Scientists identify more than 1,000 sharks and rays tangled in ocean waste

Scientists at the University of Exeter have turned to Twitter for a better picture of the effect ocean plastics are having on sharks and rays.

The scientists looked for Twitter reports of sightings of sharks and rays entangled in plastic as well as similar incidents reported in other scientific research. Summing these reports, they identified more than 1,000 individual sharks and rays seen entangled in ocean waste.

A university press release says the total number of entangled individuals to date "is likely to be far higher" as, clearly, reliance on social media reports will never lead to a comprehensive figure, even when combined with formal studies. To date, the research into plastic entanglement of sharks and rays has been far from comprehensive.

However, the scientists point out that entanglements probably aren't an existential threat to shark and ray species. By contrast, commercial fishing poses a much greater threat, whether caught deliberately or accidentally.

It's perhaps for this reason that the issue of entanglement has been relatively overlooked, argues the study's co-author Brendan Godley.

But the scientists are concerned about the welfare of sharks and rays due to the suffering caused by entanglement, which, in nearly three quarters of cases, is due to so-called "ghost" fishing equipment such as nets and lines which have been lost or thrown overboard.

Discarded polypropylene packaging strapping is the next biggest cause, involved in 11 percent of cases. Incidents with polythene bags and rubber tires have also been recorded. Bad cases are known to have led to the deaths of the animals involved.

"One example in the study is a shortfin mako shark with fishing rope wrapped tightly around it," the university's Kristian Parton says in the press release. "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."

The study is the first to use data from Twitter, which led to reports of entanglements in species and locations that hadn't been recorded in prior research.

The Twitter data encompassed 559 animals from 26 shark and ray species, great white, basking, tiger, and whale sharks.

Meanwhile, other research identified 557 individuals from 34 species. Nearly 60 percent of cases involved spotted dogfish, spiny dogfish and spotted ratfish.

Species that inhabit the open oceans and ocean floor are most at risk. In the latter case, sharks and rays can be attracted to sunken fishing nets laden with dead fish.

And those that migrate are also at greater risk as they're more likely to come across ocean waste. Body shape is also a factor, with sharks generally more at risk than rays, according to the research. Species like basking sharks, manta rays and sawfish (also a type of ray) can be more at risk due to the unusual protrusions.

The researchers are calling for reports of entanglement to be standardized to better assess the scale of the issue and identify hotspots where many entanglements occur.

Though ocean waste remains a lesser threat than fishing, the research is a welcome reminder that, as custodians of the planet, we have a responsibility not just for the survival of species, but their wellbeing too.

The research has been published in the journal Endangered Species Research. You can read the full version in this online PDF file.

Source: University of Exeter