## 4 Discussion  
A variety of factors complicate efforts to document the movements of acoustically tagged fish in the marine environment, ranging from the technical limitations associated with acoustic telemetry to the logistical challenges often encountered particularly when working under remote or adverse conditions. These factors can severely limit the ability to repeatedly locate and identify the tagged individuals. The inherent difficulties associated with tracking multiple fish or highly mobile species over vast areas can exacerbate these problems. Advances in AUV technology can enhance efforts to collect acoustic telemetry data on marine fish. In addition to functioning autonomously, making it possible for researchers to focus on other tasks, AUVs equipped with payload control have the potential to substantially improve the quality and expand the type of information obtained. Maneuvers conducted in response to detection events can enhance the accuracy of location estimates, and the ability of the AUV to move closer to the fish to collect proximal information on habitat use and the associated environmental conditions has obvious advantages. AUVs can also work in cooperative pairs [46], and eventually swarms, to enhance the information collected [47,48]. During this study, the reactive sampling conducted by the AUV under payload control was effective at repeatedly detecting and estimating the location of the tagged fish. The vehicle was able to maintain contact even when multiple individuals were present, with most fish located repeatedly over the course of a mission. Although the actual locations of the fish were unknown, the general agreement between the paired (i.e., temporally similar) AUV and vessel-based estimates suggests that the information provided by the AUV was credible. Further developments in software routines and algorithms (e.g., maneuvers that further enhance the location estimates, refined methods for processing the telemetry data collected when multiple fish are present) will likely lead to further improvements in performance. Additional information and insights on the movement patterns of marine fish may also be useful to enhance the pre-programmed routes used to make initial contact with wide-ranging or highly mobile species based on a series of informed search models [49]. For example, large fast  
Figure 7: Histogram of the distribution of dispersal rate plus error (\(\gamma\)) of acoustically tagged juvenile Chinook salmon in marine waters of Port Walter and Big Port Walter, Alaska. moving fish such as adult salmon or sturgeon (_Acipenser_ spp.) may swim a greater distance (>10 m) during the time needed to create a synthetic aperture compared to more sedentary species such as crab (_Cancer_ spp.) or flounder (_Paralichthys_ spp.). As a result, the accuracy of the location estimates may be substantially less, although the estimates may still provide useful information depending on the research objectives of the study and the spatial resolution needed. Knowing something about the behavior and movements of the fish would make it possible to tailor the parameters of the search routes and maneuvers to reflect the desired scale and enhance survey results [50; 51; 52]. Differences were observed between the two survey methods used during the study. The AUV was much more effective at repeatedly locating the fish, whereas we had considerable difficulty detecting and maintaining contact during the vessel-based surveys, particularly when greater numbers of fish were present within the area. Our initial approach in response to this problem was to continue to track the individuals detected from the boat, ranging out after contact was lost in search of other fish. This approach is biased toward more Lagrangian movements, which focus on more extended observations of an individual over space and time rather than a comprehensive inventory of fish present within the area.