Every day, marine animals that have become tangled with plastic marine debris are losing their lives.
For example, fishing lines, fishing nets, or six-pack rings that have been carelessly discarded (or intentionally tossed) end up entangling animals.
Victims that become strangled by these nets include whales, seabirds, sea turtles, seals, dolphins, manatees, sharks, big fish, corals, etc. The list is simply endless.
Once the nets entangle animals, they trail them for several days, or sometimes even for more than a decade. Subsequently,they can cause the animal to drown, starve to death, choke, or suffer from infected wounds.
Sometimes these animals are attacked by predators while they are caught up in the net and cannot move, leading them to be killed instantly or die very slowly with great suffering（Laist 1997, Ceccarelli 2009, Gilardi et al. 2010）.
These ghost nets are discarded fishing nets that roam the sea. It does not matter if these were thrown away due to unavoidable circumstances or intentionally; all of these discarded fishing nets are called ghost nets.
They are given this name because they float in the sea and there is nobody to control them, and thus they keep entangling marine animals, including fish, just like a ghost.
Almost all modern fishing nets are made of plastic, because this allows them to be light (bouyant) and durable (for long lasting use). However, at the same time, these features make it difficult for caught animals to escape.
There is no accurate estimation of how much fishing gear is lost annually all over the world. The seashore in northern Australia is known as one of the places with the most ghost nets, reaching 3 tons per kilometer in a year.
The main reason for this is the discarding and loss of fishing nets by fishing boats (including illegal ones) operating on the Arafura Sea and the Timor Sea to the north of Australia. In northern Hawaii, it is known that more than 52 tons of ghost nets are discarded every year（Gilardi et al. 2010）.
Although ghost nets occupy less than 10% by volume of all marine debris, these pose a significant threat to marine animals（Macfayden et al. 2009）.
The following are some examples. More than 300 large whales died due to being tangled in ghost nets in the northwest Atlantic Ocean between 1970 and 2009; most of these deaths happened after 1990（van Der Hoop et al. 2012）.
Researchers inspecting the wounds on humpback whales in the Gulf of Maine, USA, reported that half of them had experienced being strangled by fishing gear（Robbins & Mattila 2004）. Apparently, juvenile whales are more likely to get caught by a ghost net than adults, accelerating the decrease in the survival rate（Cassoff et al. 2011）.
Seals and sea lions get caught by fishing gear and fishing lines on their neck and body, depriving them of the ability to be mobile and seek out food, eventually choking or starving them to death. Once they get caught, they struggle in pain, making the net tighten even more.
Juvenile seals and sea lions are very curious and like to play with ghost nets, and the consequences are nothing but tragedy（Laist 1997）. When a nursing mother gets caught by ghost nets, she cannot return to her offspring.
Sea turtles fare much worse. In northern Australia, 8,690 discarded fishing nets were collected between 2005 and 2012. The estimated number of sea turtles killed by them is more than 14,600 in a year（Wilcox et al. 2015）.
Furthermore, ghost nets are a threat to all species, regardless of whether they were the fishing gear’s original target or not. It is said that the combination of the danger posed by ghost nets and the increasing pressure from fisheries may have a huge negative influence on some particular kinds of animals.