Ghosts in the Ocean
Updated: Dec 3, 2021
In this day and age, we are all becoming increasingly aware that single-use plastic is a terrible pollutant. In the marine environment plastics can now be found in every ocean, throughout the entire water column; from the surface, to the deep sea trenches. Today, it is estimated that as much as 12.7 million tonnes of plastic find their way into the marine environment each and every year (Parton et al, 2019).
Plastic is particularly damaging for marine habitats because it degrades much more slowly than other forms of litter and can persist in the environment for a long time - potentially hundreds of years. Decomposition of plastics is especially slow in the oceans due to low oxygen levels and little microorganism activity. However, when plastic does degrade, it can even then be extremely problematic because it persists as “microplastics” (Andrady, 2015).
There are two different forms of plastic litter which affect marine life, such as sharks. 1. Land-based plastics; this refers to items which are used and discarded on land, but find their way into the ocean, such as drinks bottles, plastic bags or straws. 2. Fishing-based plastics; this refers to plastic debris which has come from the fishing industry and has either been abandoned or lost at sea.
Land-based plastic pollution can cause unnecessary shark mortalities. Loop-shaped plastics (such as those found on packs of canned drinks or long, rigid strips used to pack boxes and crates) can be trapped around the body of sharks and cut into the animal increasingly severely as it grows. In the short-term this can limit a shark’s movement and affect its ability to hunt and, over the long-term can lead to bleeding or severe infections. In once incident, long-term entanglement of a shortfin mako shark (Isurus oxyrinchus) disturbed natural growth so severely that the animal suffered spinal malformation (Kühn et al, 2015). Entanglement in plastic is commonly around the gills, as long strips get caught in the gill slits. This can severely inhibit the shark’s ability to pass oxygen over the gills and can lead to suffocation (Parton et al, 2019).
Fishing-based plastic litter, however, can be dangerous to sharks on a much larger scale. “Ghost fishing gear” are nets or equipment no longer under the control of the fishing professionals, which passively drift across oceans and continue to entangle marine animals. It is estimated that 6.4 million tons of fishing gear are lost in the world’s oceans every year! If a shark is lucky, entanglement by ghost fishing may just lead to injury and/or stress, but more commonly it causes immobilisation, and ultimately death by predation, starvation or suffocation (Matsuoka et al, 2005). In a recent review, ghost fishing gear was responsible for 74% of entanglements of sharks and rays in the scientific literature studied (Parton et al, 2019). Sharks are particularly vulnerable to ghost fishing because they often bite objects in the water to investigate them (we call this “mouthing”) and curiosity of novel objects like nets may often be the cause of the initial entanglement.
Certain species of “Elasmobranchs” (sharks, skates and rays) are more susceptible to entanglement than others. Sharks with an elongated body shape with protruding appendages (like those of the Carcharhiniformes family) are more likely to become trapped compared to those “dorso-ventrally flattened” (like the rays). Whatsmore, ghost fishing gear is more commonly found in oceanic environments (as opposed to near the coast), so “pelagic” shark species are are more likely to encounter them (Parton et al, 2019).
Mortality from marine plastic debris represents a significant threat to certain groups of sharks, many of which are already considered vulnerable to extinction. BUT, not only should we be concerned about the unnecessary losses this litter causes, but we need to recognise that it represents an animal welfare issue. Entanglement in plastic debris causes sharks (and other marine animals) considerable pain and distress, and if they do eventually die as a result, it is an extended, cruel death. We have a moral obligation to clean up the mess that we created: ensuring that all plastic debris is removed from our oceans and that we have better systems in place to ensure that continued littering is kept to a minimum.
But what can you do to help!?
If you would like to help, you can attend (or even organise your own!) beach clean-up effort.
Or if you would like your outrage at ghost fishing to be heard, you can sign the petition.
Andrady AL (2015). Persistence of Plastic Litter in the Oceans. In: Bergmann M, Gutow L & Klages M (Eds.). Marine Anthropogenic Litter. Springer, Cham Swutzerland. pp 57-72. Access online.
Kühn S, Bravo Rebolledo EL & van Franeker JA (2015). Deleterious Effects of Litter on Marine Life. In: Bergmann M, Gutow L & Klages M (Eds.). Marine Anthropogenic Litter. Springer, Cham Swutzerland. pp 75-72. Access online.
Matsuoka T, Nakashima T & Nagsawa N (2005). Review of Ghost-Fishing; Scientific Approaches to Evaluation and Solution. Fisheries Science, 4:71, 691-702. Access online.
Parton KJ, Galloway TS & Godley BJ (2019). Global review of shark and ray entanglement in anthropogenic marine debris. Endangered Species Research, 39:4, 173-190. Access online.