Over the past years different activities related on marine ecosystems monitoring techniques have been carried out at OBSEA observatory. The OBSEA is a cabled observatory placed at 4 km from the coast of Vilanova i la Geltrú, Barcelona (Spain), and at 20 m depth, which has been in operation for more than 10 years. The special characteristics of the OBSEA platform (e.g. unlimited power supply, high bandwidth communication, and easy access) offers an extraordinary opportunity to develop and test different acoustic monitoring techniques. In this framework, many methods have been deployed and tested on target monitoring techniques, which goes from hydrophones surveillance, to target tracking using acoustic range-only methods by the use of autonomous underwater vehicles.