Over the last 150 years, large>>

stretches of salmon habitat have

been eliminated by human activity:

Line mining, livestock grazing, timber

(5) harvesting, and agriculture as well

as recreational and urban devel-

opment. The numerical effect is

obvious: there are fewer salmon

in degraded regions than in pris-

(10) tine ones; however, habitat loss

also has the potential to reduce

genetic diversity. This is most

evident in cases where it results

in the extinction of entire salmon

(15) populations. Indeed, most

analysts believe that some kind

of environmental degradation

underlies the demise of many

extinct salmon populations.

(20) Although some rivers have

been recolonized, the unique

genes of the original populations

have been lost.

Large-scale disturbances in

(25) one locale also have the potential

to alter the genetic structure of

populations in neighboring areas,

even if those areas have pristine

habitats. Why? Although the

(30) homing instinct of salmon to their

natal stream is strong, a fraction

of the fish returning from the sea

(rarely more than 15 percent)

stray and spawn in nearby

(35) streams. Low levels of straying

are crucial, since the process

provides a source of novel

genes and a mechanism>>

by which a location can be>>

(40) repopulated should the fish>>

there disappear. Yet high rates

of straying can be problematic

because misdirected fish may

interbreed with the existing stock

(45) to such a degree that any local

adaptations that are present

become diluted. Straying

rates remain relatively low when

environmental conditions are

(50) stable, but can increase dramati-

cally when streams suffer severe

disturbance. The 1980 volcanic

eruption of LACE>Mount Saint HelensLACE>,

for example, sent mud and debris

(55) into several tributaries of the

Columbia River. For the next

couple of years, steelhead trout

(a species included among the

salmonids) returning from the

(60) sea to spawn were forced to

find alternative streams. As

a consequence, their rates of

straying, initially 16 percent,

rose to more than 40 percent

(65) overall.

Although no one has quantified

changes in the rate of straying

as a result of the disturbances

caused by humans, there is no

(70) reason to suspect that the effect

would be qualitatively different

than what was seen in the

aftermath of the Mount Saint

Helens eruption. Such a dra-

(75) matic increase in straying from

damaged areas to more pristine

streams results in substantial

gene flow, which can in turn lower

the overall fitness of subsequent