Environmental context. Inorganic arsenic, a well-known human carcinogen, represents a major worldwide environmental problem because contaminated water supplies have lead to widespread human exposure. This study investigates the arsenic content of freshwater fish from arsenic-contaminated and non-contaminated sites in Thailand, and reports high arsenic concentrations and significant amounts of inorganic arsenic in the edible muscle tissue. The data suggest that freshwater fish may represent a significant source of inorganic arsenic to some human populations.

Abstract. Striped snakehead (Channa striata), carnivorous freshwater fish that serve as popular food in Thailand, were collected from a reference site (1.4 µg As L^–1) and from two arsenic-contaminated ponds (Pond A, 550 µg As L^–1; Pond B, 990 µg As L^–1) in southern Thailand and analysed for arsenic by inductively coupled plasma mass spectrometry (ICPMS) and for arsenic species by HPLC-ICPMS performed on aqueous methanol extracts of muscle, liver and gill (n = 3 fish from each site). Mean total arsenic concentration in muscle tissue of C. striata collected from the reference site was 1.9 µg As g^–1 (dry mass) while fish from the contaminated sites contained 13.1 µg As g^–1 (Pond A) and 22.2 µg As g^–1 (Pond B). Liver and gill tissues showed similar increasing arsenic concentrations on going from the reference site to Ponds A and B, with Pond B showing the highest levels. Speciation analysis on the three tissues showed that, although arsenate was the major extractable arsenical in reference fish (e.g. 0.73 µg As g^–1 in muscle tissue), dimethylarsinate was by far the dominant arsenic species in fish from the two contaminated sites. Three non-carnivorous fish species (Danio regina, Rasbora heteromorpha and Puntius orphoides), collected from Pond B only, had lower arsenic concentrations (7.9–11.3 µg As g^–1 in muscle tissue) than did C. striata, and contained appreciable amounts of trimethylarsine oxide together with two other major arsenicals, arsenate and dimethylarsinate, and smaller quantities of arsenite and methylarsonate. The study shows for the first time a clear effect of water arsenic concentrations on natural fish tissue arsenic concentrations, and is the first report of a freshwater fish species attaining arsenic concentrations comparable with those found in marine fish species. Furthermore, the high concentrations of toxic inorganic arsenic (predominantly arsenate) in the muscle tissue of the edible fish C. striata have human health implications and warrant wider investigations.