The turnover ratio of South Korean stocks soared to a three-year high in 2015 as market volatility increased amid unfavorable external conditions and a protracted economic downturn, the bourse operator said Monday.

The turnover ratio of shares listed on the main bourse, the KOSPI, came to 319.13 percent, up from 199.84 percent the previous year according to data by the Korea Exchange (KRX).

The corresponding ratio for shares in the tech-heavy KOSDAQ market was tallied at 637.23 percent in 2015, while that for 2014 was 387.06 percent, according to the KRX.

The ratios mean that each listed share on the KOSPI changed hands 3.2 times a week, and stocks for the KOSDAQ did so nearly 6.4 times per week, the bourse operator noted.

They are the highest level since 2012 when the turnover for the KOSPI came to 342.35 percent and that of the KOSDAQ stood at 683.39 percent.

"The surge is attributable to the overall growth in stock trading last year," a KRX official said.

In 2015, a total of 262.8 billion shares were traded on the KOSPI and KOSDAQ markets, up from the previous year's 155 billion.

"In addition, investors were probably nervous amid unfavorable external conditions such as ample signs of an economic slowdown in China and possible policy changes in the U.S.," he added. 

Of 707 KOSPI-listed firms, 41 companies had a turnover ratio of more than 1,000 percent last year, more than double the previous year's 18. Among 966 KOSDAQ firms, 128 were found to have the level, up from 78 companies in 2014, according to the KRX. (Yonhap)