This story originally appeared in The Guardian and is part of the Climate Desk collaboration.

Dolphin numbers in the Indian Ocean may have dropped by more than 80 percent in recent decades, with an estimated 4 million small cetaceans caught as “by-catch” in commercial tuna-fishing nets since 1950, according to a study.

As many as 100,000 cetaceans-mainly dolphins-were caught in commercial gill nets as by-catch in 2006, with current annual numbers at about 80,000.

Published in the journal Endangered Species Research, the study used the changes in the number of dolphins caught in gill nets as a way to calculate changes in dolphin populations in the Indian Ocean.

The authors say gill-net fishing in the Indian Ocean is “effectively unmanaged” and potentially the biggest unresolved issue facing cetaceans today.

The study, led by Charles Anderson of the Manta Marine organization in the Maldives, estimated that dolphin numbers had likely dropped to only 13 percent of their levels before 1980, when large-scale fishing efforts for tuna in the Indian Ocean began.

Puti Liza Mustika, a coauthor of the study from James Cook University’s College of Business, Law, and Governance, told Guardian Australia that most of the dolphins caught accidentally in gill nets did not get reported and were likely discarded over the side of boats.

About a third of the tuna pulled from the Indian Ocean is caught in gill nets that hang in the water column-a method that has long concerned conservationists, because as well as catching the valuable tuna, they also entangle other species, including dolphins, whales, sharks, and turtles.

In the study, the authors acknowledge the estimates of the impacts on dolphins are “not definitive” and are “subject to much uncertainty.” There was no attempt to differentiate the different dolphin species impacted.

But the authors added: “Nevertheless, they do highlight the potential impact of Indian Ocean gill-net fisheries on regional cetacean populations, and the need for much improved monitoring, mitigation, and management.”

Mustika said some reliable data was available from observers on boats and from previous studies. While the study looked at all cetaceans-dolphins, porpoises, and whales-the vast majority being caught were dolphins.

“That by-catch number is alarming, but there are a lot of uncertainties, because the data sets are insufficient,” Mustika said.

Results taken from 10 programs to reliably count by-catch between 1981 and 2016-carried out in Australia, Sri Lanka, India, and Pakistan-were used as the basis to extrapolate how many cetaceans were caught across all Indian Ocean fisheries covering 24 countries.

Iran, Indonesia, India, Sri Lanka, and Pakistan were five countries catching the most tuna using gill nets. Together with Oman, Yemen, UAE, and Tanzania, those nine countries accounted for almost all the dolphin by-catch.

Iran and Indonesia did not report by-catch, Mustika said, while the limited figures that were available from other countries were likely unreliable.

About 4.1 million small cetaceans had been caught between 1950 and 2018 in Indian Ocean gill nets.

The study said Iran’s average annual catch of 214,262 tons of tuna-the greatest haul across the Indian Ocean gill-net fisheries-was likely catching 30,302 cetaceans a year.

The study found that for every 1,000 tons of tuna currently being caught, about 175 cetaceans were also being caught.

Mustika said the vast majority of those animals die. “It’s a painful death. Dolphins are clever, but because the net is very thin in the water, the dolphins’ sonar misses them.”

She said the fishers should not be seen as “dolphin killers,” and those she had spoken with were unhappy when dolphins became caught in their nets.