Significant metal accumulation in fish in a metal-contaminated river without detectable effects on fish and...

Excess metal accumulation in organisms can result in adverse impacts at the levels of the individual, population, and community. A detectable increase of metal concentrations in organisms does not necessarily imply that there are such impacts, but to our knowledge, no field study has directly tested this hypothesis. To test this hypothesis, we investigated the accumulation of six elements (Cu, Zn, Cd, Pb, As, and Se) in masu salmon (Oncorhynchus masou, Salmonidae) at nine study sites in a metal-contaminated river receiving mine discharge and in a nearby reference river. Multiple fish community surveys in 2018 and 2019 consistently indicated that the abundance and condition factors of the four dominant fish species, including masu salmon, were comparable in the two rivers. In contrast, despite sample sizes of only five individuals per site, statistically significant increases in the concentrations of Cu, Cd, Pb, As, and Se in the muscle of masu salmon were observed at multiple sites in the metal-contaminated river, where no detectable impacts on the abundance or condition factor of the fish were observed. The muscle concentrations of Cu, Zn, Cd, Pb, As, and Se at these sites were 1.4-2.5, 1.5-1.9, 188-520, 4.6-68.0, 1.1-3.9, and 2.8-3.5 times, respectively, the mean concentrations at the reference sites, although the increase of the Zn concentration in muscle at these metal-contaminated river sites was not significant. These results provide clear empirical evidence that a detectable increase in metal accumulation does not necessarily imply that population and community-level impacts are simultaneously occurring in the environment.