(1995) has pointed out that because ~90% of new mutations are unconditionally deleterious (and
therefore do not contribute to future evolution), a more realistic long-term minimum viable
effective population size may be closer to 5,000 per generation. Using similar arguments, Lynch
(1996) has suggested that an effective size of ~1,000 is usually large enough for a population to
maintain genetic variation. Based on the probability of losing rare alleles, Waples (1990a) has
suggested that 100 effective breeders/year is necessary to maintain genetic variation in salmon
populations in the short-term. These general recommendations have some limitations that must
be understood before they are used to help determine levels of abundance necessary for viable
salmonid populations. First, they are based on models of single, reproductively isolated
populations. As the term is used in this document, a population is substantially reproductively
isolated, but may receive low levels of gene flow from other populations. Gene flow, like
mutation, is a source of genetic variation, so it is likely that populations connected by gene flow
will have somewhat smaller minimum genetically viable population sizes than completely
isolated populations. Second, the genetic parameters that form the basis for the Franklin (1980)
and Lande (1995) recommendations were estimated from data obtained from only one species
(Drosophila melanogaster), and must therefore be regarded as preliminary. Nonetheless, these
recommendations may be reasonable as starting points for determining the minimum abundance
necessary for long-term genetic viability, especially in the absence of additional information.
In order to convert these recommendations of effective population size per generation to
salmon spawning abundance per year, it is necessary to know the ratio of the effective number of
breeders to the observed number of breeders (N
b
/N ratio) and the generation time for the
population in question. Several studies suggest that a N
b
/N ratio of 0.3 is approximately correct
for salmon and steelhead in general (see following AAssessment Methods@). With this ratio, the
recommended minimum long-term genetically viable population sizes discussed above range
from 1,670/generation (Franklin 1980 and Soulé 1980) to 16,700/generation (Lande 1995). The
minimum spawning size recommended by WDFW (1997) falls in this range (3,000/generation).
For populations that spawn at multiple age classes, the spawners/generation value must be
divided by the generation length (median age of reproduction) to obtain the corresponding
numbers of spawners per year. For example, many chinook salmon populations have about a
four-year generation time (reviewed by Healey 1991). A range of ~417 to ~4,170 breeders per
year, therefore, may be reasonable minimum values for maintaining sufficient genetic diversity
to ensure long-term persistence of chinook salmon populations. Based on genetic evidence,
Allendorf et al. (1997) concluded that salmon populations with N
e
below 500 (or N below 2,500)
per generation would be at high risk and populations with N
e
below 50 (or N below 250) per
generation would be at very high risk. Wainwright and Waples (1998) noted that if demographic
factors were included, thresholds for these categories would be higher, but they did not suggest
specific values.
Demographic stochasticity
Demographic stochasticity refers to the effects of apparently random events associated
with mate choice, fecundity, fertility, and sex ratio and tend to cause higher extinction risks in
small populations than in large ones. Mathematical theory for extinction due to random variation
in birth and death rates among individuals in a population was first developed in the 1930s (see
Goodman 1987, Gabriel and Bürger 1992), but was first widely applied in developing island
biogeography theory (MacArthur and Wilson 1967, Richter-Dyn and Goel 1972). The