Featured Application: Using autonomous underwater vehicles equipped with telemetry-based payload control to locate marine fish tagged with acoustic transmitters.Abstract: An autonomous underwater vehicle (AUV) under payload control (PC) was used to map the movements of juvenile Chinook salmon (Oncorhynchus tshawytscha) tagged with acoustic transmitters. After detecting a tag, the AUV deviated from its pre-programmed route and performed a maneuver designed to enhance the location estimate of the fish and to move closer to collect proximal environmental data. Nineteen fish were released into marine waters of southeastern Alaska. Seven missions with concurrent AUV and vessel-based surveys were conducted with two to nine fish present in the area per mission. The AUV was able to repeatedly detect and estimate the location of the fish, even when multiple individuals were present. Although less effective at detecting the fish, location estimates from the vessel-based surveys helped verify the veracity of the AUV data. All of the fish left the area within 48 h of release. Most fish exhibited localized movements (milling behavior) before leaving the area. Dispersal rates calculated for the fish suggest that error associated with the location estimates was minimal. The average movement rate was 0.62 body length per second and was comparable to marine movement rates reported for other Chinook salmon stocks. These results suggest that AUV-based payload control can provide an effective method for mapping the movements of marine fish.Autonomous underwater vehicles (AUVs) are increasingly being used to supplement the use of surface vessels for measuring the physical, chemical, and biological characteristics of the marine environment, and when equipped with acoustic receivers can be used to determine the distribution and movements of fish tagged with acoustic transmitters [13,14]. This approach has definite advantages over tracking fish from surface vessels, particularly for demersal or deep-water pelagic species [15]. Proximal data obtained from AUV-based sensors and sonar also make it possible to compare the spatial and temporal distribution of the fish with the associated environmental conditions, benthic characteristics, and the presence of other fish species and untagged con-specifics.Despite these advantages, AUVs have their own constraints. Unlike a piloted vessel that can easily deviate from its existing course to enhance location estimates, AUVs mapping the movements of acoustically tagged fish were initially programmed to simply follow a predetermined route and record the transmitters detected. Position estimates were limited to kernel density or sound-pressure-level (SPL) estimates of proximity, which were often bimodal [16,17]. Positioning errors frequently resulted from the presence of tagged fish on the periphery of the pre-programmed route and the unfavorable geometry of the hyperbolic curves used to estimate fish locations [15]. Survey routes for unmanned vehicles (both surface and submerged) have become much mor...