Choi Yoon, emeritus professor at Gunsan University's Department of Marine Biological Resources

Interview on CBS Radio's 'Park Jae-hong's Hanpan Seungbu'

"Bull sharks tend to dive below the surface when threatened"

Coast guard and fire department rescue boats spray water jets to guide Bukang-i toward the open sea at Bukhang Waterfront Park in Dong-gu, Busan, on Tuesday. Rescue authorities deployed vessels and water pumps in the park's artificial waterway that day in an attempt to push the shark out of the channel, but the effort was unsuccessful. [Yonhap]
Coast guard and fire department rescue boats spray water jets to guide Bukang-i toward the open sea at Bukhang Waterfront Park in Dong-gu, Busan, on Tuesday. Rescue authorities deployed vessels and water pumps in the park's artificial waterway that day in an attempt to push the shark out of the channel, but the effort was unsuccessful. [Yonhap]

A shark expert who has studied the animals for nearly three decades says the first rescue attempt to guide "Bukang-i" — a shark that has taken up residence in the artificial waterway of Bukhang Waterfront Park in Busan — out to sea failed because the animal dove beneath the surface, a behavior consistent with the species' ecological traits.

Choi Yoon, an emeritus professor in the Department of Marine Biological Resources at Gunsan University, said in an interview on CBS Radio's "Park Jae-hong's Hanpan Seungbu" on Wednesday that the outcome came as a surprise. "I thought the best possible method had a good chance of succeeding, so the failure was quite unexpected," he said, citing a 2004 case in which a great white shark was successfully guided out of a coastal lake in Massachusetts using the same water-pump method.

Coast guard and fire department rescue boats spray water jets to guide Bukang-i toward the open sea at Bukhang Waterfront Park in Dong-gu, Busan, on Tuesday. Rescue authorities deployed vessels and water pumps in the park's artificial waterway that day in an attempt to push the shark out of the channel, but the effort was unsuccessful. [Yonhap]
Coast guard and fire department rescue boats spray water jets to guide Bukang-i toward the open sea at Bukhang Waterfront Park in Dong-gu, Busan, on Tuesday. Rescue authorities deployed vessels and water pumps in the park's artificial waterway that day in an attempt to push the shark out of the channel, but the effort was unsuccessful. [Yonhap]

Choi said he had been thinking carefully about why the water jets failed, and attributed it to a key difference between great white sharks and bull sharks. "In the case of the great white, its prey are mammals — dolphins and seals. It was an extremely large adult, about 4 meters long. Because it hunts at the surface and comes up to breathe, it tends to stay near the surface even when faced with a sudden threat," he explained.

Bull sharks — the species Bukang-i is believed to belong to — behave differently, he said. "They feed near coral reefs in tropical waters or close to the seabed. In Korean waters, they hunt schooling fish like yellowtail and mackerel around Jeju Island and along the southern coast. When danger suddenly appears, they tend to move downward."

He said that instinct to hide below likely explains the rescue failure. "Because it shows this behavior of hiding downward, I think what happened this time is that when the water pumps created a threatening situation, it sank to the bottom — it hid beneath the surface — and that made things quite difficult," he said.

Choi noted that in the Massachusetts case, the great white shark remained near the edge of the lake for about a week longer, and the entire rescue operation took roughly two weeks in total. He advised against rushing the next attempt. "We should not be too hasty," he said.

"For the next attempt, if it hides, we should wait a little for it to rise back up and then proceed gradually," he added. "The artificial waterway is estimated to be about 500 meters long. It would be better to work after dark, and even if it takes two to three days, slowly pushing it out would be the better approach."

When asked about reports that Bukang-i had been seen swimming even while the water pumps were operating, Choi said, "It seems to have adapted somewhat," and added, "It still appears to be in good health."

On the use of nets — the second method announced by the Busan Coast Guard and the National Institute of Fisheries Science — Choi said it was "the best option available at this point." He cautioned, however, that the approach carries risks. "Using nets has a clearly higher chance of success than water pumps, but there is a possibility of physical contact with the shark, and there is also the risk of it becoming entangled. The downside is that it poses a somewhat higher risk to the shark," he said.

Asked about the possibility that Bukang-i might return to the sea on its own, Choi said the chances were good. "The artificial waterway at Bukhang is much shallower and narrower than the open sea, so when water temperatures drop, it gets far colder than the ocean. Once the water temperature falls below 20 degrees Celsius — around November — conditions become extremely difficult for bull sharks to survive. So there is a very real possibility that it will gradually make its way back to warmer waters on its own," he said.

He added, however, that the shark could not feed in the current waterway in the meantime. "We also have to keep in mind the possibility of it dying over the long term, which is why it is advisable to continue rescue efforts now," he said.


jshan@heraldcorp.com