Here, the words "unlike" and "as" signal that this question is testing COMPARISONS. We always want to ask, "are the two things we're comparing really comparable?" Think APPLES to APPLES.

(A) There are no legal limits, as there are for cod and haddock, on the size of monkfish that can be caught, a circumstance that contributes to their depletion through overfishing.
This isn't an elegantly written sentence, but the comparison here is correct: "there are... as there are" is a comparable comparison. "A circumstance" accurately modifies the preceding clause. There is some awkwardness/ambiguity with the use of "their," but remember that the GMAT does sometimes allow pronoun ambiguity.

(B) There are no legal limits on the size of monkfish that can be caught, unlike cod or haddock, a circumstance that contributes to depleting them because they are being overfished.
Here, "unlike" is comparing the SIZE of monkfish to cod and haddock themselves. "Depleting them" is both awkward and ambiguous.


(C) There are legal limits on the size of cod and haddock that can be caught, but not for monkfish, which contributes to its depletion through overfishing.
Here, "limits ON the size of cod" cannot be compared to "FOR monkfish." It should be "on monkfish." The pronoun "its" is also ambiguous here.


(D) Unlike cod and haddock, there are no legal size limits on catching monkfish, which contributes to its depletion by being overfished.
If a sentence begins with the modifier "unlike cod and haddock..." then the subject following must be a noun comparable to "cod and haddock." For example, "unlike cod, monkfish is being depleted..."

(E) Unlike catching cod and haddock, there are no legal size limits on catching monkfish, contributing to their depletion because they are overfished.
If a sentence begins with the modifier "unlike catching..." then the subject following must be a gerund comparable to "catching." For example, "unlike catching fish, shooting ducks is..."

So, the OA is A. It's not a perfect answer, but it's the least terrible of the bunch.