Background: Polycyclic aromatic hydrocarbons (PAHs) such as naphthalene (NAP) are organic pollutants that have spread widely in littoral marine ecosystems. We aimed to study the effect of acute and prolonged exposure to naphthalene (NAP) on plasma cortisol and thyroid levels in mature and immature Liza klunzingeri mulet.

Methods: In acute stress experiment, the treatment group received 2μl g^-1 sunflower oil containing NAP (50 mg kg^-1) intraperitoneally and the controls were injected with sunflower oil alone (2μl g^-1). Blood samples were obtained from both groups after 3 h. In prolonged stress experiment, 10 μl g^-1 of coconut oil containing NAP (50mg kg^-1) was implanted and blood samples were obtained 72 h after injection.

Results: Both the acute and prolonged exposure induced significant increase in cortisol and a significant decrease (P<0.05) in thyroid hormone T4 levels. Thyroid T3 hormone levels only decreased significantly (P<0.05) after prolonged exposure. Acute exposure resulted in significant decrease (P<0.05) in T3/T4 ratio only in immature fish. On the other hand, prolonged stress increased T3/T4 ratio in immature fish.

Conclusion: Changes in the plasma levels of these hormones indicate low physiological capacity and survival potential of fish in waters polluted with naphthalene.