Fishing fleet support vessels and factory/processing ships tend to stay in international waters, outside the reach of any particular nation's coast guard or naval forces. Many such vessels rarely, if ever, visit a port to avoid actions like boarding and seizure. And when they do, they try to dock at a port in a jurisdiction that's unlikely (or expressly made it clear) they aren't going to take any sort of police action against the vessel and its crew.

While these vessels are out in international waters, one can't simply board/seize/sink such vessels without causing a major diplomatic incident with economic consequences at best. At worst, it would be perceived as an act of war and invite military response.