This paper presents the technical design and validation of an autonomous surface vehicle (ASV) system capable of robotic search, localization and autonomous tracking of freeranging fish carrying acoustic transmitter tags. The system consists of three identical differential-thrust catamaran vehicles equipped with time-synchronized acoustic telemetry receivers capable of accurate timestamping and decoding of signals received from individually coded tags. Multi-vehicle signal receptions allow time-difference of arrival (TDoA) measurements and are used in position estimators to enable collaborative fine-scale tracking and mapping of the movements of individual fish.The performance of the system was validated through several sea trials culminating in a deployment over several days in a Norwegian fjord where migrating wild Atlantic salmon smolts and sea-run brown trout had previously been tagged using acoustic transmitters in a nearby river. The three vehicles were able to operate autonomously over the entire deployment, carrying out coordinated searches along shorelines, keeping stations at hot-spot locations, and undertaking formation control while pinpointing fish positions. For the total of 5112 tag detections made during the deployment, 17 unique tag identifiers (fish) were registered at least twice, contributing significantly to the ongoing study of migrating salmonids in the area.