Nowadays, the use of autonomous vehicles for ocean research has increased, since these vehicles have a better cost/performance ratio than crewed vessels or oceanographic ships. For example, autonomous surface vehicles can be used to localize underwater targets. Whereas different research works are focused in target tracking using acoustic modems (or USBL), in this paper a new method called Area-Only target tracking is presented, which uses the signal generated by acoustic TAGs. This document, the first tests are presented and their results discussed, which were conducted in the Monterey Bay.