Rising sea temperatures cause fish to belly up en masse

As seawater temperatures have risen significantly due to the recent scorching heat, fish in the fish farms of the East Sea have perished en masse.

Approximately 176,000 fish at 14 fish farms on the east coast went bellies-up as seawater temperatures rose, according to North Gyeongsang Provincial Government on Monday.

Around 93,000 fish at eight fish farms in Pohang, 11,000 fish at four farms in Yeongdeok and 72,000 fish at two farms in Ulsan, all located in North Gyeongsang, were reported dead. North Gyeongsang Provincial Government estimated 1.1 billion won ($957,000) in damages.

“About 90 percent of dead fish are temperature-sensitive starry flounder,” said Seo Seong-bae, a fisheries official from the headquarters in charge of fish farms in the East Sea in North Gyeongsang.

The optimal water temperature for starry flounder is around 20 degrees Celsius (68 degrees Fahrenheit), which is far below the current water temperature in the East Sea. Ever since a high water temperature alert was issued on July 24, coastal water temperatures in Pohang and Uljin have been hovering around 26.7 degrees Celsius.

If the heat wave does not soon cool off, the fish deaths are expected to continue.

In collaboration with the involved cities and provinces, North Gyeongsang introduced support measures for suffering fish farms.

First, the local governments strongly advised workers to sell all their starry flounder as soon as they reach salable size, which is generally around 350 grams.

Second, the local governments promised measures to cool down seawater. Pohang city provided large amounts of ice to fish husbandries to immediately lower the water temperature.

Large bouts of fish deaths in fish farms is not uncommon during the hotter months in Korea. Three out of 14 fish farms that use seawater from the East Sea implemented modern systems that bring cooler seawater from below to the surface in order to maintain a stable overall temperature of around 16.5 degrees Celsius.

But, with water temperatures rising earlier than forecast and as the heat wave continues, even the currently utilized method of cooling is not enough to lower the overall water temperature.

“It is difficult to completely escape the heat, even with the new system,” said an official from North Gyeongsang. “Such technologies prove frail in the face of a natural disaster.”

Measuring Korean seawater temperatures every July for eight years, the Ministry of Oceans and Fisheries published a 2018 study which showed a 2.89-degrees Celsius increase with 0.34-degrees Celsius hike rate annually.

The surface layer temperature of Korean seas from 1980 to 2020 rose approximately 1.23 degrees Celsius, 2.6 times higher than the global ocean’s temperature rise — 0.48 degrees Celsius — during the same period.

BY LEE JIAN, KIM YOUN-HO [lee.jian@joongang.co.kr]