Thankfully, in our modern world, we are all becoming increasingly more aware of the damage that we are causing to our environment. One-such issue which is now prevalent in the public consciousness, is that of plastics. We now all know we should be reducing / reusing / recycling plastics. But plastics are still a major environmental issue, especially in our oceans. So what effect do plastics have in the marine world? Are they really so damaging? And do they affect our beloved sharks and rays?
Life In Plastic, It's Fantastic
Plastics are an especially serious environmental issue because they are made from finite natural resources, and many products are used maybe once or twice and are then thrown away. These buoyant plastics can make their way into lakes and rivers and eventually the ocean, where they take hundreds of years to degrade. 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, so plastics can now be found in every ocean, throughout the entire water column; from the surface, to the deep sea trenches (Wabnitz & Nichols, 2010; Tiktak et al, 2020).
There are two different forms of plastic litter which can affect marine life, such as sharks.
1. Land-based plastics; this refers to items which are used and discarded on land, such as drinks bottles, plastic bags or straws.
2. Fishing-based plastics; this refers to plastic debris which has come from the fishing industry (Wabnitz & Nichols, 2010; Tiktak et al, 2020).
Fishing-based plastics are plastic nets, pots and traps, which have been lost or discarded by the fishing industry and continue to fish indiscriminately for many years (Wabnitz & Nichols, 2010; Tiktak et al, 2020). To learn more about this issue you can check out Ghosts in the Ocean.
Plastic Smile
You might be surprised to hear that something described as ‘land-based’ can affect marine life, but it certainly does! Land-based plastics are regularly washed into lakes and rivers, and eventually find their way into the ocean, where they cause many problems (Wabnitz & Nichols, 2010; Tiktak et al, 2020).
Land-based plastic litter 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 wrap around their body 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 move and hunt and, over the long-term can lead to bleeding or severe infections. 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 (Kühn et al, 2015; Parton et al, 2019; Afonso & Fidelis, 2023).
Scientists discovered one incident where a shortfin mako shark (Isurus oxyrinchus) entangled in plastic debris had suffered such disturbed growth, that it's spine was actually malformed (Parton et al, 2019). And in Brazil entanglement in strips of plastics have been shown to deform tiger sharks (Galeocerdo cuvier) (Afonso & Fidelis, 2023).
We Are Everywhere
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).
"Microplastics" are tiny fragments of plastic (less than 5 mm in length), which enter natural ecosystems as a result of human activity. Microplastics can be created when larger plastics begin to degrade and break down into smaller fragments, and they are also sloughed from clothing and produced during industrial processes. They are also found in many cosmetic products, such as exfoliation creams and body scrubs (Valente et al, 2019).
Small but Mighty
You might imagine that something so small could not be that harmful… but microplastics are now prevalent in the marine environment. It is thought that within the next few years there will be more pieces of plastic than there are fish in the sea! As microplastics are so abundant and wide-ranging, scientists suspect they are likely to have an impact on a diverse array of marine species (Valente et al, 2019).
Sharks can either directly ingest microplastics or they can be indirectly ingested, when microplastics have accumulated in the tissues of the sharks' prey (this process is known as "bioaccumulation"). Once plastics are ingested, they can either be excreted or accumulated. So they either pass out of the animal in their faeces or vomit, or they can become trapped in the gastrointestinal tract or they may be absorbed into tissues (Valente et al, 2019).
Scientists have discovered that microplastics are a problem for many species of sharks and rays which filter-feed. These creatures' anatomies are not effective enough to filter out such tiny objects, so they accidentally ingest microplastics. Scientists have learned that whale sharks (Rhincodon typus) in Indonesia ingest anywhere between 63 and 137 individual pieces of plastic every hour when they are feeding! (Germanov et al, 2019).
The ingestion of plastics can disrupt regular gut functioning - possibley even blocking the intestines up entirely. Necropsies of whale sharks which have washed up in Brazil, Thailand, the Philippines and Malaysia have confirmed that large amounts of plastics are accumulated within their bodies and it has been suggested this may have contributed to their deaths (Germanov et al, 2019; Valente et al, 2019).
