Historical changes in the functional connectivity of rivers based on spatial network analysis and the past occurrences of diadromous species in Portugal

The disruption of longitudinal connectivity caused by artificial barriers in rivers is considered to be a major threat to freshwater fish communities at the global scale. In Portugal, the construction of several big dams all through the 20th century was a major cause of disappearance or decline of long distance migratory fish species from various rivers. In this study we analysed the historical changes of functional connectivity for diadromous fish at the Tagus basin river network (Portugal), using a spatial graph approach that integrates both structural connectivity and habitat suitability. Due to the presence of many artificial barriers, the current distribution of species does not reflect natural environmental conditions. Therefore, historical data on the occurrence of species is paramount to assess the potential habitat suitability. The compilation of historical data on the distribution of diadromous species [Sea Lamprey-Petromyzon marinus (Linnaeus 1758) and Allis shad-Alosa alosa (Linnaeus 1758)] was based on geographical dictionaries and geo-referenced information for Portugal dated between 1700 and 1900. Habitat suitability was expressed as the probabilities of occurrence of empirical predictive models based on the historical records and a set of regionalized environmental data. The loss of connectivity caused by big dam construction between 1928 and 2004 was then analysed using a spatial graph approach. The output of this analysis will aid decision-makers to optimize targets of connectivity restoration actions aiming at enhancing the functional connectivity of the river network for diadromous species, using the historical situation as a benchmark.