New research finds that thousands of sharks and rays could be entangled in the plastic polluting Earth’s oceans.
Scientists at the UK’s University of Exeter examined existing scientific literature and took to Twitter to find documented instances of shark and ray entanglements. They ended up finding reports of more than 1,000 entangled animals — and they say the actual number of sharks and rays snarled in plastic is likely to be far higher, as there have been few studies that focused specifically on the issue.
According to the study, published in the journal Endangered Species Research this month, the researchers’ literature review discovered documentation of 557 animals from 34 different species that had become ensnared in plastic waste. Close to 60 percent of the animals were lesser spotted dogfish, spotted ratfish, or spiny dogfish.
These entanglements occurred across all three major ocean basins, but the vast majority of cases were in either the Pacific (49 percent) or the Atlantic (46 percent) oceans.
Some 44 percent of the entanglements occurred in the waters of the USA, while 30 percent were found in UK waters, and 10 percent were found in South Africa’s waters.
“The most common entangling objects were ghost fishing gear (74% of animals) followed by polypropylene strapping bands (11% of animals), with other entangling materials such as circular plastic debris, polythene bags and rubber tyres comprising 1% of total entangled animals,” the authors write in the study.
The researcher’s Twitter investigation, meanwhile, uncovered another 74 cases of entanglement involving 26 different species, including basking sharks, great white sharks, tiger sharks, and whale sharks.
Nearly 90 percent of those entanglements occurred in the Atlantic Ocean, and ghost fishing gear — nets, lines, and other fishing tackle that has been lost or discarded at sea — was again found to be the most common culprit, entangling 94.9 percent of animals.
“Entanglement in marine debris is symptomatic of a degraded marine environment and is a clear animal welfare issue,” the authors write. But they add that entanglement is “likely a far lesser threat” to shark and ray populations than the threat posed by commercial fishing.
The study’s lead author, Kristian Parton of the University of Exter’s Centre for Ecology and Conservation, highlights the impacts to animal welfare by pointing to the case, included in the study, of a shortfin mako shark that had become tightly wrapped in fishing rope.
“The shark had clearly continued growing after becoming entangled, so the rope — which was covered in barnacles — had dug into its skin and damaged its spine,” Parton said in a statement. “Although we don’t think entanglement is a major threat to the future of sharks and rays, it’s important to understand the range of threats facing these species, which are among the most threatened in the oceans.” Entanglement “can cause pain, suffering and even death,” Parton added.
In the study, Parton and co-authors identify a number of factors that put some species at higher risk of entanglement than others. Habitat is one important factor, as sharks and rays in the open ocean are apparently more likely to become ensnarled in plastic. So are species that dwell on the ocean floor, thanks to fishing gear like nets that sink to the bottom of the sea while full of dead fish, attracting predators that end up getting ensnarled. Species that migrate long distances also face heightened risk of being snared by plastic waste, per the study. And body shape appears to play a role, as well: sharks are at greater risk than rays, especially those sharks with unusual features, such as manta rays, basking sharks, and sawfish.
Study co-author Brendan Godley, a University of Exeter professor, said that entanglement is an issue that deserves far more attention, and that the study’s results show it is far more widespread than is currently understood.
“Due to the threats of direct over-fishing of sharks and rays, and ‘bycatch’ (accidental catching while fishing for other species), the issue of entanglement has perhaps gone a little under the radar,” Godley said in a statement. “We set out to remedy this. Our study was the first to use Twitter to gather such data, and our results from the social media site revealed entanglements of species — and in places — not recorded in the academic papers.”
CITATION
• Parton, K. J., Galloway, T. S., & Godley, B. J. (2019). Global review of shark and ray entanglement in anthropogenic marine debris. Endangered Species Research, 39, 173-190. doi:10.3354/esr00964
This article by Mike Gaworecki was first published by Mongabay.com on 16 July 2019.
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