Confinement during field studies may jeopardize antioxidant and physiological responses of Nile tilapia to contaminants

• Confinement can be a confounding variable in field studies with Nile tilapia.
• Caging leads to a decrease in antioxidant defenses and blood glucose and lactate.
• Exposition to a contaminated site produced marginal responses.
• Caging effect needs consideration in biomarker responses to contaminants.

This work evaluates the effects of caging, a known confinement stress, in Nile tilapia (Oreochromis niloticus) during an environmental study in Cubatão river, southern Brazil. Caging animals for 7 days, regardless of being at the reference or at a contaminated site, resulted in lower levels of antioxidant-related defenses (glutathione, glutathione S-transferase, glucose-6-phosphate dehydrogenase) in liver and physiological parameters (blood glucose and lactate) as compared with free-swimming animals. Higher hepatic glutathione reductase activity and elevated Hb content could be associated to contaminant exposure. In conclusion, the confinement stress in caged Nile tilapia biochemical and physiological disturbances, acting as a confounding factor in field studies.