One of the critical issues in species invasion ecology is the need to understand and evaluate the dimensions and processes of aquatic organisms transfer with vessels ballast water. The assessment of the quantity of ballast water discharged as the medium of transfer is one of the basic elements of the decision making process in ballast water risk assessment and management. The possibility to assess this in advance of the vessel's arrival to a port enhances the management process and gives port authorities a decision supporting tool to respond in time with adequate measures. A new generic ballast water discharge assessment model has been prepared. The model is based on vessel cargo operation and vessel dimensions. The model was tested on real shipping traffic and ballast water discharge data for the Port of Koper, Slovenia. The results show high confidence in predicting whether a vessel will discharge ballast water, as well in assessing the quantity of ballast water (to be) discharged. Highlights? A model to predict ballast water discharge was developed. ? The model was applied to the Port of Koper and verified on reported data from vessels. ? The model assesses vessel specific ballast water discharges. ? Ballast water discharge information is crucial for risk assessment and management. ? The model is a decision support tool for a more effective management in shipping.