ÖOver the past half century, ocean or open ocean shark and ray populations have declined by 71 percent, according to a study published Jan. 27 nature, a trend that coincides with the increasing fishing of these species.
“Knowing that this is a global number, the results are strong,” said co-author Nick Dulvy of Simon Fraser University in a statement. “If we don’t do anything, it will be too late. It’s much worse than other animal populations we’ve looked at. It’s an incredible rate of decline, steeper than most declines in elephants and rhinos, and these animals are iconic when it comes to advancing conservation efforts on land. “
There are 31 species of shark and ray in the open ocean, 24 of which are critically endangered according to the standards of the Red List Index of the International Union for Conservation of Nature (IUCN). Three species – the oceanic whitetip, the scalloped hammer head, and the large hammer head – are threatened with extinction.
Shark declines have already been documented, but to get a global feel for the animal’s situation, the team used long-term data from the Red List along with the Living Planet Index, a project between the World Wildlife Fund and the Zoological Society of London that is tracking the biodiversity. These datasets quantify the subpopulation sizes of species in different geographic areas over time, and the team used these to extrapolate global numbers.
For example, following the introduction of protection in 2005, the great hammer head experienced a resurgence in two areas. Other regions have seen declines, including a 99.9 percent decline in the Mediterranean since the mid-1980s. Taking into account all regions in which this species occurs, the IUCN data included in the analysis showed a reduction of more than 80 percent over a period of three generations of sharks. The fisheries figures indicate how much vessels are bringing in and how many sharks have been ingested as bycatch, which gives the team an idea of how the fishery correlates with population size.
These cartilaginous fish are harvested for their meat and fins, although most die as bycatch in other fisheries, especially large fish like tuna, which the sharks hunt themselves.
“You drop a fishing line in the open ocean and often it is sharks first – whether or not they are the primary target,” says Stuart Sandin, a marine biologist at the Scripps Institution of Oceanography, who was not involved in the research, says the Associated Press.
New technologies have made it easier for fishermen to meet market demands, including improved sonar technology for finding fish and stronger, faster boats with increased capacity for used bait hooks and nets. As a result, annual shark catches in 1970 are 18 times higher than in 1970, the authors write in the newspaper, putting a tremendous burden on these populations, which are slow to mature and do not produce many offspring at the same time.
“That’s the driver behind the 70% reduction over the past 50 years,” says University of Exeter co-author Richard Sherley BBC News. “For every 10 sharks you had in the open ocean in the 1970s, you would have an average of three across those species today.”
To combat these nosedive numbers, the authors recommend governments around the world to take and enforce scientifically based protective measures and reach limits so that an irreversible collapse of the population does not become a reality.
“The United States is one of the few countries that has really managed to reverse a severe population decline through management,” says co-author Sonja Fordham, President of Shark Advocates International science. “It’s difficult; the fishing industry is under heavy pressure to protect short-term economic interests.”
In the future, the team will investigate the status of sharks and rays that occupy reefs and coastal waters.