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.
Option Elimination -
(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. - Okay. "They're" referring to "monkfish." The reference to a collective noun can be singular or plural based on the context. Let me share an example.
If we say, "The Oneida is a Native American tribe in New York," we refer to "The Oneida" as a collective entity.
But if we say, "The Oneida are known for their long-standing cultures and traditions," we refer to the group's members (individually).
Likewise, here, we refer to the group's (monkfish's) members/fish.
Moreover, had this been a single monkfish, we would have had an article "a" for general monkfish or "the" for a specific monkfish, which we don't have here.
"a circumstance ....." is a flexible modifier that modifies the earlier clause: "There are no legal limits....."
(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. - unlike "cod or haddock" is compared with "There are no legal limits on the size" - wrong.
(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. - When referring to the individual members of the collective noun, we need to use "their" and not "its" as used here. Wrong. Moreover, "which" here refers to "monkfish." So, does "monkfish" contribute to its depletion? Doesn't make sense. Wrong.
(D) Unlike cod and haddock, there are no legal size limits on catching monkfish, which contributes to its depletion by being overfished. - The same issue as B. "which" again refers to "monkfish."
(E) Unlike catching cod and haddock, there are no legal size limits on catching monkfish, contributing to their depletion because they are overfished. It's not exactly the same, but it's a similar issue as in B.