As essential components of aquatic ecosystems, fish has been widely used in environmental monitoring, ecotoxicology studies, and risk assessments. Environmental stressors can either evoke adaptive response in fish or threaten the fish's health and well-being. Early life stages of fish are more susceptible to environmental stress and the impaired fish development at their early life stages may have significant impact on their population fitness with significant ecological consequences. Changes in fish behavior can serve as sensitive and important endpoints for toxicological study and risk assessment. In this talk, I will introduce fish in the ecosystem, early development of fish, fish ecotoxicology case studies, their potential implications in ecological outcomes, and some biomonitoring and risk assessment applications.