A new analysis published last Thursday in the journal Science reveals that overfishing has caused populations of chondrichthyan fishes – sharks, rays, and chimaeras – to decline by more than 50% since 1970. To determine the consequences, a team of researchers developed an aquatic Red List Index (RLI) which shows that the risk of extinction for chondrichthyan has increased by 19%. The study also highlights that the overfishing of the largest species in nearshore and pelagic habitats could eliminate up to 22% of ecological functions.
Chondrichthyans are an ancient and ecologically diverse group of over 1,199 fishes that are increasingly threatened by human activities. Overexploitation by target fisheries and incidental capture (bycatch), compounded by habitat degradation, climate change and pollution, has resulted in over one-third of chondrichthyans facing extinction. Here, the RLI was used to track the status of these species over the past 50-years. “The shark and ray RLI shows how 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 at Simon Fraser University, Canada. “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”.
These widespread documented declines are expected to have significant consequences on other species and aquatic ecosystems. “Sharks and rays are important predators, and their decline disrupts food webs throughout the ocean. 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”, said Dr. Nathan Pacoureau at the European Institute for Marine Studies, Brest University, France. “Rays, meanwhile, are important foraging animals that mix and oxygenate sediments, influencing marine productivity and carbon storage”.
Despite these alarming trends, the team emphasizes the positive development in the appreciation and conservation of sharks and rays. “This analysis points to solutions” said Professor Colin Simpfendorfer, 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”.
“The Red List Index has been a tool used by governments for monitoring conservation progress on land, but until now, yet no equivalent existed for the ocean.” said Dr Rima Jabado, Deputy Chair of the IUCN Species Survival Commission (SSC) and Chair of the IUCN SSC Shark Specialist Group. “This new shark and ray RLI will help track progress toward ocean biodiversity and sustainability targets, identify the species and places most at risk and guide future conservation efforts”.
