Shark populations around the world have seen an alarming and continual drop in the past 70 years, according to a new study.

The findings suggest that despite conservation efforts, many species have become threatened and endangered and remain at risk because of overfishing and habitat loss, researchers say.

Continued improvements are needed in many parts of the world to slow and reverse declines, including for those that use coral reefs, they say.

In a comprehensive study published in Nature, the team deployed more than 15,000 baited remote underwater video stations on 371 coral reefs in 58 countries.

Surprisingly, they detected no sharks in almost 20% of locations surveyed and they were almost completely absent from coral reefs in several nations.

The study supports the belief that demand for products, such as fins and meat, and bycatch (those found in nets by fishermen seeking other types of fish) strongly contribute to the widespread declines in numbers around the world, the researchers say.

“The study globally assessed sharks at coral reefs, which included 59 different species in the Atlantic, Pacific, and Indian Oceans,” says Philip Matich, a marine biologist at Texas A&M-Galveston. “These ranged from Caribbean reef sharks and bull sharks to hammerheads, tiger sharks, and many other species, including those not tied to coral reefs.

“But sharks were completely or nearly absent from some countries, which was not expected considering the importance sharks play in maintaining the stability of marine food webs.

“Many of the nations that lacked sharks were characterized by low socioeconomic status, which can affect conservation and management due to available resources, including finances, personnel, food security, education, and infrastructure.”

Matich and colleagues detected no sharks on any of the reefs in six nations: the Dominican Republic, the French West Indies, Kenya, Vietnam, the Windward Dutch Antilles, and Qatar.

The study shows without corrective steps in regions where management is still ineffective, continued depletion is highly likely, particularly for those with slow growth rates, late age at maturity, and low reproductive output, Matich says.

Numbers are important because they can be a barometer of overall ocean health and ecosystem vitality.

“Sharks have important roles in marine ecosystems, but disturbance can alter this role,” he says. “A major disturbance to sharks and their ecological roles is habitat deterioration—as habitats are damaged, the resources they provide, like food and shelter, can change, often negatively.

“In turn, changes in shark populations can further affect the health of ecosystems because they help regulate prey populations by eating and scaring them, affecting behavior and abundance when present.”

Coral reefs are also in decline in many parts of the world, adding to the problem, the researchers say.

The study notes that some countries—especially the Bahamas—provide sanctuaries, prohibiting fishing and harvesting to combat the problem. The study shows these measures protect fish that use coral reefs, with the Bahamas supporting some of the healthiest populations across the world.

“Change takes time, and like many other management and conservation issues that we currently face, it’s unclear if the number of nations without sharks inhabiting their coral reefs will increase, decrease, or remain stable,” Matich says.

“I’m personally optimistic based on my interactions with communities and the value they place on live sharks over dead sharks, but there is still a lot of change that needs to happen.”