'Buchang-i' goes silent for 7th day — has the shark finally left for open sea?

A shark nicknamed "Buchang-i" that spent more than a month in an artificial waterway at Busan's North Port waterfront park has not been seen for seven days, raising hopes that it has returned to the open sea.

No citizen sightings of Buchang-i had been reported as of Monday, according to the National Institute of Fisheries Science and other agencies involved in monitoring the shark.

Buchang-i was guided out of the waterfront park's waterway by authorities on Oct. 21.

The fisheries institute had been monitoring the shark's location and health in the waters around North Port, but Buchang-i disappeared from view Tuesday of last week and has not been spotted since. The institute stopped on-site monitoring Friday and shifted to a standby approach, dispatching personnel only when a citizen tip comes in. No tips have been received.

Experts say the development may signal that Buchang-i, having grown accustomed to the narrow, shallow waterway, is now ready to move into a wider environment.

However, there remains a strong possibility that the shark is diving deep in the waters just off North Port. Experts also say it may have already passed through Busan Port and reached the open sea.

Authorities plan to keep a barrier — installed to prevent Buchang-i from re-entering the waterfront park waterway — in place for now.

Removal of the barrier will be considered once officials are more confident the shark has moved to open waters.

"We will make a decision at an appropriate time after consulting with the National Institute of Fisheries Science and other relevant agencies," a Busan city official told Yonhap.

Buchang-i was first spotted in the waterfront park waterway on Sept. 18. After staying for well over a month, it was guided out of the waterway by authorities on Oct. 21.

Dropping water temperatures may also have contributed to Buchang-i's recent disappearance, experts say.

When Buchang-i was first spotted in the waterfront park waterway, the surface water temperature off North Port stood at around 25 degrees Celsius. As the weather turned colder, it fell below 20 degrees for the first time Wednesday.

Experts say the optimal water temperature range for blacktip sharks — the species Buchang-i belongs to — is between 18 and 26 degrees Celsius.

The surface temperature off North Port is now near the lower end of that range and is expected to drop to around 15 degrees by mid-November.

As surface temperatures fall, Buchang-i may have descended to warmer, deeper waters.

Meanwhile, Buchang-i — which sustained wounds during its time in the narrow, shallow waterway — is believed to be in good health now that it is outside the channel.

While fish such as mullet were present in the waterway as potential prey, the open sea offers a far better feeding environment for a shark. Buchang-i was never observed feeding during its time in the waterway.