Cotton Thread (Rotting Thread) Product Description "Rotting thread" for traps is a cord that is designed to biodegrade  over time. This is a safety feature that creates a legal escape route for trapped lobsters/crab/fish if a trap is lost at sea, so they can eventually get out instead of remaining indefinitely. While most of the trap's construction uses strong, rot-resistant synthetic ropes and netting, this specific cord is used in a location that is intended to fail.