The area of ocean under formal protection has expanded substantially, while the area effectively protected has grown far less. The gap is enforcement, and it is the variable that determines outcomes.
Designation and protection are different things
A marine protected area is a legal boundary with a set of rules, which may prohibit everything or may permit most existing activity to continue.
Many designations allow commercial fishing within the boundary, so the classification alone says little about what actually happens there.
Fully protected zones where extraction is prohibited make up a small fraction of the total, and they are where the measurable ecological effects occur.
The ocean is difficult to watch
Boundaries at sea are invisible, distances are large and a patrol vessel covers a small area relative to the zones it is responsible for.
Fishing at night or in poor weather is hard to detect, and vessels can disable position-reporting equipment when entering restricted areas.
Satellite monitoring and vessel tracking have improved detection considerably, though they work best against larger vessels that are required to carry transponders.
Consequences have to outweigh the catch
A penalty smaller than the value of an illegal catch functions as an operating cost rather than a deterrent.
Effective systems combine a realistic probability of detection with penalties that scale to the value involved, including gear seizure and licence loss.
Prosecution capacity is often the weak link, since a detected incursion still requires evidence, jurisdiction and a court willing to act.
Local support does most of the work
Reserves designed with the fishing communities around them are enforced partly by those communities, because the boundary protects a shared asset.
Where a reserve is imposed without consultation, the same people become the primary source of incursion, and no realistic patrol budget compensates.
Co-management arrangements that grant local exclusive access outside the no-take zone tend to produce the strongest compliance.
The effects appear quickly when it works
Well-enforced no-take areas accumulate larger and more numerous fish within a few years, and larger fish produce disproportionately more eggs.
Adults and larvae move across the boundary, which is the spillover effect that makes reserves defensible to fishers working the surrounding water.
The contrast with a nominally protected area of similar habitat is usually visible to anyone swimming over both, which is the most direct evidence that enforcement is the variable.