The recently rediscovered spinach pipefish. SHOAL

On a vast network of rivers flowing through the jungles of Papua New Guinea, researchers have rediscovered the spinach pipefish, a relative of the seahorse, which had not been recorded in more than 40 years. The fish has long been sought after by conservationists, who feared it had gone extinct.

On an expedition that unfolded like a search for a missing person, researchers spent two weeks traversing the Sepik River and its tributaries, scouring villages and markets for evidence of the missing pipefish. They showed its photo to fishers, vendors, and village elders and asked where it might be found.

The task was daunting. The Sepik River extends across 700 miles of forests, floodplains, and wetlands along the northern coast of Papua New Guinea. “Travel is almost entirely by motorized boat or canoe, often taking many hours between villages,” said Jerrica Agebigo of the New Guinea Binatang Research Centre.…