Histopathology-based health assessment of two tilapia species in the Shongweni Dam, and metal accumulation, and human health implications.

Abstract

The Shongweni Dam serves as a repository for pollutants from the upper uMlazi River catchment in KwaZulu-Natal. The uMlazi River catchment is characterized by wastewater treatment plant, agricultural, and industrial activities. Despite the aforementioned anthropogenic activities in the catchment, little is known about their impact on the aquatic biota in the Shongweni Dam and human depending on these biotas for their livelihood. Therefore, this study aims to assess the health of the commonly occurring fish species, Coptodon rendalli and Oreochromis mossambicus in relation to water quality, metal accumulation and human health risks associated with the consumption of these two species. Sampling was conducted during dry (July – August) and wet seasons (November and December) in 2021. Sampling involved collecting surface water, sediment, and fish samples. The water was analysed for physical properties, nutrients and metal concentrations. Metals were also analysed in sediment to get a comprehensive understanding on their behaviour between the two matrices. Fish were sampled and each individual was weighed and total lengths were measured. Fish were euthanized by severing the spinal cord and opened ventrally. The length and weight measurements were used to calculate the condition factor. Liver and gonads were also weighed and their weights were used to calculate hepatosomatic and gonadosomatic indices, respectively. The general health of fish was assessed using the Health Assessment Index (HAI) protocol. The liver, gill and gonads were cut out, and fixed in 10% buffered neutral formalin for histopathological examinations. A piece of muscle was wrapped with aluminium foil and frozen for metal analyses. Most physical variables were within the guidelines whereas some nutrients; nitrates (NO3-) and orthophosphates (PO43-), exceeded the guidelines indicating a eutrophic condition. Metal concentrations such as aluminum (Al), iron (Fe) and lead (Pb) exceeded guidelines whereas all metals exceeded the guidelines in sediment. Moreover, most metals in the water were higher during dry seasons whereas a contrasting trend was observed for sediment. Biometric indices and health assessment index showed no significant difference between species (p > 0.05) and seasonal variation was observed for each species (p > 0.05). Histopathology revealed regressive changes and circulatory disturbances in gills and livers were prominent and no seasonal variation was observed for both species (p > 0.05). However, gonadal lesions varied between species (p < 0.05), with pigmented melanomacrophages being the most prevalent. Among organs, gills were most affected, followed by the liver. Muscle exhibited significant concentration of chromium (Cr) and Pb exceeding the permissible limit for safe consumption for both species. Moreover, the non-carcinogenic health risks showed the Target Hazard Quotient (THQ) > 1 for Pb and Cr, with arsenic (As) and antimony (Sb) being on the verge of reaching THQ of 1. Cancer risk (CR) values, exceeding the 10-4 to 10-6 range for carcinogens As, Cr, Pb, and cadmium (Cd) indicated unsafe levels for human consumption regardless of season. It is evident that anthropogenic activities in the uMlazi River catchment are impacting the water quality and fish are also responding to the water quality deterioration. Moreover, consumption of fish from this dam should be exercised with caution as frequent consumption could result in human health risks.