Until the 1970s, people routinely dumped toxic chemicals and other waste products into the ocean with little understanding of the consequences. We assumed that the vast ocean had an almost unlimited capacity to dilute and disperse our rubbish – out of sight, out of mind. Unfortunately, our steady creation of novel chemicals and long-lasting products, such as plastics, is creating a larger and larger problem for generations to come.
Persistent organic pollutants, heavy metals, crude oil, and marine debris (such as plastic waste, and lost or discarded fishing gear) are the most common ocean pollutants. Some of these substances are used for disease and pest control, or in manufacturing and industrial processes. Others are accidental by-products of waste incineration, vehicle emissions, or forest fires. Pollutants can enter the marine environment from a variety of sources, such as discharge and runoff from agricultural and urban areas, from fishing or transport vessels, and even from winds depositing atmospheric waste onto the ocean surface.
Sharks and rays, many of which are top predators in marine and freshwater ecosystems, are highly susceptible to environmental pollution. Pollutants typically bioaccumulate, where the amount in the shark’s body grows faster than their ability to excrete it. This is compounded by biomagnification, where sharks unavoidably ingest the pollutants within their prey species too. When people, in turn, rely on sharks and rays as a source of protein, the pollutants can be passed on to them and their families.
In this Fact Sheet, we identify the main sources of ocean pollution and how these are likely to impact sharks and rays, with a particular focus on those listed on the Convention on the Conservation of Migratory Species of Wild Animals (CMS), and explore some of the downstream health implications for people.
Toxic chemicals and heavy metals
High concentrations of toxic pollutants, including organic (e.g. PCBs, DDTs, and organochlorines) and inorganic substances (e.g. heavy metals, including mercury), are now routinely found in sharks and rays. Research on the impact of these pollutants on sharks is still at an early stage, but studies on marine mammals and teleost fishes have found neurological disorders, structural damage to organs and gills, reduced fertility, developmental effects, and cancers, at levels of pollutant exposure similar to those reported from Blue Sharks (Prionace glauca), White Sharks (Carcharodon carcharias), Shortfin Mako (Isurus oxyrinchus), Common Threshers (Alopias vulpinus), and Whale Sharks (Rhincodon typus). While shark physiology does not necessarily respond in the same way to other animals, these results provide cause for concern as increasing pollutant loads are documented.
Sharks can also, inadvertently, transfer pollutants to their developing pups. A study on Common Threshers found that an adult female transferred 29–54% of the mercury and organic contaminants in her body to the near-term embryos. Similarly, high organochlorine levels have been found in young White Shark pups, presumed to be transferred from their mother’s tissue. The elevated levels of pollutants in these young sharks point to a heightened future risk of ill effects, as they will continue to bioaccumulate these contaminants throughout their lifetime. While studies of pollutant effects on reproduction have so far focused on sharks that give birth to free-swimming pups, such as those listed above, the permeable eggs of other species may mean that their embryos cannot avoid exposure to waterborne pollutants during development.
“Red Tides,” a toxic bloom of Karenia spp. dinoflagellates associated with nutrient run-off from agriculture, are increasing in frequency along the southern coast of the United States. In 2000, a large bloom led to the mass death of hundreds of Blacktip (Carcharhinus limbatus) and Atlantic Sharpnose (Rhizoprionodon terraenovae) sharks in northwest Florida. Examination of the dead sharks found that they also transferred brevetoxinsfrom the algal bloom to their embryos, showing that such maternal transfer can take place for a wide variety of pollutants. Red Tides are an ongoing problem for a number of shark species in this region, with a probable Whale Shark death also reported from Florida in 2018.
Oil spills
Oil and gas extraction is a huge industry worldwide, including in the ocean. The first offshore oil drilling platform went live in 1947 and, since then, over 12,000 platforms have been constructed on the continental shelves of 53 countries. As engineering improves, platforms are being built in deeper and deeper waters. The potential threat to sharks and rays comes primarily from large-scale oil spills, either from platform blowouts or tanker accidents.
The “Deepwater Horizon” oil spill in 2010 was the largest accidental spill in history and also the best-studied incident when considering potential effects on sharks and their relatives. The platform was located in the northern Gulf of Mexico, 66 km off the US coast, with the blowout occurring at 1,500 m depth. An estimated 750 million liters of oil were spilled, covering over 180,000 km2 of surface waters, affecting over 2,100 km of coastal habitats, and contaminating surrounding deepwater areas.
Around 80 species of sharks and rays live in the Gulf of Mexico. Their distributions, habitat preferences, biology and ecology, conservation status, and likely exposure to spilled oil, have been modeled to create vulnerability indices to regional oil spills. Sharks had higher overall vulnerability scores than teleost fishes, with the Whale Shark, Giant Manta Ray (Mobula cf. birostris), and the Scalloped Hammerhead (Sphyrna lewini) the most susceptible overall. The Gulf of Mexico is a globally important feeding area for Whale Sharks and Giant Manta Rays, both of which filter-feed on the surface, leading to concerns that both oil spills and the chemical dispersants used as a treatment for spills could damage their respective gill structures. Neonate and small juvenile Scalloped Hammerheads, on the other hand, use coastal nursery areas that could be affected by spills. All three species are globally Endangered or Critically Endangered on the IUCN Red List of Threatened Species.
