Detrimental effect of the harmful dinoflagellate Karenia mikimotoi on Japanese common squid Todarodes pacificus

Japanese common squid Todarodes pacificus was exposed to the red tide-causing dinoflagellate Karenia mikimotoi, collected from a red tide event in Hakodate Port, in seawater at four densities (0, 90, 320, and 1900 K. mikimotoicells mL⁻¹) for 24 h. The number of squids that sunk to the bottom every hour and the survival of squids were recorded every 3 h. Squids in the group exposed to the highest cell density of K. mikimotoi began to sink to the bottom within 1 h of the start of the exposure test, and the frequency of anomalies (sinking or death) was significantly higher in this group than in the other groups (Wilcoxon signed-rank test with Bonferroni correction, P-values < 0.001). Hazard ratios for squid mortality were affected by K. mikimotoi cell density (Cox proportional hazards model; P-values < 0.01). These results suggest that K. mikimotoi is harmful to the Japanese common squid and that even short-term exposure to high densities of this alga causes debilitation and loss of swimming ability, leading to death. The debilitation and death of the Japanese common squid by K. mikimotoi has the potential to cause a loss of freshness and a decline in their commercial value, as has been observed during red tide events.