Ghost fishing normally occurs with passive fishing gear such as longlines, gill nets, entangling nets, trammel nets, traps and pots, etc. as opposed to active fishing gear such as trawls and seines. The catching process of active fishing gear generally ceases when the gear is no longer attached to the vessel. However, any type of lost or abandoned fishing gear on the surface is a danger to passing vessels by becoming entangled in the propeller and disabling the vessel, particularly in bad weather. Fishing vessels are especially at risk because the lost fishing gear may be brought to the surface if entangled in the vessel's own fishing gear.
Having established that there is likely a problem of ghost fishing with a particular fishery, the next logical step would be to quantify it. How much does ghost fishing contribute to the decrease of the stocks of fish, cetaceans, etc. and how many nets are floating around? The answer is not easy as nets do not float or remain moored in the same position forever. Driftnets are subject to the effects of wind and currents and can lose their straight, rectangular, curtain-like shape to form an irregular shape that finally becomes self-entangled into a floating mass. Local wind and current conditions determine the speed at which this occurs. In addition, over a longer period of time, marine organisms foul the netting, making it visible, so that the catching efficiency of the net decreases.
Furthermore, large fish or cetaceans approach the net to feed on the smaller entrapped fish and can also get caught. In a struggle to free themselves they contribute more and more to the decreasing catching area of the net. In any of these intermediate stages the net can be washed ashore by coastal or oceanic currents where it will disintegrate due to the abrasive environmental conditions of sandy or rocky shores as well as to the effect of sunlight. The net might also foul sensitive areas for some time before it disintegrates. If the net remains floating on the surface it will eventually degrade due to the effects of sunlight.
The amount of fish and other animals which will have been "caught" during this time depends on the various factors listed above. But whatever the estimate, it is generally agreed that measures to avoid ghost fishing are needed and, in some countries, have already been initiated.
Traps and pots
Traps and pots operate slightly differently from gill nets and entangling nets. They are almost always moored on the bottom with a line and float to mark their position. Fish and crustaceans are usually attracted to the trap or pot by means of bait. Once in the trap or pot, the target species - as well as other species - cannot escape. In the case of lost gear, the trapped fish do not die immediately but once the food supply runs out it they may starve.
However, the trapped fish may become the "bait" for larger fish. It is this 'rebaiting' of the trap by captured fish or crustacea that forms the basis of ghost fishing by the gear. In a recent experiment in Canada where 'ghost fishing' was simulated with traps targeting Dungeness crab, it was estimated that losses due to ghost fishing accounted for seven percent of the total landings.
Where ghost fishing is believed to be a problem, methods to deactivate the fishing gear after an appropriate interval have been suggested. This includes the use of biodegradable twines in the framing of gill nets or in special panels in pots. Similar results can be achieved with the use of 'pop-ups'.
There have been remarkably few surveys to estimate the prevalence of ghost fishing gear despite the availability of relatively low-cost video camera equipment. Measures reducing the effects of ghost fishing and national and international initiatives that have been taken to reduce the environmental damage that can be caused by ghost fishing and lost gear. For example, Canada and Norway have conducted gear retrieval or clean-up programmes to recover or destroy lost gill nets, particularly in deep waters. This is because in deep water, fishing gear made of synthetic fibers degrade much slower than in shallow water.