The Worst... Beware of The Plastics!
Updated: Jan 13, 2022
"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. Microplastics find their way into the ocean when they are flushed into sewage systems and eventually are washed into waterways. It is thought that within the next few years there will be more pieces of plastic than there are fish in the sea! But what damage can something so tiny really cause in such an enormous ocean?... The answer... A LOT!
Microplastics are now prevelent in the marine environment, but we are only just beginning to understand how this might affect marine organisms, like sharks. As microplastics are so abundant and wide-ranging, it is thought they are likely to have an impact on a diverse array of marine species. It is hypothesised that microplastics might affect sharks in 2 different ways: 1. through direct ingestion of microplastics, 2. from indirect ingestion of microplastics, which have been accumulated in the tissues of the sharks' prey (this process is known as "bioaccumulation") (Valente et al, 2019).
Scientists have discovered that microplastics are a problem for many species of sharks and rays which filter-feed, as they are often not filtered out of the water and are directly ingested. In Indonesia, scientists used visual and trawl surveys to count how many items of floating plastic and how many microplastic particles were found in the water at the Komodo National Park, the Nusa Penida Marine Protected area and Pantai Bentar in East Java. The goal of the work was to assess how plastic pollution might affect filter-feeding whale sharks (Rhincodon typus) and manta rays (Mobula alfredi), which use the protected areas for foraging (Germanov et al, 2019).
They found that plastic abundance was as high as 449,000 pieces in every square kilometre! The vast majority (78%) of these were microplastics. Calculating the feeding rates of whales sharks and manta rays, they estimated that manta rays would ingest as many as 63 pieces of plastic every hour when feeding at Komodo and whale sharks would ingest 137 pieces every hour when foraging in Java! This means that, even though these habitats are protected, human activity may still have an impact on sharks and rays in the area (Germanov et al, 2019).
These scientists suggested that the ingestion of plastics may disrupt regular gut functioning and may also leach toxic chemicals, which could progressively poison whale sharks and rays (Germanov et al, 2019). 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 (Valente et al, 2019).
Whatsmore, it has been found that whale sharks retain high levels of plastic contaminants (such as organochlorine compounds (PCBs, DDTs) and polybrominated diphenyl ethers (PBDEs)) in their tissues, but is not yet known, what physiological impacts this may have on the health of these sharks in the long-run (Fossia et al, 2017).
It is not only filter-feeders as risk though. Predatory sharks can also ingest plastics. In order to assess how plastics accumulate in the gut of sharks, a recent study analysed three deepwater shark species: the blackmouth catshark (Galeus melastomus), lesser-spotted dogfish (Scyliorhinus canicula) and the velvet belly dogfish (Etmopterus spinax) (Valente et al, 2019).
This study found that all three species had microplastics within their gastrointestinal tracts.
See image:
a = velvet belly dogfish
b = lesser-spotted dogfish
c = blackmouth catshark
These species all feed on crustaceans (crabs and lobsters) and cephalopods (octopi and squid), which live on or near to the ocean floor. The researchers suggested the sharks might have accidentally ingested the plastics, after mistaking them for prey (Valente et al, 2019).
This study suggests that very diverse shark species are at risk of ingestion of microplastics and that seafloor habitats will be equally at risk from microplastics as shallow, coastal areas (Valente et al, 2019).
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.
If plastics block the digestive tract they can cause serious physical damage and reduce how much the animal can eat. This can affect their growth and reproduction, or even lead to starvation. Furthermore, accumulation of plastics in tissues can cause altered immune system functioning and energy depletion (Valente et al, 2019). More ecotoxicology studies are needed to fully explore how plastics degrading within the bodies of sharks might release harmful chemicals and to assess how plastics contaminants might bioaccumulate from prey species, up the food chain, to sharks and other top predators. It is very possible the effects of microplastics might be worse then we can have anticipated.
These studies highlight that 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 shark species. 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
Fossia MC, Bainia M, Pantia C, Gallia M, Jiménez B, Muñoz-Arnanz J, Marsilia L, Finoiac MG &Ramírez-Macíasd 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.
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.
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.
By Sophie A. Maycock for SharkSpeak.