2017 Volume 83 Issue 1 Pages 41-51
Mass mortality of cultured bluefin tuna Thunnus orientalis, with the loss of approximately 1,800 individuals, occurred in the coastal area of Tsushima Island, Nagasaki in September 2015. A red tide of harmful unarmored dinoflagellate Cochlodinium polykrikoides and turbid water were observed simultaneously with the fish mortality. Since bluefin tuna farming has not long been established, the impacts of red tide and water environment on cultured bluefin tuna are not fully understood. To understand the cause of bluefin tuna mortality, we examined cell densities of C. polykrikoides and environmental factors at Asou Bay and offshore from west Tsushima Island. At the time of the mortality of cultured bluefin tuna, C. polykrikoides were 172-795 cells mL−1 around the cages. The cell density of C. polykrikoides red tide was lower than that of previous cases of mortality of different fish species. Moreover, mortalities of bluefin tuna were obvious at the cages exposed to turbid water caused by river water inflow due to heavy rain, 3.8-7.6 FTU. These facts suggest that cultured bluefin tuna is rather sensitive to C. polykrikoides red tide and turbid water.
