Impact

The Threat of Ghost Nets – When Marine Animals Become Entangled in Plastic Fishing Gear

There is a YouTube video showing a humpback whale that has become tangled up in fishing gear near the island of Tonga in the South Pacific Ocean. The crew of the ship who found the entangled whale are determined to rescue it, but they have to give up on the attempt when their ship runs low on fuel.

As this video conveys, fishing gear that has been discarded in the sea, especially fishing nets, are entangling whales and other marine life species. Such lost or abandoned fishing devices are called “ghost nets”, and they create serious issues with various marine animals.

A ghost net continues to fish and trap marine organisms, entangling and potentially killing marine life in a process known as ghost fishing. These ghost nets seriously damage the marine environment by dragging along the seabed (Breen 1990). 

These nets can be entangled in dolphins’ and whales’ bodies, flippers, and/or flukes (Moore et al. 2013, Van der Hoop et al. 2013). An observation of 626 southern right whales conducted in the North Atlantic Ocean revealed that 83% of these whales had experienced entanglement in fishing gear (Knowlton et al. 2012).

Pinnipeds, such as seals and sea lions, tend to become captured by ghost nets because of their sense of curiosity (Laist 1997, Page et al. 2004), and juveniles are especially vulnerable to net entrapment (Laist 1997, Lucas 1992, Allen et al. 2012).

When California sea lions are young, for example, they are very curious and playful but less savvy about ghost nets. Because of this playful and curious nature, they are more likely to become entangled in marine debris (Zavala-González & Mellink 1997, Hanni & Pyle 2000).

Some baby seals continue to survive even though fishing gear has become tangled around their necks. However, when the baby grows, the gear will tighten, strangling them and preventing them from feeding. Once seabirds have had their beaks, feathers and/or feet entangled in a fishing line, they become deprived of the ability to eat and fly (Camphuysen 2001, Rodríguez et al. 2013). 

If these marine animals cannot be freed from the entanglement, the consequences are most likely death by drowning or starvation (Laist 1997, Pecci et al. 1978). They can also be more easily attacked and eaten by predators because they cannot move swiftly to escape (Kaiser et al. 1996, Stevens et al. 2000, Hébert et al. 2001). However, after these predators have attacked the victims, they too may become tangled in the very same nets.

Most plastics used for fishing gear are light and buoyant. If the victim of a ghost net is a large animal, such as a whale, and it dies, the net sinks to the bottom of the ocean due to the weight of the carcass. Then, when the dead whale has been eaten by benthic organisms and it is nothing but a pile of bones, the ghost net regains its buoyancy again. It starts floating in the sea and entangles more whales or other sea creatures, creating a deadly cycle that continues to cause tragedies over and over again. 

When marine animals are badly injured by a ghost net, their movement will be restricted and, of course, they cannot live long (Lucas 1992, Arnould & Croxall 1995, Laist 1997, Moore et al. 2009, Allen et al. 2012).  

Eventually, the mesh of a ghost net is clogged by the various organisms attaching themselves, and it becomes heavy and is unable to drift any further. Until then, a ghost net will continue to tangle creatures, causing serious damage to the environment (Erizini 1997, Humborstad et al. 2003, Sancho et al. 2003). Scientists estimate that a single ghost net will entangle victims for around 30 to 570 days, depending on its size, shape, and location (Matsuoka et al. 2005). 

However, they also believe that in the deep ocean the ghost fishing process takes longer than their estimated time as adhering organisms find it difficult to attach to the ghost net (Breen 1990, Humborstad et al. 2003, Large et al. 2009).

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