Property rights in the ocean

An article about lobstermen in Maine talks about how they have unofficial property rights to various parts of the ocean. It does not explain how this overcomes the problem of the commons.

Lobster fishermen have feuded for generations over who can set traps, and where. To protect their fishing grounds, the lobstermen here have been known to cut trap lines, circle their boats menacingly around unwelcome vessels and fire warning blasts from shotguns.

This year there has been violence as outsiders have infringed on those informal property rights.