Health assessment of tilapia (Oreochromis spp.) aquaculture systems in the northern provinces of South Africa

Abstract

This study evaluated eighteen commercial farms within Gauteng, Northwest and Limpopo provinces, where a representative sample of grow-out fish from each farm, was humanely euthanased, weighed, measured, and each fish’s overall health assessed through microscopic examination of skin and gills. A full necropsy and histo-pathological evaluation of all key organs followed. Farm production parameters were assessed by means of a questionnaire with a detailed history and a comprehensive water analysis that included water temperature, dissolved oxygen, CO2, ammonia, nitrite, nitrate, pH, hardness and alkalinity. These production parameters, together with stocking density and underlying nutrition, were compared with the macro- and microscopic findings for positive and negative correlations / relationships. Significantly high burdens of ecto-parasites and very poor water quality, with compromisingly low dissolved oxygen and temperatures, and high carbon dioxide and nitrite, were found in association with severe gill pathology. This was compounded by inadequate filtration for the density of fish stocked, with resultant toxic nitrogenous waste accumulation. Other key abnormalities observed were chronic-active hepatic lipid oxidation, low hepatocellular lipid, evidence of secondary opportunistic infectious disease, and extremely poor growth. Poor farm management practises were prevalent, with evidence of uneconomical fish sex ratios, and poor implementation of biosecurity and disease management. The study serves to highlight the factors that currently dominate as critical issues affecting overall health and growth of aqua-cultured tilapia in the South African context.