A new study finds that overfishing has caused populations of sharks, rays and chimaeras to decline by half since 1970.
A new study has shown that populations of sharks, rays and chimaeras have declined by more than 50 per cent over the last 50 years.
Chondrichtyes – the class of cartilaginous, or chondrichthyan fish – are among the most ancient of jawed animals, appearing in the fossil record of the early Silurian period around 439 million years ago – 20 million years earlier than modern bony fish.
Today, more than 1,199 species of sharks, rays and chimaeras inhabit almost every marine environment, but are increasingly threatened by human activities, especially overfishing but also through pollution and climate change.
A 2021 re-assessment of the IUCN Red List status of chondrichthyan fish found that one-third of all sharks, rays and chimaeras were at risk of extinction; a figure which the new study’s authors suggest has increased by a further 19 per cent.
The new analysis – completed as part of the collaborative Global Shark Trends Project (GSTP) – drew on the 2021 data to create an aquatic Red List Index (RLI), tracking the status of each species over the last 50 years.
By developing the 50-year RLI for chondrichthytes the study’s authors say they have shown how extinction risk ‘has expanded across geographic, bathymetric, and biodiversity axes’ as a result of overfishing; initially in rivers and the near-shore environment before eventually spreading to the deep ocean.
‘The shark and ray RLIs show declines first occurred in rivers, estuaries, and nearshore coastal waters before spreading across the oceans and then down into the deep sea,’ said Professor Nicholas K Dulvy of Canada’s Simon Fraser University.
‘The sequential depletion of the largest and most functionally important species – such as sawfishes and rhino rays – was followed by the decline of large stingrays, eagle rays, angel sharks, hammerheads and requiem sharks. Eventually, fisheries turned to deepwater sharks and skates for the liver oil and meat trade.’
The decline of the chondrichthyans as predatory fish has significant consequences for other species across a wide range of aquatic ecosystems. The study highlights that overfishing of the largest species could eliminate up to 22 per cent of the ecological functions necessary for sustaining life within a given environment.
‘Sharks and rays are important predators, and their decline disrupts food webs throughout the Ocean,’ said Dr Nathan Pacoureau at the European Institute for Marine Studies, Brest University, France.
‘Larger wide-ranging species connect ecosystems, for example, reef sharks are vital in transferring nutrients from deeper waters to coral reefs, helping to sustain those ecosystems.
‘Rays, meanwhile, are important foraging animals that mix and oxygenate sediments, influencing marine productivity and carbon storage.’
Although the decline of shark, ray and chimaera populations is of global concern, the team behind the study has also noted that there have been positive developments in the way that sharks and rays are considered within the ecosystems they inhabit.
For many years – largely thanks to the mainstream media – sharks in particular have been characterised as dangerous to humans and therefore perceived as legitimate targets for hunting, despite a huge amount of evidence to the contrary.
Sharks and rays have been illegally targeted for their fins, gill-rakers and meat by industrial fishing operations, but many are also caught as bycatch, the result of indiscriminate fishing methods such as gillnets and longlining.
Since the 1990s, however, the conservation of sharks and rays has been increasingly recognised within Regional Fisheries Management Organizations and international wildlife treaties, particularly the Convention on International Trade in Endangered Species of Wild Flora and Fauna (CITES), to which more than 100 endangered species have now been added.
Studies have shown that proper fishery management has a dramatic effect on preserving shark and ray populations, and it is hoped that the new Red List Index will help further progress in the sustainability of ocean biodiversity by targeting the species most at risk to guide future conservation efforts
‘This analysis points to solutions,’ said Professor Colin Simpfendorfer of James Cook University, Australia. ‘Nations can reduce the extinction risk by lowering fishing pressure to sustainable levels, strengthening fisheries governance, and eliminating harmful subsidies.
‘Progress has already created bright spots of hope for chondrichthyans, including in Australia, Canada, New Zealand, United States, and parts of Europe and South Africa.’
‘There has been notable progress in the appreciation and conservation of sharks and rays,’ conclude the report’s authors. ‘Yet chronic underassessment and undermanagement of fisheries is widespread, particularly in countries with weaker governance.
‘Science-based fisheries catch limits and measures to minimize incidental catch, including spatial protections, are essential to ensure sustainability and to recover species to their ecological, social, and economic potential. However, these limits need to be applied at scale, enforced, and tailored to species’ biology.’
The complete paper, ‘Ecological erosion and expanding extinction risk of sharks and rays’ by Nicholas K Duvaly et al is published (paywalled) in the online journal Science.
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