Some deepwater sharks and ghost sharks (chimaeras) have also been identified as highly vulnerable to regional oil spills, particularly the Blotched Catshark (Scyliorhinus meadi), Caribbean Roughshark (Oxynotus caribbaeus), and Smallfin Catshark (Apristurus parvipinnis). Between 0.5–25% of the Deepwater Horizon oil spill is estimated to have been deposited on the seafloor, where a sudden influx of organic hydrocarbon can overwhelm natural microbial biodegradation. Field studies have detected polycyclic aromatic hydrocarbons (PAHs), indicative of oil exposure, in deepwater sharks within 100 km of the spill site. Detrimental effects may be particularly high for the species whose egg cases develop over prolonged periods on the seafloor.
Oil concentrations in coastal areas of Louisiana remained an order of magnitude higher than baseline when surveyed 8 years after the spill. With the oil now sequestered in anoxic sediments, levels are expected to remain significantly above background for decades, with ongoing impacts on sharks and their prey species in the region. Oil and gas exploration and drilling also increase vessel traffic, which poses an additional collision risk to large surface-feeders like Giant Manta Rays and Whale Sharks.
Ocean plastic
Rubbish in rivers, beaches, and the open sea is a glaringly obvious problem for anyone that uses these environments. The majority of this rubbish consists of plastic debris. Plastic is cheap to make, lightweight, and durable. Unfortunately, that has led to mass production of disposable packaging which, coupled with poor waste management, adds an estimated 14 million tons of discarded plastic to the ocean each year. Based on current trends, the quantity of plastic trash entering the ocean is expected to triple by 2040.
These plastics are widely distributed by winds and currents, traveling out to sea, washing up on distant beaches, settling into deep-sea trenches, and almost everywhere in between. This waste is projected to take hundreds of years to degrade. The major threat to sharks from plastic comes from entanglement, particularly in discarded fishing gear (‘ghost nets’), along with internal injuries and pollutant offloading from ingested plastic.
Around 6.4 million tonnes of fishing gear is lost in the world’s oceans each year. Ghost fishing gear commonly consists of synthetic nylon nets that can passively drift in currents over large distances. These nets are, by design, hard for ocean wildlife to detect, and they can trap and kill animals for many years. Migratory sharks and rays, some of which swim thousands of kilometers each year to feed and breed, are one of the worst affected groups. Oceanic species, such as Silky Sharks (Carcharhinus falciformis), Whale Sharks, White Sharks, and Giant Manta Rays, are particularly vulnerable to entanglement as they feed in frontal zones, where huge quantities of drifting rubbish also accumulate.
Plastic waste, regardless of whether it was originally a fishing net or a toothbrush, does not disappear over time – rather, it will break up into smaller and smaller pieces. These tiny, toxic pieces of plastic, now ubiquitous throughout the ocean, are impossible for animals to avoid. While plastic fragments have been found in the stomachs of many sharks and rays, accidental ingestion by large filter-feeders such as Manta Rays, Whale Sharks, and Basking Sharks (Cetorhinus maximus) is a particular concern. These species feed in areas where zooplankton are swept together by ocean currents and tidal flows; unfortunately, drifting plastics are along for the same ride. Studies of plastic fragments (opens in a new tab) at Reef Manta Ray (Mobula alfredi) and Whale Shark feeding areas in Indonesia estimated there to be 20,000–449,000 pieces of plastic per km2, leading to estimated ingestion rates of up to 63 items per hour for Manta Rays, and up to 137 items per hour for Whale Sharks. Manta Rays were estimated to be ingesting up to 980 g of plastic per kilogram of plankton. Basking Sharks in the Mediterranean Sea were estimated to be ingesting 540 plastic pieces per hour.
Blockages and internal injuries from ingestion can be lethal. Inspection of a dead whale shark that washed ashore in Thailand found it had been killed by a single hardened plastic straw that perforated its esophagus. Dead animals are unlikely to be found in the wild, and are rarely the subject of detailed examinations, so mortalities from plastic have so far only been documented due to unusual circumstances. Two other separate plastic-related deaths in Whale Sharks in Japan were identified because they died in a rehabilitation center, 201 and 297 days after their respective arrivals, from intestinal damage caused by ingested plastic pieces that were not available to them within the facility.
That said, plastic ingestion by sharks and rays, especially small pieces, will usually result in the fragments passing through the intestinal tract without causing damage. A concern, though, is that individuals will increasingly suffer from malnutrition – such as in the case of Reef Manta Rays in Indonesia, referred to above, where they could be physically ‘full’ with only 52% zooplankton in their stomach. An additional area of current research is whether ingested plastics will offload pollutants to sharks and rays. Plastics adsorb many of the chemical pollutants listed earlier, such as PCBs, DDT, PAHs, and heavy metals, and can concentrate them up to one million-fold that found in the surrounding water. Upon ingestion, these chemicals can leach into the animals’ tissue. In other ocean wildlife groups, such as marine mammals, this is believed to suppress their reproduction. For sharks and rays that are already threatened with extinction, such as the three megaplanktivores listed above, the possibility of a similar inhibitory effect is a significant concern.
