No abstract

Intensification of aquaculture practices resulted in increased ammonia production in the culture systems. Lack of proper management of the culture system will result in elevated ammonia, which adversely affects the culture species. Moreover, with the ongoing global warming, fish metabolism and ammonia excretion are expected to increase. This, coupled with the hypoxic condition, exacerbates the severity of ammonia toxicity. Ammonia toxicity can alter the growth and the biochemical, physiological and immunological responses of fish. Several factors such as pH, temperature, salinity, dissolved oxygen, species and life stages of fish, physical activity, feeding and stress influence the toxicity of ammonia in fish. A proper understanding of these factors is a prerequisite for successful aquaculture. The physiological changes caused by the elevated ammonia hinder homeostasis that eventually increases their susceptibility to diseases. This review discusses ammonia toxicity in fish, factors affecting the toxicity and various physiological, haematological, histological and immunological responses induced by the elevated ammonia. Furthermore, the review summarizes various management and dietary interventions to mitigate ammonia and ammonia induced stress in aquaculture.

Large-scale fish mortality was observed in flood-affected fish farms across several parts of Kerala following heavy rainfall in August 2018 and 2019—nearly 53% above the normal monsoon rain that the region receives. The affected fish had severe haemorrhages and ulcers, typical of the highly infectious disease epizootic ulcerative syndrome (EUS) caused by the water mould Aphanomyces invadans. In freshwater, snakeheads Channa spp. and in brackish water mullet (Mugilidae) and pearl spot (Etroplus suratensis) were severely affected. EUS was observed in 4 freshwater fishes for the first time: dotted sawfin barb Pethia punctata (Cyprinidae), Malabar leaffish Pristolepis malabarica (Pristolepididae), mahecola barb Puntius mahecola (Cyprinidae) and giant snakehead Channa pseudomarulius (Channidae). Histology and molecular diagnosis confirmed the cause of mortality to be EUS. Fungal hyphae were also observed in deeply ulcerated fish, revealed by lactophenol cotton blue staining. The severity of the EUS outbreak was linked to the sudden change in water quality associated with the flood, such as lower water temperature, and decreases in pH, total alkalinity and total hardness.