A comparison of physiological responses in yellowtail to fatal environmental hypoxia and exposure to Chattonella marina

Physiological responses of yellowtail to fatal environmental hypoxia were compared to those found during Chattonella exposure in normoxia, in order to clarify the roles of blood P-O2 drop in fish kill mechanism by C. marina. Blood P-O2 immediately declined in response to a fall in water P-O2. Most of the measured variables (blood pH, hematocrit, plasma ions, heart rate, blood pressure, ventilatory frequency) did not change until blood P-O2 was decreased to 30-40 mmHg. In contrast, ventilatory pressure was increased at a blood P-O2 of 80 mmHg. Responses in most variables to fatal hypoxia were indistinguishable from those observed when fish were killed by exposure to C. marina. However, plasma chloride concentration was increased to higher levels in fish killed by C. marina than in fish subjected to hypoxia at the lowest brood P-O2 levels, implying dysfunctioning of chloride cells by C. marina. Increases in hematocrit and ventilatory pressure in response to lowered blood P-O2 were attenuated in fish killed by C. marina. Overall, we suspect that the fall in blood P-O2 played a crucial role in fish kill mechanism by C. marina, at least under our experimental conditions, and most other physiological disorders were secondarily brought about by the P-O2 drop.