Oceans in imminent danger of losing a quarter of all sharks and stingrays

Tony Gilbert

What would happen if our oceans lost a significant portion of cartilaginous fish species? Unfortunately, we may not have to wait long to find out. A new major study has performed the first global systematic survey of these fish and has found that a quarter of sharks, rays, and chimaeras could be wiped out within the next few decades. The study was led by Nick Dulvy of the International Union for Conservation of Nature (IUCN) and was published in the journal eLife.

IUCN is the conservation organization that facilitates dialogue between scientists, government officials, and conservation organizations around the globe to protect biodiversity on Earth. They might be most well known for the IUCN Red List of Threatened Species, which provides detailed information about the organisms’ conservation status, including population trends and biggest threats to their survival. This helps threatened species get legal protection in an attempt to preserve their numbers.

Cartilaginous fish have deep evolutionary roots and contain sharks, rays, skates, and chimaera. Many previous studies of these fish have been local or regional, but this report was the first to look at their conservation status on a global scale. Of the 1,041 confirmed species, 181 of them are considered to be threatened. About 23% were deemed “least concern” and have, for the time, stable populations. 

The team was unable to get sufficient data on all species for several reasons, but not for a lack of trying. In these situations, a classification of “data deficient (DD)” is made. For this study, nearly 47% of the species were deemed DD. Given the data they have, however, it is estimated that over half of these DD species are actually threatened. This would bring the total number of threatened species up to 249; one quarter of all cartilaginous fish species. Nearly 30% are believed to be near threatened, while only 37.4% are believed to be least concerned. The remaining 8.7% did not meet population  and geological range metrics necessary to make an educated estimation, and keep the “data deficient” designation.

Overwhelmingly, habitat destruction and overfishing are to blame for these dwindling numbers. The species most affected by habitat destruction are those living in coastal waters where human activity has destroyed reefs that typically host the type of fish they eat. Overfishing is a two-fold problem, as some threatened species are caught on purpose, while others may have been on accident, in what is known as bycatch. This also includes the fish that are caught in shark control nets or discarded fishing nets and are never actively caught by humans.

For many of these species who are targeted by fishermen, some are only wanted for their fins. A market exists for meat and liver oil from some of these species, though fins fetch the most money. In addition to sharks, rays that resemble sharks are also prime targets for fishermen. The report finds that one third of threatened sharks and rays are still being targeted, despite their conservation designation. 

Climate change is also to blame for these decreasing populations, as some species are incredibly sensitive to small changes in climate. In addition to the increasing temperatures, the ever-increasing amounts of atmospheric CO2 dissolves in the ocean and increases  water acidity.

This study is the culmination of 20 years of study from over 300 researchers. The metrics used to make the conservation designations include: abundance, population trends, habitat, historical and current range, and current threats. Without committed intervention, the oceans could lose at least a quarter of all of its cartilaginous fish, including many sharks which serve as apex predators.

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