## 1 Introduction  
Acoustic telemetry (defined here as the detection of fish or other aquatic fauna tagged with acoustic transmitters) is a useful tool for collecting detailed information on the distribution, movements, and habitat use of marine fish [1; 2; 3]. However, the principal advantage of this method--the ability to repeatedly locate and identify tagged individuals--is often diminished by adverse marine conditions (e.g., ocean stratification, ambient noise), limited reception range (typically less than a kilometer), and the inherent tradeoffs between transmitter size, transmitting power, and operational life [1; 4; 5]. Various approaches have been used to address these constraints, ranging from arrays of acoustic receivers [6; 7; 8] to mobile surveys using a variety of vessel-based methods [9; 10; 11; 12]. 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 more sophisticated and are typically designed to provide coverage based on the information needed and the physical features of the surrounding area, such as topography or current patterns [18; 19; 20]. In the case of fish telemetry, the reception range of the transmitters is also an important consideration in planning the mission parameters and the configuration of the route [15]. The use of AUVs to locate acoustically tagged fish has been enhanced by advances in payload control (PC) hardware and software, which allow the vehicle to alter its mission in response to information from an on-board sensor (i.e., reactive sampling), in this case telemetry data from an integrated acoustic receiver. This approach has been used to track the Lagrangian movements of a single acoustically tagged fish [14; 21]. However, repeatedly locating multiple fish over a vast area remains a major challenge. In previous papers, we have discussed the development of payload control software [22] and its utility for mapping the location of multiple targets (reference transmitters at known locations and depths) during a single mission [23]. Here, we examine the use of AUV-based payload control to synoptically map the movements of a number of highly mobile marine fish. Chinook salmon (_Oncorhynchus tshavytscha_) are a wide-ranging anadromous species that spend several years in the marine environment before returning to coastal rivers throughout the northern Pacific Rim to spawn. These returns support important commercial and subsistence fisheries. Chinook salmon returns were relatively stable until the late 1990s, when dramatic declines in abundance were reported [24].