Accumulation of methylmercury in the farmed and wild freshwater fish from the Pearl River Delta Region

Mercury (especially methylmercury) is easily accumulated by fish, thus posing a great threat to human health. Fish consumption is the main pathway for human exposure to MeHg, thus the MeHg accumulation in fish is critical for food safety. In the Pearl River Delta, bosh fish farming and wild fishing are well developed, leading to be the highest fishery production in China. MeHg in fish is mainly derived from food consumption. Compared with wild fish, farmed fish have more stable food sources and more simple food composition, resulting in a much higher rate of growth. However, the MeHg accumulation in fish from Pearl River Delta has not been thoroughly understood, and the effects of fish food (composition and MeHg concentration) on MeHg levels in fish have not been reported. This study investigated the bioaccumulation of MeHg in farmed and wild freshwater fish in Pearl River Delta and evaluated the MeHg exposure risk by fish consumption, aiming to figure out the key factors determining MeHg levels in fish. The farmed and wild fish (with fish food for farmed fish) were collected from 15 sites located in the Pearl River Delta and three tissues (muscle, liver and intestine) were sampled. The fish tissues and fish food samples were digested by 25% KOH/methanol solution at 80^oC for 4 h, and MeHg in the digested solution was ethylated by 1% NaBEt₄ solution and then detected by Gas Chromatography-Cold Vapor Atomic Fluorescence Spectrometry (GC-CVAFS). ① MeHg concentrations in fish tissues in this area were generally low and did not exceed the limit in the Food Safety Standard of China (GB 2762—2017) (Carnivore fish: 1 000 ng/g dw, others: 500 ng/g dw). Particularly, MeHg concentrations in muscle tissues of farmed fish (mean value: 61.53 ng/g dw, ranged from 0.26 to 331.27 ng/g dw) were significantly lower (P<0.05) than those of wild fish (mean value: 110.77 ng/g dw, ranged from 0.71 to 1 006.05 ng/g dw). ② Significant positive correlations were observed between MeHg concentrations in fish food and MeHg levels of farmed fish muscle (R²=0.39, P<0.05), and between trophic levels of wild fish and MeHg levels of wild fish muscle (R²=0.23, P<0.05). This suggested that MeHg accumulation in fish was mainly affected by food composition and the MeHg level in fish food. ③ Risk assessment showed that the MeHg exposure to human through fish consumption was low in this region. However, it should be noted that the MeHg intake through fish consumption was higher in wild fish than that of farmed fish. Fish is generally safe for human consumption in Pearl River Delta. However, MeHg levels in wild fish were higher than farmed fish, leading to a potential risk of MeHg exposure by consuming some specific species of wild carnivorous fish (e.g. Siniperca chuatsi). For farmed fish, the MeHg concentration in fish food was the key factor determining MeHg level in fish. Thus, feeding fish with clean fish food can be an effective practice to diminish MeHg contamination in farmed fish.