Had a different thought process that led to the same outcome, so thought I'd share.
(A) Fishing vessels often exceed their fishing quotas for cod and therefore often underreport the number of tons of cod that they catch.
If they underreport, then how is the estimate derived from fishing vessels increasing? Eliminate.
(B) More survey vessels are now involved in the yearly sampling effort than were involved 10 years ago.
If there are more vessels, then they should theoretically catch more cod, all else equal. Note that while the final estimate is an average, the number of cod caught by research vessels and reported is not an average. In any case, more vessels means larger number of caught cod, which is contrary to the stem. Eliminate.
(C) Improvements in technology over the last 10 years have allowed commercial fishing vessels to locate and catch large schools of cod more easily.
This looks good, keep. Basically implies that the commercial vessels are catching all the cod throughout the year and leave nothing for the research vessels.
(D) Survey vessels count only those cod caught during a 30-day survey period, whereas commercial dishing vessels report all cod caught during the course of a year.
This basically restates a premise. Not useful. Eliminate.
(E) Because of past overfishing of cod, fewer fishing vessels now catch the maximum tonnage of cod each vessel is allowed by law to catch.
This makes the discrepancy go the other way - eliminate.