Gorillas and chimpanzees may be twice as numerous in western Africa as previously thought, but the apes are still endangered, declining fast and in dire need of protection, an international study found Wednesday.

The new count uses mathematical modeling to project that western lowland gorillas in western equatorial Africa number nearly 362,000, up from prior estimates of 150,000-250,000 individuals, said the report in the journal Science Advances.

However, the study found a 19 percent plunge in that gorilla population in eight years.

And chimpanzees in the area are expected to number almost 129,000, up from earlier counts of 70,000 to 117,000.

Researchers spent a decade trudging through an area of forest that's about the size of the state of Washington — or Ireland and Scotland combined — looking for lowland gorillas, chimpanzees and nests in what scientists said is the most accurate count for the apes in this primary region where they live.

The population still qualifies for the IUCN critically endangered red list because the animals are on pace to lose more than 80 percent of their population in three gorilla generations, which is a key threshold, said study author Fiona Maisels of the Wildlife Conservation Society and the University of Stirling in Scotland.

At the current rate, 80 percent or more of the gorillas will be gone by end of the century, said University of Illinois primate expert Paul Garber, who wasn't part of the study but praised it. He said in the five years since the 2013 count, the loss has likely accelerated.

"That is a doomsday scenario, and we need to reverse this immediately," Garber said in an email.

Gorillas are hunted as food, and Maisels blames that for much of the population drop. Four out of five gorillas live in an area not protected from hunting, the paper found. Maisels also said forest loss could be huge in the future.

To conduct the census, pairs of researchers surveyed thick forest areas looking up, down and sideways for both gorillas and chimpanzees, averaging about 1.2 miles (2 km) a day, Maisels said. Overall, researchers covered about one-quarter of the 290,000-square-mile (750,000-square-km) area of Angola, Cameroon, Central African Republic, Equatorial Guinea, Republic of Congo and Gabon and then used computer simulations to account for the rest.

"We used around 20,000 nests," Maisels said, referring to both species. "Nobody else has done that."

While two other outside experts praised the study, Volker Sommer at the University College London said the counting method is flawed because researchers can't tell if nests they find are day or night nests, new or old, single or multiple. He said in an email that then adding computer modeling to it "takes a questionable approach to new heights" so "we cannot expect standardized results from such an approach."

Duke University conservation scientist Stuart Pimm called the paper important, especially in the wider context of assessing how a species is doing beyond old pure population estimates.