Congress enacted the MSA. The MSA and subsequent amendments extended the jurisdiction of the United States to 200 nautical miles beyond the U. S. territorial sea and claimed “exclusive fishery management authority over all fish” within that area, known as the “exclusive economic zone.” D administers the MSA under a delegation from the Secretary of Commerce. The MSA established eight regional fishery management councils composed of representatives from the coastal States, fishery stakeholders, and D. The councils develop fishery management plans, which D approves and promulgates as final regulations. The MSA requires that certain provisions-such as “a mechanism for specifying annual catch limits . . . at a level such that overfishing does not occur,”-be included in these plans. Relevant here, a plan may require that “one or more observers be carried on board” domestic vessels “for the purpose of collecting data necessary for the conservation and management of the fishery.” The MSA specifies three groups that must cover costs associated with observers: (1) foreign fishing vessels operating within the exclusive economic zone (which must carry observers); (2) vessels participating in certain limited access privilege programs, which impose quotas permitting fishermen to harvest only specific quantities of a fishery’s total allowable catch; and (3) vessels within the jurisdiction of the North Pacific Council, where many of the largest and most successful commercial fishing enterprises in the Nation operate. The MSA expressly caps the relevant fees at two or three percent of the value of fish harvested on the vessels in categories 2 and 3. It authorizes the Secretary to impose “sanctions” when “any payment required for observer services provided to or contracted by an owner or operator . . . has not been paid.” With respect to the Atlantic herring fishery, D's Rule eventually created an industry-funded program that aims to ensure observer coverage on 50 percent of trips undertaken by vessels with certain types of permits. Vessel representatives must “declare into” a fishery before beginning a trip by notifying D of the trip and announcing the species the vessel intends to harvest. If D determines that an observer is required, but declines to assign a Government-paid one, the vessel must contract with and pay for a Government-certified third-party observer. D estimated that the cost of such an observer would be up to $710 per day, reducing annual returns to the vessel owner by up to 20 percent. Ps challenged the Rule claiming that the MSA does not authorize D to mandate that they pay for observers required by a fishery management plan. The District Court concluded that the MSA authorized the Rule, but noted that even if Ps’ “arguments were enough to raise an ambiguity in the statutory text,” deference to D’s interpretation would be warranted under Chevron. The D. C. Circuit affirmed. The majority addressed various provisions of the MSA and concluded that it was not “wholly unambiguous” whether D may require Atlantic herring fishermen to pay for observers. The court proceeded to Chevron’s second step and deferred to the agency’s interpretation as a “reasonable” construction of the MSA. In a second case, Ps use small-mesh bottom-trawl gear and can freeze fish at sea, so they can catch more species of fish and take longer trips than other vessels. They generally declare into multiple fisheries per trip so they can catch whatever the ocean offers up. If the vessels declare into the Atlantic herring fishery for a particular trip, they must carry an observer for that trip if D selects the trip for coverage, even if they end up harvesting fewer herring than other vessels-or no herring at all. Their court deferred to D’s interpretation under Chevron and thus granted summary judgment to D. The First Circuit affirmed. It rejected Ps' argument that the express statutory authorization of three industry funding programs demonstrated that D lacked the broad implicit authority it asserted to impose such a program for the Atlantic herring fishery. Both sets of Ps appealed.