What is claimed to be the first study to rank marine mammals by their vulnerability to macroplastics pollution has found that Hawaiian monk seals are the leading contender for extinction.
Co-authored by scientists at Washington-based Ocean Conservancy, Arizona State University and the Shaw Institute, the study considers the risks posed by ocean-borne plastics items larger than 5mm.
It concludes that following Hawaiian monk seals it is African manatees, Australian sea-lions, vaquita porpoises and Mediterranean monk seals in descending order that are most at risk globally from plastics ingestion or entanglement.
And, assessed by order rather than species, it is sirenians such as manatees and dugongs that are most vulnerable to plastics pollution.
The International Union for Conservation of Nature (IUCN) classifies 125 species as marine mammals. Those living primarily in freshwater habitats (such as hippopotamuses) or on land (polar bears) were excluded from the study.
Of the 117 species evaluated, more than one in three are IUCN Red Listed as Vulnerable, Endangered or Critically Endangered. Of the 22 marine mammals in the highest-risk group, 17 fall into one of these three categories.
Sensitivity to plastics
“All marine mammals are affected by plastic pollution, but we wanted to understand which ones we should be most worried about,” said Dr Erin Murphy, Ocean Conservancy’s manager of ocean plastic research and a study co-author.
The researchers scored species according to 11 traits, reflecting likelihood of exposure, relative sensitivity to plastics and population resilience, and ranking the animals’ vulnerability as high, medium-high, medium, medium-low and low.
The remaining 15 species most at risk (in descending order) are the Florida manatee, Indo-Pacific bottlenose dolphin, sei whale, North Atlantic right whale, Hector’s dolphin, Indo-Pacific humpback dolphin, dugong, grey whale, North Pacific right whale, Irrawaddy dolphin, Atlantic spotted dolphin, Atlantic humpback dolphin, Risso’s dolphin, common bottlenose dolphin, Baird’s beaked whale, Arnoux’s beaked whale and the Indo-Pacific finless porpoise,
“It is not surprising that Hawaiian monk seals emerged at the top of this list because they are curious fish-eaters that have been found tangled in fishing gear,” said Murphy. “They also have a small population located near the Great Pacific Garbage Patch, exposing them to a lot of plastic.
“Local organisations have been conducting targeted debris-removal programmes to help protect the Hawaiian monk seal, and studies have shown that this has helped the population rebound. This gives us a lot of hope for how to protect the species at the top of the list.”
Sixth of a football
Murphy and her Ocean Conservancy colleagues Dr Britta Baechler (its ocean plastics research director) and Nicholas Mallos last year co-authored a complementary study that quantified the extent to which a range of types of plastic kill seabirds, sea turtles and the marine mammals that consume them.
Based on data from more than 10,000 necropsies, they found that ingesting the plastics contained in less than a sixth of a football is enough to kill one in two of the smallest marine mammals: harbour porpoises.
An estimated 11 million tonnes of plastics enter the ocean each year. Of the 7,000 or so marine mammals studied that had died with plastics in their digestive systems, 72% had eaten fishing debris, 10% soft plastics, 5% rubber, 3% hard plastics, 2% foam and 0.7% synthetic cloth.
“It’s been energisng to see the impact our research has made, from inspiring volunteers to influencing policy-makers to take action,” said Baechler. Ocean Conservancy organises the International Coastal Cleanup, which it says is the world’s biggest single-day beach and waterway clean-up, involving hundreds of thousands of volunteers annually, as well as the Global Ghost Gear Initiative.
The macroplastics study has just been published in Conservation Biology.
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