To catch turtles in Brunei Bay, Juanita Joseph used nets called 'kabat' to trap turtles at the mouth of estuaries. Joseph learned the method from local fishermen. (Credit: Juanita Joseph)

In the Murky Waters of Brunei Bay, Turtles Feed

[ICCB 2019] There were two key facts that turtle expert Juanita Joseph of the Borneo Marine Research Institute wished she had known about the murky waters of Brunei Bay. The first was that hundreds of green turtles fed in the bay; the second was that crocodiles swam in the same water.

Today, the turtles keep drawing Joseph back to the bay, and the crocodiles keep her out of the water — unless necessary.

(Photo: To catch turtles in Brunei Bay, Juanita Joseph used nets called ‘kabat’ to trap turtles at the mouth of estuaries. Joseph learned the method from local fishermen. Credit: Juanita Joseph)

Since 2011, Joseph’s team has studied Brunei Bay as an important site on the migratory route of green turtles in the South China Sea. They found that green turtles in Brunei Bay partly originated from rookeries in the Sulu Sea, the Celebes Sea, and the east coast of Peninsular Malaysia.

Because most green turtles in the bay were adults or sub-adults, they must have spent their juvenile years elsewhere.

Already, Joseph’s team has matched the genetics of adult green turtles in Brunei Bay to the juveniles in Layang-Layang Island and Mantanani Island, off the northwestern coast of Sabah.

Brunei Bay (4°45′–5°02′N,114°58′–115°10′E) straddles the northeastern coast of Brunei, and the Malaysian Borneo states of Sarawak and Sabah. Near the coast, the water is turbid from river sediment. But natural treasures abound. Fishermen prize the area for its rich fish stock. Dolphins and dugongs visit the bay.

The marine mammals attracted scientists from Universiti Malaysia Terengganu. But when they began seeing more turtles than dugongs, they invited Joseph who was then at the same institute, to join them.

Joseph’s team captured turtles, tagged them, and collected blood samples for health checks and genetic analyses. They had expected Brunei Bay to be a foraging ground for a small number of turtles.

But “for three years we tagged the turtles, none of them was recaptured again, so that indicates it’s a big foraging ground,” Joseph tells Macaranga after her presentation on 22 July. By 2019, they had tagged 116 green turtles and one juvenile hawksbill turtle.

While green turtles feed in Brunei Bay in numbers, capturing one was by far the biggest challenge, says Joseph. They couldn’t see in the extremely murky water. Snorkeling or diving to catch turtles was impossible.

Their attempts didn’t go unnoticed though. When local fishermen learned what the team was trying to do, they heaped praise on the team.

“They told us ‘wow, you are so brave to swim here because it’s full of crocodiles,’” Joseph recalls.

Fortunately, the fishermen offered them a safer and more effective way to catch turtles: a traditional fishing method using a net called ‘kabat’ installed along the mouth of the estuary during the highest tide at night.

By day, turtles returning to sea from the estuary would be caught in the net. Locals told Joseph that they catch fish with kabat and any trapped turtles are always released.

So Joseph paid locals to deploy the kabat for turtles. It proved effective. One night, so many turtles were caught that they tore the net and escaped. “We don’t like catching turtles because it stresses them,” says Joseph. “We do it only to collect samples.”

Joseph’s work highlights the need to conserve Brunei Bay. “If there is disturbance in Brunei Bay, it would affect sea turtles in the South China Sea,” says Joseph.

But Brunei Bay isn’t just about sea turtles.

There are also the dugongs, dolphins, and the understudied rich coastal ecosystem. An ecosystem that is already stressed by sediment from illegal logging upstream of rivers, heavy shipping traffic and fishing. An upcoming petrochemical refinery plant in the area, announced in June 2019, just adds to the environmental concern.

There is little time to lose.

This is a part of our reporting on the International Congress for Conservation Biology, July 21-25, 2019, Kuala Lumpur.