GMAT Sentence Correction (SC) | Around 1900, fishermen in the Chesapeake

I understand why the past perfect "had reduced" is correct here - but how can the simple past tense "blocked" in the relative clause be correct? Doesn't this imply that the sequence of events is: "landings are reduced" followed by "the culverts blocked shad migrations"? But how is this logical? First the migrations are blocked by the culverts and subsequently (or perhaps at the same time) landings are reduced, so the verb tense for "blocked" should
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