Fish Diseases in Wastewater Aquaculture and Remedial Measures

Abstract

Water quality and biological factors strongly affect the growth of fish in aquaculture ponds. Deterioration of water quality and adverse biological factors, regardless of the nature of aquaculture ponds, would cause poor ecosystem health and disease occurrence in the cultured fishes. Wastewater-fed aquaculture is a well-established climate-resilient practice that contributes substantially to inland fish production in India and elsewhere. Enhancement of fish production in such systems is, however, limited by suboptimal conditions of water quality and disease occurrences. Investigations in wastewater fish culture wetlands revealed various stressors that affect fish growth and production. These stressors are (1) suboptimal diurnal as well as seasonal water quality with DO level fluctuating from 0 to 18.0 mg/l, high CO2 (nil–16.0 mg/l), high unionized ammonia (0.11–0.42 mg/l) and low transparency (<14 cm) throughout the culture period and (2) biological stressors manifested by the abundance of urceolariid ciliates (Trichodina, Tripartiella spp.) in the hyper-mucus-secreting fish gills. The stress caused by the multiple stressors are physiologically manifested in the resident fish populations by significant changes in the levels of stress-sensitive blood parameters such as haematocrit, plasma cortisol, cholesterol, glucose, chloride and lactic acid levelss. Morphological alterations are exhibited in the form of hyperplasia, hypertrophy and oedema in the gills, proliferation of mucous cells and decrease in chromatophores in fish skin. These factors affect the growth of fish as reflected by the reduced condition factor. Stressed fish in such systems become prone to various infectious and noninfectious diseases such as fin and tail rot, dropsy, bacterial gill disease, saprolegniasis, trichodiniasis, myxosporean diseases, dactylogyrosis, argulosis, ergasilosis, hypoxia and algal toxicosis. Rapid assessment of the fish health needs to be conducted using the health assessment index (HAI) method and necessary remedial measures be adopted.