Fish are considered to be good indicator organisms to show the negative effects of migration barriers. One aim of the EFI+ project was to develop a specific metric based on historical reference data. For altogether 16 diadromous species data on the historical distribution in European rivers were compiled. While most attempts undertaken up to now focused on the former presence of species on a catchment scale our main objective was to identify particular locations of past occurrence on the level of river segments. Data sources employed stem mainly from the 18th and 19th century. We used commercial fish registers, topographic and geographical literature as well as fish biological publications from this period. The number of data found for each of the selected diadromous fish species varied considerably. Apart from the differences in spatial distribution the importance for fisheries but also the possibilities of correct determination on species level were important factors. Although the EFI+ data set on the historical distribution of diadromous fish is not complete it is an important European-wide attempt to collect such information on the scale of river segments. Apart from information on past occurrence data on the present distribution of diadromous species were compiled. For the sampling sites used for the assessment method development in the EFI+ project a metric was calculated using the total number of diadromous species at present and historically. The metric shows a reaction in particular to the existence of downstream barriers on catchment level. In the future further metrics tests, e.g. on the reaction of single fish species, and improvements of the basic data, in particular in terms of compiling more data on historical distribution, are needed. A second goal of the work on historical diadromous species occurrence was a test of approaches to compensate missing information on the historical distribution. Several methods were tested to develop models using environmental conditions of sites where the historical presence is known. After, these models can be applied to rivers and river sections where historical information is missing. Data for the historical and present occurrence of diadromous fish species were provided by numerous people from the EFI+ group. In particular we would like to mention for Finland: T. Vehanen & A. Romakkaniemi, France: J. Belliard, Germany: Ch. Wolter, Hungary: B. Halasi-Kovacs, Italy: G. Maio & E. Marconato, Lithuania: T. Virbickas, Netherlands: T. Buijse & M. Beers, Poland: P. Debowski, Portugal: A. P. Avelar, A. P. Rodriguez & T. Ferreira, Romania: Kl. Battes & Ka. Battes, Sweden: U. Beier & B. Sers, Spain: D. Garcia Jalon & J. Gortazar, Switzerland: E. Schager, United Kingdom: R. Noble