Global Warming Imperils Rare Fish In Lake Biwa

March 31, 2008

Release from: Masaki Takakura
Daily Yomiuri (Japanese)

OTSU - We are often warned about the deterioration of our living environment caused by global warming or cross-border pollution and its effects on the habitats of animals and plantlife.

Another reminder of this deterioration came with the news that a rare fish is threatened in Lake Biwa.

The fish, chaenogobius isaza, or isaza in Japanese, is a goby species that live in schools in the depths of Lake Biwa.

The fishing season for isaza lasts from autumn to spring. On a recent day, five small boats set out to catch some. Fishermen on board the boats pulled up their fishing nets from the favored depth of the fish of about 60 meters. Inside the nets some little fish about five to six centimeters long could be seen. These were indeed isaza, a seasonal delicacy that tastes good when boiled in soy sauce with sugar or cooked in miso soup.

In December, an underwater camera installed in an unmanned submersible vessel belonging to the Lake Biwa Environmental Research Institute captured images of many dead fish on the bottom of the 90-meter-deep lake.

Institute Researcher Toshiyuki Ishikawa and his colleagues examined the images and counted more than 1,800 dead isaza. It was the first time a die-off of this kind had been discovered since the institute started observing the bottom of the lake about seven years ago.

Around the time this finding was made, readings taken from the lake showed that the minimum concentration of dissolved oxygen at the lake's bottom was only 0.6 milligram per liter, much lower than the 2-milligram mark usually needed to guarantee a living environment for fish.

The research institute concluded it likely that the fish had died from a lack of oxygen.

The bottom layer of the lake's water is typically short of oxygen because sunlight does not reach that deep. But in winter, the layer of water near the lake surface cools and becomes denser, resulting in the surface layer of the lake mixing more with the bottom layer of the lake thereby distributing oxygen to the lower layer. This phenomenon, called "deep breathing," is an important part of the annual circulation cycle of the lake's water.

According to observations made by Prof. Shuichi Endo at Shiga University, the average water temperature in Lake Biwa has risen more than 2 C over the past 20 years.

These findings suggest that "water temperatures in winter are not dropping sufficiently, obstructing the regular circulation of the lake's water," Ishikawa said, speaking at the research institute.

The concentration of dissolved oxygen also decreases if there is an increase in nutrients sparking the growth of aquatic plant life that consumes oxygen, a process known as eutrophication.

According to the prefectural fisheries research section, the concentration of dissolved oxygen in the lake was nine milligrams in 1950. By the 1970s onward, it had decreased to the four- to six-milligram range, in which it stabilized after sewage system improvements. In 2002, however, it plunged to a record low of two milligrams.

Researchers are concerned that global warming has become a new burden on the lake's ecosystem.