Olive flounder farming in South Korea was initiated in the mid-1980s on Jeju Island, which offers turbulent marine conditions and underground sea water temperatures of around 1 7°C throughout most of the year, providing an ideal environment olive flounder aquaculture. As of 2022, Jeju Island contributed a pproximately 51% of national flounder production in South Korea, yielding 23,337 metr ic tons valued at $261,870,000. Nevertheless, with the growth of the olive flounder aquaculture industry , there has been an increase in disease caused economic loss. In recent years, Enteromyxum leei ( E. leei ) infection has caused emaciation symptoms and mass mortality in olive flounder . E. leei proliferates within the intestine during digestion, penetrates the intestinal wall in substantial quantities, impairs intestinal function, obstructs digestion, and hinders nutrient absorption. The situation exacerbates when bacterial diseases coincide with E. leei infestations.
The objectives of this study were to identify the seasonal incidence patterns, affected fish sizes, and the relationship between of E. leei infestations and bacterial diseases. We conducted a total of 1,164 disease examinations for four years, from 2017 to 2020. The results re vealed a total of 165 cases of E. leei infection, indicating an incidence rate of 14.2%. Seasonally, 40 cases occurred in November, 29 cases in October, and 26 cases in December, primarily in the autumn and early winter. A total of 23 cases transpired during the six months from March to August, affirming its seasonality. The incidence of E. leei infection varied by the size of the fish, with a 1.2% infection rate for juvenile fish less than 20 cm, a 13.7% infection rate for fish between 21 and 30 cm, a 25.2% infection rate for fish sized between 31 and 40 cm, and a relatively lower incidence rate of 7.6% observed for fish over 40 cm. Co-infections of bacterial diseases among the fish infected with E. leei were as follows: 51.5% for Vibriosis , 10.9% for Tenacibaculosis , 23.6% for Streptococcosis , and 19.4% for Edwardsiellosis. In comparison to the incidence of bacterial diseases, the rate of co- infection with E. leei was significantly higher for Edwardsiellosis and Vibriosis. In conclusion, this study highlights the significant impact of Enteromyxum leei infestations on olive flounder aquaculture in Jeju Island, South Korea, revealing seasonal incidence patterns, size-dependent infection rates, and the association with bacterial diseases, underscoring the need for effective management strategies to mitigate economic losses in the industry.