A new method is proposed to build an autonomous water pollution monitoring map by fish robots using ubiquitous sensor networks with sonar localization. Autonomous fish robots are introduced instead of fixed data logging stations to track pollutant sources as well as monitoring. Autonomous tracking is one of important functions in mobile underwater vehicles which monitor water pollution indices. When fish robots find obstacles on its path, proper direction changes to avoid collision are necessary. Otherwise, they should follow the given tracks as close as possible to obtain a pollution map. For efficient tracking performance and procedures of surveying water areas, fish robots use GPS and a sonar system to find exact localization. Although GPS is a fundamental tool to obtain positional information for large areas, it is not the best choice in accurate applications due to intolerable errors. In this paper, we propose to employ USN motes with sonar system to transmit sonar signals to calculate precise positional information in a given area. Our experimental results in a lake show that fish robots obtain more detailed positional information and make better tracking performance in the real situation.