Fisheries are one of the main reasons for ocean waste. Ghost nets that have been lost accidentally or abandoned are the main factor in plastic waste in the sea.
Fishing nets and ropes used in fisheries are lost or abandoned for whatever reason. Fish lines used in recreational fishing are also often abandoned at sea.
However, this rate differs depending on the location. For example, in a study in Korea it was shown that ghost nets occupied half or three quarters of annual marine debris (Jang et al. 2014).
In the open sea, 50-90% of marine debris could be ghost nets (Hammer et al. 2012).
Over the past several decades, fisheries and fishing grounds have been expanding, meaning that ghost nets are also increasing rapidly.
The place with the worst level of ghost nets in the world is the Arafura Sea and the Timor Sea, located to the north of Australia (Wilcox et al. 2013).
Consider the following picture. The yellow dots indicate the quantity of plastic waste from ships. You can see the wide range that is a darker color in the sea to the north of Australia.
In this area, massive fishing nets are abandoned or lost by fishing boats, including those involved in illegal operations (Wilcox et al. 2013)．Therefore, the coast off north Australia has the largest incidence of ghost nets in the world, estimated at 3 tons annually per square kilometer (Wilcox et al. 2013). This causes the death of 14,000 sea turtles annually (Wilcox et al. 2013).
Almost all the fishing nets, ropes and lines used in fisheries are made of plastic. Generally speaking, these are not broken up by microorganisms in the sea.
Thus, unless ghost nets are found and retrieved, they remain permanently in the ocean.
More than 200 kinds of ocean animals, such as ocean mammals, sea birds, sea turtles and fish, become entangled by marine debris, including ghost nets (Kühn et al. 2015).