Determination of LD50 of some essential oils and histopathological changes in short-term exposure to one of them in rainbow trout ( Oncorhynchus mykiss )

In this study, the toxicity of thymol, the essential oil (EO) of thyme plant, cumin seeds, and caraway seeds on rainbow trout ( Oncorhynchus mykiss ) was investigated and compared through a static method. The acute and short-term exposure study were conducted at a pH of 7.4 and a temperature of 15°C. In Acute toxicity test, concentrations of agents that killed 50% of rainbow trout (LC50) within 96-h for EOs of thyme, thymol, cumin, and caraway were 6.6, 2.6, 35, and 14 mg L −1 , respectively. Changes in fish behavior were restless, aimless swimming, and imbalances that were the same for all agents in acute toxicity test. In short-term study, histopathological changes consisted of hyperemia and edema in most organs. But most of the changes were observed in gill and brain of fish that included cellular hyperplasia and fusion of lamellae in the gills and necrosis and inflammation in the brain in addition to hyperemia and edema. The results showed that EOs used in this study were likely to exert their effects through competition with oxygen insolubility in the water and, for this reason, most of the changes were seen in the gills and brain that are more sensitive to the amounts of oxygen. This study concluded that the acute toxicity of these EOs is significant and the use of these substances in the aquatic industry should be cautious.