

population size can lead to extinction of a small population even if the population is growing, on
average, over a longer period.
Many SONCC coho salmon ESU populations have declined to such a low point that they are
likely influenced by multiple, interacting processes (e.g., Shasta River, Middle Mainstem Eel
River, Mainstem Eel River, Upper Mainstem Eel River, and Mattole River populations), that
make recovery of the SONCC coho salmon ESU difficult. These random processes can create
alterations in genetics, breeding structure, and population dynamics that may interfere with
persistence of the species. Random processes can be expressed in four ways: genetic,
demographic, environmental, and catastrophic events (Shaffer 1981, Lande 1993, McElhany et
al. 2000, Reed et al. 2007).
Genetic stochasticity refers to changes in the genetic composition of a population unrelated to
systematic forces (selection, inbreeding, or migration), i.e., genetic drift. Genetic stochasticity
can have a large impact on the genetic structure of populations, both by reducing the amount of
diversity retained within populations and by increasing the chance that deleterious recessive
alleles may be expressed. The loss of diversity will likely limit a population's ability to respond
adaptively to future environmental changes. In addition, the increased frequency with which
deleterious recessive alleles are expressed (because of increased homozygosity) could reduce the
viability and reproductive capacity of individuals.
Demographic stochasticity refers to the variability in population growth rates arising from
random differences among individuals in survival and reproduction within a season. This
variability will occur even if all individuals have the same expected ability to survive and
reproduce and if the expected rates of survival and reproduction don't change from one
generation to the next. Even though it will occur in all populations, demographic stochasticity is
generally important only in populations that are already small (Lande 1993, McElhany et al.
2000). In very small populations, demographic stochasticity can lead to extinction
(Shulenburger et al. 1999).
Environmental stochasticity is the type of variability in population growth rates that refers to
variation in birth and death rates from one season to the next in response to weather, disease,
competition, predation, or other factors external to the population (Melbourne and Hastings
2008). Catastrophic events are sudden, rare occurrences that severely reduce or eliminate an
entire population in a relatively short period of time (McElhany et al. 2000). For example, the
1964 flood in northern California significantly degraded many watersheds and reduced the
abundance of many SONCC coho salmon ESU populations.
These stochastic processes always occur; however, they don’t always significantly influence
population dynamics until populations are small. Due to the low abundance of most SONCC
coho salmon ESU populations, stochastic pressure is likely to be one of the most significant
threats to their persistence. Stochastic events have likely contributed to population instability
and decline for many SONCC coho salmon ESU populations, which likely explain why recent
adult returns remain low despite improved ocean conditions since 2007 and significant
reductions in bycatch mortality from commercial and recreational fishery closures enacted more
than 15 years ago.
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