Complete discussion is provided at the below mentioned link
Over the last 150 years, large stretches of salmon habitat have
been eliminated by human activity: mining, livestock grazing,
timber harvesting, and agriculture as well as recreational and
urban development. The numerical effect is obvious: there are
fewer salmon in degraded regions than in pristine 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 populations. 1ndeed, most analysts believe that some kind
of environmental degradation underlies the demise of many
extinct salmon populations. Although some rivers have been
recolonized, the unique genes of the original populations have
Large-scale disturbances in one locale also have the potential to
alter the genetic structure of populations in neighboring areas,
even ifthose areas have pristine habitats. Why? Although the
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 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 repopulated
should the fish there disappear. Yet high rates of straying can be
problematic because misdirected fish may interbreed with the
existing stock to such a degree that any local adaptations that
are present become diluted. Straying rates remain relatively low
when environmental conditions are stable, but can increase
dramatically when streams suffer severe disturbance. The 1980
volcanic eruption of Mount Saint Helens, for example, sent mud
and debris into several tributaries of the Columbia River. For the
next couple of years, steelhead trout [a species included among
the salmonids] returning from the 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 overall.
Although no one has quantified changes in the rate of straying as
a result of the disturbances caused by humans, there is no 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 dramatic 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
According to the passage, human activity has had which of the following effects on salmon population?
1. An increase in salmon population numbers in some previously polluted rivers.
2. A decline in salmon numbers in some rivers.
I answered #1, which is apparently wrong:
While human efforts may have been able to clean up some rivers and encourage some salmon to return to them, the passage does not discuss them.
My Argument: Clearly the correct answer has to be about rivers, which are mentioned only once in the passage (highlighted in bold) - and the one mention discusses recolonising or increasing population numbers.
The correct answer is apparently #2:
According to the passage, there are fewer salmon in areas that have been degraded by human activity.
My Argument: The question asks about rivers, not areas. The passage mentions regularly the declining population in "areas", "locales" and "habitats" - which are not limited to rivers. The passage does not explicitly discuss reducing populations in rivers.
Please correct me.