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Around 1900, fishermen in the Chesapeake Bay area landed more than seventeen million pounds of shad in a single year, but by 1920, over-fishing and the proliferation of milldams and culverts that have blocked shad migrations up their spawning streams had reduced landings to less than four million pounds.

(A) that have blocked shad migrations up their spawning streams had reduced landings to less
(B) that blocked shad from migrating up their spawning streams had reduced landings to less
(C) that blocked shad from migrating up their spawning streams reduced landings to a lower amount
(D) having blocked shad from migrating up their spawning streams reduced landings to less
(E) having blocked shad migrations up their spawning streams had reduced landings to an amount lower

OA: B

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Below is my analysis. I have posted few questions. Experts help.

A) that have blocked shad migrations up their spawning streams had reduced landings to less - Since we are talking about a fact that has already happened in the past we need simple past tense to talk about the events that blocked. Also, not very sure about the usage of their spawing streams. Shouldn't we just use up spawing streams.
(B) that blocked shad from migrating up their spawning streams had reduced landings to less -
(C) that blocked shad from migrating up their spawning streams reduced landings to a lower amount - We need the usage of past perfect here due to the usage of by 1920.
(D) having blocked shad from migrating up their spawning streams reduced landings to less - Again we need past perfect. What is the issue with using having. How does it bring about a change in the meaning.
(E) having blocked shad migrations up their spawning streams had reduced landings to an amount lower - Same question as in D.

[crackverbal]

Let us look at each of the answer options -

In A - the usage of present perfect is incorrect. Let us look at this part - "overfishing and culverts that have blocked". Present perfect is used when an action that started in the past has some bearing on the present.
B - correct answer. Let us look at this example from GMAT -
In 1981 children in the United States spent an average of slightly less than two and a half hours a week doing household chores; by 1997 that figure had grown to nearly six hours a week.
Here in the first clause a specific time is given (1981). Hence, the past tense is used.
In the second clause we have two times - 1997 and before 1997. (Notice the by 1997). The time spent doing homework by kids up to 1997 - six hours.
Similarly, in option B - the first clause is referring to a specific time period - 1900. Hence, past tense is used. In the second clause - 'but by 1920, ..." we have two time lines -
1. Before 1920
2. 1920.
"reduction in landings" - this started before 1920 and was continuing in 1920 (that's why the use of perfect tense).
C - usage of reduced is incorrect.
D and E - usage of "having blocked ..." is incorrect.
Let us look at a correct usage of "having ..." from GMAT -
Having finally reached a tentative labor agreement with its company's pilots, the airline's board of directors must now determine how the airline can both increase profits and compete more effectively for customers than it did in the past.

"having finally reached ..." describes an action that was completed before the actor does another action. Here, "reaching an agreement" was done by the board before they did something else (determining how the airline can both increase profits and compete more effectively). No such usage in D and E. (this implies that after blocking shad migrations, overfishing and culverts then proceeded to reduce landings - does not make sense.)

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Hi Crack verbal,

I wanted to know for option D and E is there any difference between the usage of comma + having and having without comma? The usage that you have shared is applicable for comma + having.

Also, can I say that having without comma directly modifies the preceding noun Culverts instead of the entire noun phrase and that is why it is incorrect?