A Fish-Oriented Heavy Metal Pollution Index for the Danube River

The Heavy Metal Pollution Index (HPI) usually includes the metals that have a regulatory standard rather than those most relevant to fish. Its usual weights also give most influence to the metals with the strictest standards. This study developed a fish-oriented HPI for the Danube River. A data-coverage screen and an expert evaluation with the PAPRIKA method selected eight metals: Cd, Pb, Hg, Cu, Ni, Zn, As and Cr. They were combined with equal weights so that each selected metal contributes to the index. The index was calculated for 6467 sampling events at 43 stations in eight countries (1998–2022), and values below detection limits were estimated by multiple imputation. A generalised additive mixed model showed that the index increased about fourfold from the upper to the lower Danube and tended to decline over time. The downstream increase persisted under most alternative methods. In a Serbian reach surveyed for fish in 2022, the index in 2020–2022 was in the upper third of the basin, mainly because of Ni and Cu, and the fish community was dominated by eurytopic and non-native species. Selecting metals by their relevance to fish, and weighting them so that this selection matters, offers a transferable approach for fish-relevant contamination indices in large rivers.