Levels of Total Mercury in Farmed and Wild-caught Oreochromis Niloticus Niloticus (Nile Tilapia), Pond Sediments and Water in the Migori Gold Mining Belt, Kenya

Abstract

Mercury is a well-known toxicant with a myriad of ill effects on human health. It occurs naturally in the environment at basal levels. Human activities, such as mercury use in artisanal and small scale gold mining is one of the major sources of environmental mercury pollution. Migori is renowned for artisanal gold mining. Inland fish farming is also practised in this area with Nile tilapia (Oreochromis niloticus niloticus) is the main fish reared. Studies have shown that up to 90% of the mercury used during gold panning in Migori is lost to the environment. Consequently, the mine tailings, soils and waters in these areas are heavily contaminated with mercury. Mercury is washed off to the nearby streams and rivers by run-water, thus extending the pollution farther to the water system. Inland fish farms and Lake Victoria draw their waters from these polluted streams and rivers with mercury being deposited in soil sediments, water and eventually taken up by planktons, insects and other lower organisms which form a major part of the diet for the tilapia fish. Mercury gets absorbed into the fish through feeds, skin and gills. In fish, the mercury is partitioned differently across various tissues depending on the partition coefficients of the tissues to the different mercury forms. Tilapia brain, liver and muscle tissues were selected for this study. Tilapia brain and liver have high-fat content hence are likely to concentrate high amounts of methyl- and other organic forms of mercury which are lipophilic. The liver is also the main organ for metabolism and elimination of the mercury from the fish. Tilapia fish muscle is the major part of consumed by man. Thus it is a tissue of interest in this study since its contamination poses a risk to human health. This study assessed the levels of total mercury (T-Hg) in fish pond sediments, water and tissues of farmed and wild-caught Nile tilapia (Oreochromis niloticus niloticus) in the Migori gold mining belt. The correlation between the mean T-Hg levels in the tilapia fish tissues and the mean T-Hg in pond water and sediments was evaluated. The potential risk to human health from the consumption of the fish was also

determined. Ten locations in Rongo and Nyatike sub-counties in Migori with known artisanal gold mining and inland fish farming activities were conveniently selected for the study. Five tilapia fish (irrespective of sex) were sampled from each site except Minyenya (where four fish were sampled). Two replicate samples of pond water and sediment were collected from each site except for the lake (soil and sediment not sampled). Each fish sample yielded one sample of brain, liver and muscle tissues. A 0.3 - 0.5g portions of the samples were homogenised and acid-digested to reduce all the mercury forms to mercury metal (this yield is referred to as total mercury (T-Hg) which was analysed using cold vapour atomic absorption spectroscopy and the mean T-Hg levels recorded in μg/g wet weight. All the data generated were organised, aggregated and mean measures established. Microsoft Excel (2016) and Statistical Package for the Social Sciences (SPSS, version 20.0) were used for statistical analysis. Data for mercury analysis was expressed as the mean± standard deviation. One-Way Analysis of Variance (ANOVA) was used to analyse the levels of T-Hg in fish tissues across the sites. Tukey's HSD test was used as a post-hoc test. Pearson's rank correlation and the t-test were used to determine whether there were any relationships between the various parameters in the study. Values of p≤0.05 were considered significant in all cases. Sediment quality was evaluated using a geo-accumulation index (IGEO) while the estimated daily intake of fish per meal (EDIm), target hazard quotient (THQ), and the maximum allowable fish consumption rate (CRmw) were used as human health risk indices. Concentrations of mean T-Hg in sediments ranged from 0.208±0.000 to 1.113±0.008 μg/g wet weight (n=8, 95% CI); with six of the eight sites sampled being moderately polluted (1≤IGeo˂2), whereas two sites (Minyenya and Kokaka) being strongly polluted (3≤IGeo˂4). Mean T-Hg in the water samples ranged from 0.002±0.000 to 0.004±0.001 μg/ml wet weight (n=8, 95% CI) with all the sites having higher values (up to 40 times higher)

for T-Hg than the maximum contaminant level of 0.0001 μg/ml allowable for mean T-Hg in unpolluted surface water set by the Food and Agriculture Organization (FAO). The concentrations of mean T-Hg were highest in the tilapia brain tissues, ranging from 0.128±0.021 to 3.798±1.421 μg/g wet weight (n= 49, 95% CI); with the highest proportion (78%, 38/49 samples) having mean T-Hg levels above (up to eight times higher) the limits of 0.5 μg/g wet weight recommended as safe by WHO for consumption by the general human population. The mean T-Hg in tilapia muscle tissues ranged from 0.179±0.020 to 0.595±0.065 μg/g wet weight (n= 49, 95% CI) with 31% (15/49) of fish muscle tissues tested having the levels above 0.5 μg/g wet weight. Mean T-Hg levels were lowest in tilapia liver tissues, ranging from 0.103±0.118 to 0.588±0.374 μg/g wet weight (n= 49, 95% CI) with only 27% (13/49) of fish liver tissues tested having the levels above 0.5 μg/g wet weight. However, most of the tilapia fish samples (87.8% (43/49) of brain, 69.4% (34/49) of liver and (68.7% 34/49) of muscle tissues respectively had mean T-Hg above the 0.2 μg/g (wet weight) level recommended by WHO for at-risk populations (frequent fish eaters, people with renal and liver diseases, pregnant mothers and developing children). There were positive correlations between mean T-Hg levels in tilapia brain and muscle tissues and the mean T-Hg levels in fish pond sediments (r=0.528, p<0.05 and r=0.524, p<0.05 respectively). However, there was no significant correlation between the mean T-Hg content in soil sediments and the mean T-Hg level in fish liver tissues. There was a positive correlation between mean T-Hg levels in tilapia brain tissues and mean T-Hg levels in pond water (r=0.402, p<0.05) as well as between mean T-Hg levels in tilapia muscle tissues and mean T-Hg levels in pond water (r=0.616, p<0.05). However, there was no significant correlation between the mean T-Hg content in pond water and the mean T-Hg level in fish liver tissues. The estimated daily intake of fish per meal (EDIm) and target hazard quotient (THQ) for human consumption ranged

from 2.43-15.84 μg/g and 24.3-158.4 μg/g respectively while the maximum allowable fish consumption rate for humans in meals/week (CRmw) ranged from 1-4 whole fish. These findings show that the levels of mean T-Hg in tissues of Nile Tilapia in the Migori gold mining belt are above-recommended limits. Consumption of Nile tilapia, therefore, bears a significant risk of mercury exposure in frequent fish-eaters, pregnant women and children of developmental age in the Migori gold mining belt, but is safe for the general human population.