A Comprehensive Review on Cadmium Toxicity in Freshwater Fish: Physiological, Ecological and Health Implications

Cadmium (Cd), a highly poisonous and non-essential heavy metal, is a severe danger to freshwater ecosystems because of its persistence and bioaccumulation. Cd enters aquatic systems by industrial effluents, agricultural discharge, and urban runoff, accumulating in fish organs such as the gills, kidneys, liver and gonads. Its toxicological effects manifest as physiological, behavioural, biochemical, haematological, histopathological, and genotoxic changes. Behavioural signs such as irregular swimming, increased mucus production, and respiratory distress are prevalent. Necrosis, cellular degeneration and vacuolization are among the histological alterations observed in major organs. Furthermore, cadmium exposure causes chromosomal abnormalities and DNA damage, showing a high genotoxic potential. This review underlines cadmium contamination's ecological and toxicological significance, as well as the need for tougher environmental controls and additional study into species-specific effects and long-term effects on freshwater fish fitness and biodiversity. In aquatic species, behavioural alterations are a sensitive signal of chemically induced stress. At similar waterborne Cd concentrations, it has been reported that Cd can change dominance behaviours in young rainbow trout. Signs of stress, such as irregular fin movement, hyperactivity, trouble breathing, and surface attachment of fish followed by death, along with excessive mucous secretion on the body surface, were seen in the higher concentration during 72 and 96 hours of cadmium exposure. Changes in haematological indices, such as haemoglobin content, haematocrit, and the number of erythrocytes, can be used to track stress brought on by pollutants such as heavy metals since fish blood is susceptible to pressure from pollution. Moreover, histopathological changes due to cadmium have been studied in different freshwater fish, and the changes have been traced to various organs, gills, kidneys, liver, ovaries, testis and brain, etc. The conclusions of this analysis underscore the critical necessity for strict regulatory limits on industrial discharge and constant monitoring of aquatic ecosystems. Future research ought to investigate species-specific responses, long-term exposure consequences, and potential mitigation techniques such as bioremediation and governmental interventions. In recent years, experts have concentrated on the environmental concerns of microplastics, and microplastics promote cadmium transport in the food chain, which should be seriously regarded.