Investigating eco-hydrological relationships between fishes and flow regimes in Spanish rivers

Abstract: There is currently a need in Spain for monitoring the effects of the environmental flows. The use of eco-hydrological relationships to design both environmental flows regimes and the monitoring programs that track their effects on riverine ecosystems is highly recommended. Fishes are the biological element more sensitive to flow regime changes, thus the relationship between fishes and flow dynamics becomes especially relevant to design environmental flows and the required monitoring programs. This study presents an analysis of the relationships between flow regimes in 33 Spanish rivers, defined using a set of annual, monthly, seasonal, and daily hydrological metrics, including the Indicators of Hydrological Alteration, and fish assemblages, characterized through a combination of taxonomic community composition and ecological traits metrics. A selection of fish and flow metrics that were statistically relevant and non-redundant was conducted, through a Principal Component Analysis and correlation analysis. The potential biological implications of the fish-flow relationships that were found in the statistical analysis were discussed according to the findings in the literature. The results showed that larger rivers tended to have a greater fish richness and a lower fish abundance than smaller rivers. More specifically, rivers that have their annual maximum flows in April were related to a higher fish richness, showing the importance of the timing, and not only the magnitude, of the flow regime. Furthermore, this flow condition may enhance native species richness and control the spread of alien species in rivers downstream from dams. The number of flow reversals was especially suitable when studying fish-hydrological relationships at a national scale, and they were negatively correlated with fish abundance in rivers. In contrast, flow skewness was positively correlated with fish abundance in rivers, and they may promote fish recruitment. Other correlations among some hydrological metrics and fish metrics related to ecological traits were observed and discussed too.