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

This article investigates the relationship between metal accumulation in masu salmon (Oncorhynchus masou) and potential ecological impacts in a metal-contaminated river in northern Hokkaido, Japan, affected by legacy mine discharge. Despite significantly elevated concentrations of metals such as copper (Cu), cadmium (Cd), lead (Pb), arsenic (As), and selenium (Se) in salmon muscle tissue—up to hundreds of times higher for some metals compared to a nearby reference river—no detectable adverse effects were observed on fish abundance, condition factors, or macroinvertebrate community richness and abundance. The study provides empirical evidence that increased metal accumulation in fish does not necessarily correspond with population- or community-level ecological impacts, highlighting the importance of considering biological and ecological context when assessing environmental risks of metal contamination. These findings contribute valuable field-based data for ecological risk assessment and management of metal pollution in freshwater ecosystems.