Indeed, in just the past few months, researchers have used
  that data to [...] identify supply chains where vessels
  offload catches to each other at sea, and to show that the
  creation of marine protected areas can lead to a wave of
  pre-emptive overfishing.

How long until some fishermen begin to claim a right to privacy? I don't mean that flippantly. Logically speaking, it's a stretch to go from a personal right of privacy as the EU is slowly enshrining to one that protects the economic activity of commercial actors. But as a practical and political matter I don't think it's insurmountable. In America we're experts at reframing these rights issues in a way that benefits corporations. Europe might not be immune to these pressures, simply behind the curve.

>One prominent study said 55 percent, its critics say 4 percent, and they both used the same data.

TL;DR - The first study (55% fished) divided the ocean in 160,000 squares (each 3,100 square kilometers) with 55% containing fishing activity in 2016.

The second study divided the ocean into much smaller squares (123 square kilometers)and found only 4% had fishing activity in 2016.