A) The present-perfect tense of have blocked
inappropriately describes an event that
caused something to happen before 1920.
In addition, migrations up their spawning
streams is incorrect.
B) Correct. The comparison of poundage
is efficiently explained, and the sequence
of tenses makes sense. Despite a possible
superficial appearance of a comparison
between countable things (pounds), less
is more appropriate than fewer for the
comparison. The fishermen landed different
amounts of fish; they did not land the
number of pounds in terms of which those
amounts are measured.
C) The comparative expression to a lower
amount is unnecessarily wordy. The past-
perfect form had reduced would make the
temporal relationships somewhat clearer
than does the past tense reduced.
D) The present-perfect participial phrase,
having blocked . .. streams, should be set off in
commas; as it stands, it does not make sense.
The past-perfect form had reduced would
make the temporal relationships somewhat
clearer than does the past tense reduced.
E) The present-perfect participial phrase must
be set off with commas; the pronoun their,
which is also in (A), nonsensically refers to
migrations, and the comparative expression
to an amount lower is unnecessarily wordy