1. Caused by natural conditions
(1) Changes in water temperature: fish are cold-blooded animals. If the water temperature rises and falls sharply or the temperature is too high, it will cause fish body discomfort, which will lead to various diseases, and serious fish fry will die in large numbers.
(2) Changes in water quality: fish have an adaptation range to the pH of water, and 7-8.5 is suitable. If the pH is lower than 4.5 or higher than 9.5, it can cause fish disease and even death.
(3) the change of dissolved oxygen: the dissolved oxygen in the water is too low, the fish will die because of the floating head, the pan pool oxygen; If the dissolved oxygen in water is too high, the young fish will suffer from bubble disease. Dissolved oxygen in normal aquaculture water should not be less than 3mg/L.
2. Caused by human operation factors
Improper stocking density or unreasonable proportion of mixed breeding; inadequate, uneven or excessive bait casting; improper operation in the process of hauling net, all of these will influence fish health a lot.