It is not only filter-feeders at risk though, as predatory sharks can also ingest plastics. In fact, plastics are now so prevalent that scientists have even found microplastics in the guts of deep-water sharks that live far offshore, hundreds of metres down! (Valente et al, 2019)
Having A Break-Down
When plastics degrade they release toxic chemicals, such as organochlorine compounds (PCBs, DDTs) and polybrominated diphenyl ethers (PBDEs). These chemicals can spread far and wide and be absorbed or eaten, or they can be released directly into the shark's gut if they break-down after being ingested. Scientists have found that whale sharks retain high levels of plastic contaminants in their tissues, but we don't know what kind of effects this may have on the sharks' health (Fossi et al, 2017).
Plastic Responses
Plastic litter affects all areas of the ocean - from the surface to the ocean floor - and will have a serious impact upon a diverse range of sharks, rays and other marine animals. If we are to improve the situation, it is of critical importance that we reduce our consumption of single-use plastics, increase how much plastic is recycled, improve waste processes to reduce the amount of litter finding its way into the ocean and scale-up ocean clean-up initiatives, to remove as much plastic from the ocean as possible.
If you would like to reduce the amount of plastic pollution you are responsible for, you can:
Recycle all your household plastic waste!
Choose to only buy products with recyclable plastic. Check for the logo on all your bottles and tubs, and beware for sneaky bits which are not recyclable.
Reduce the amount of plastics you consume, by buying second-hand or upcycled products.
Switch from plastic to degradable beauty products, like bamboo alternatives. Reusable menstrual cups reduce plastic waste, girls and we can all switch to biodegradable tooth brushes, electric tooth-brush heads, dental floss....
Avoid buying beauty products in plastic packaging. You can get great toiletries through companies like Lush, which wrap many of their products in paper and recycle all their plastics.
Stop buying single-use plastics. Don't buy bottled drinks or foods wrapped in plastic. You can buy reusable plastic/bamboo/glass bottles and steel straws everywhere now, so there is no excuse!
Participate in beach clean-up efforts nearby... or why not even organise your own!
References
Afonso AS & Fidelis L (2023). The fate of plastic-wearing sharks: Entanglement of an iconic top predator in marine debris. Marine Pollution Bulletin, 194, p.115326. Access online.
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.
Fossi MC, Baini M, Panti C, Galli M, Jiménez B, Muñoz-Arnanz J, Marsili L, Finoia MG &Ramírez-Macías D (2017). Are whale sharks exposed to persistent organic pollutants and plastic pollution in the Gulf of California (Mexico)? First ecotoxicological investigation using skin biopsies. Comparative Biochemistry and Physiology Part C: Toxicology & Pharmacology, 199, 48-58. Access online.
Germanov ES, Marshall AD, Hendrawan G, Admiraal R, Rohner CA, Argeswara J, Wulandari R, Himawan MR & Loneragan NR (2019). Microplastics on the menu: plastics pollute Indonesian manta ray and whale shark feeding grounds. Frontiers in Marine Science, 6, 679. 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.
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.
Tiktak GP, Butcher D, Lawrence PJ, Norrey J, Bradley L, Shaw K, Preziosi R & Megson, D. (2020). Are concentrations of pollutants in sharks, rays and skates (Elasmobranchii) a cause for concern? A systematic review. Marine Pollution Bulletin, 160, 111701. Access online.
Valente T, Sbrana A, Scacco U, Jacomini C, Bianchi J, Palazzo L, de Lucia GA, Silvestri C & Matiddi M (2019). Exploring microplastic ingestion by three deep-water elasmobranch
species: A case study from the Tyrrhenian Sea. Environmental Pollution, 253, 342e350. Access online.
Wabnitz C & Nichols WJ (2010). Plastic pollution: An ocean emergency. Marine Turtle Newsletter, (129), 1. Access online.
By Sophie A. Maycock for SharkSpeak
