Why Many Marine Protected Areas Still Swarm With Fishing Boats

Lines in the water

If a country sets aside an ocean sanctuary, you might expect fishing fleets to pack up and leave. A vast investigation of 861 marine protected areas set up between 2014 and 2022 suggests otherwise. Across these parks, covering 14.3 million square kilometres of ocean, industrial fishing activity often kept climbing.

Governments are racing to meet global conservation targets, such as safeguarding 30 percent of the planet by 2030. To find out if these paper boundaries translate into actual ocean safety, researchers turned to satellite tracking data. By tracking ships with the Automatic Identification System (AIS)—a satellite tracking network that broadcasts a vessel's identity and location—they followed fishing effort across 12 years.

Tracking the nets

The researchers used a method called Before-After-Control-Impact (BACI). This compares fishing activity inside a sanctuary before and after legal protection against an outside buffer zone 5 to 30 kilometres away. This clever check ensures that changes in boat traffic reflect genuine sanctuary rules rather than general regional trends.

The results were startling. Across the studied reserves, 58 percent experienced an increase in industrial fishing effort after establishment. In fact, average fishing hours went up both inside and outside the boundaries, showing no net reduction across the entire global sample. Establishing an ocean reserve behaves a bit like painting a 'no-parking zone' along a quiet desert road: it looks productive on a map, but it rarely reduces real city congestion where cars actually clog the streets.

Strict rules make the difference

Real relief for ocean life came almost entirely from fully protected zones. In reserves where all fishing is forbidden, 78 percent saw fishing effort drop inside their borders, achieving a clear net reduction compared to outer waters. In contrast, minimally protected zones saw industrial fishing rise both inside and outside.

Size also mattered, as larger reserves showed greater fishing reductions. The age of the reserve, however, showed no clear link with falling pressure once protection levels were accounted for. More troubling is an issue known as residual conservation: governments frequently place high-level protection in remote waters where fishers rarely ventured in the first place. Highly protected zones averaged only 0.3 fishing hours per square kilometre each year before designation, while lightly protected waters averaged 4.3 hours. Nearly 15 percent of all analysed reserves had zero industrial fishing activity before they were ever declared protected.

What we still don't know

Satellites cannot see every net. AIS signals capture roughly 50 to 70 percent of global industrial fishing, but they miss small-scale coastal fishers entirely. Furthermore, crews fishing illegally inside sanctuaries can deliberately switch off their tracking transponders to dodge detection. We also know very little about open-ocean high seas reserves, as the researchers could only study seven of them. What remains clear is that true conservation additionality—real environmental improvement that would not have happened otherwise—requires strict rules, not just lines drawn on empty water.