Ghost nets, another term for lost or abandon commercial fishing gear, are a growing danger to marine life. This is a problem that the NOAA "Ghost Net Project" researchers are trying to address by predicting where drifting ocean debris will accumulate. The debris "convergence zones" as they are called, are caused by prevailing winds and currents. NOAA Magazine has a recent article on the work being done in this field.
Common type Trawler fishing (Source: NOAA)
The NOAA researchers have developed a new method of using Satellites and a NOAA P3 aircraft to locate possible convergence zones. In an operational test this past March NOAA researchers were able to confirm their method on one such convergence zone north of Hawaii. The area they found contained about 2000 separate pieces of debris, including balls of nets up to 10 meters across.
NOAA WP-3D Orion (Source: NOAA)
According to the Hawaii Sea Grant, a federal cleanup of Hawaiian Islands two years prior collected some 100 tons of discarded fishing debris including nets and line. This kind of debris kills tens of thousands of sea birds, dolphins, turtles and fish annually across the Pacific.
Submarines can also run into ghost nets as well. I recall an incident that happened on the USS Alabama during Post Shipyard Availability (PSA) in 1985. On returning to New London from some ops at sea I came out of the hatch forward of the sail to find the prefect pattern of a trawler net on the sail and sailplanes. The net was gone and we never knew we ran into it, but it did paint a distinctive pattern.
What about submarines and active fishing gear?
HMS Turbulent (Source: Royal Navy Website)
There have been occasions when a Submarine is accused of getting tangled in a trawler's net. In a recent incident a British Trafalgar class sub the "HMS Turbulent" was cleared of sinking a French trawler. The HMS Turbulent was tied up to the pier when the trawler sank. But, that didn't stop the French lawyers from accusing the British sub of pulling the Trawler to the bottom.
Ghost nets are a serious problem in the marine environment, where as submarine nets, are a different topic.