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 responses in fish or threaten the fish's health and well-being. Early life stages of fish are more susceptible to environmental stress, and impaired development at their early life stages may have significant impact on their population fitness with significant ecological consequences. Fish behavioral changes can serve as sensitive and important endpoints for toxicological study and risk assessment. In this talk, I will introduce the role of fish in the ecosystem, the early development of fish, case studies on fish ecotoxicology, their potential ecological implications, as well as some of their current biomonitoring and risk assessment applications.