This chapter provides a comprehensive examination of the ecotoxicological consequences of non-chemical toxic agents in aquatic ecosystems, with a particular focus on biological stressors. Non-chemical stressors pose significant threats to aquatic environments. The chapter begins with a broad classification of non-chemical stressors, encompassing physical, socio-economic, and biological factors. Special emphasis is placed on biological stressors such as pathogens, parasites, invasive species, predator–prey dynamics, and competition, detailing their mechanisms of impact and the resulting ecological repercussions. Through an exploration of how these stressors induce toxicological effects in aquatic organisms, the chapter highlights the interactions between biological stressors and other environmental factors. Management and mitigation strategies, including molecular and genetic tools, biological control methods, and conservation strategies, are discussed as crucial approaches to counteract the negative effects of biological stressors. The chapter concludes by underscoring the need for predictive modeling, integrative research, and interdisciplinary collaboration to effectively address and mitigate the challenges posed by biological stressors in aquatic ecosystems.