## 4 Discussion
In addition, mapping efforts would tend to be concentrated in areas where the fish were detected (to increase the probability of making contact) rather than conducting a more systematic search of the surrounding area. The serendipitous approach of using real-time information from the AUV substantially enhanced the vessel-based results and helped to verify the veracity of the AUV data. The advantage of using AUVs to collect telemetry data has been demonstrated in previous AUV-boat comparisons where similar survey routes were used for both methods [15]. Although less definitive in shallow waters (<20 m), detection rate and tracking success were significantly greater for the AUV for both reference tags (i.e., stationary transmitters at known locations and depth) and free-ranging animals located at deeper depths. The enhanced performance is likely due to a number of factors. The AUV is quieter when running than a surface vessel, can operate below the thermocline and halocline (and avoid the sound channeling associated with these conditions), and is less affected by signal degradation caused by sea surface noise. Although a submerged hydrophone can be towed from a boat, it is susceptible to becoming entangled in marine vegetation and debris, and signal strength is reduced due to line loss. The position of the hydrophone (a factor used to estimate transmitter locations) is also less certain depending on the length of the cable, the velocity of the current, and the speed of the vessel. Further, an AUV eliminates the human bias caused by the tendency to \"look where the light is better\", in other words a reluctance to leave an area where fish have been detected and search other areas that may (or may not) produce useful information. These results suggest that AUVs can provide a viable method for mapping the spatial and temporal distribution of acoustically tagged marine fish. A basic assumption in any tagging study is that the handling methods used do not adversely affect the fish (i.e., tagged fish behave the same as untagged fish) or that any effect is limited in severity and duration and ultimately has negligible effect [53; 54; 42]. During this study, the behavior of the fish prior to release, as well as the necropsy results for the fish tagged and sacrificed, suggest that the fish were not adversely affected by the handling methods. The initial movements of the fish, dispersing from the release site relatively soon after release, also suggest that the fish were not exhibiting latent effects from the tagging. The residency time exhibited during the study was limited, with all of the fish leaving the area within 48 h. Although one fish (Fish 12019, M20) displayed directed movements (leaving the area -2 h after release), most exhibited a series of localized movements within the area prior to departure. The average movement rate observed (0.62 BLs\({}^{-1}\)) was well within the range of what would be considered reasonable for juvenile Chinook salmon. Movement rates reported for juvenile Chinook salmon in the Salish Sea (Washington, USA) averaged 0.86 BLs\({}^{-1}\) with a maximum individual rate of 2.88 BLs\({}^{-1}\)[55]. Ocean movements of juvenile Chinook salmon in coastal waters of southern British Columbia (Canada) averaged 0.33 BLs\({}^{-1}\)[56]. [PERSON] estimated that the optimal swimming speed (i.e., minimum cost per distance traveled) for sockey salmon (_O. nerka_) was approximately one body length per second [57], and this metric is often applied to other salmonids. The dispersal rate for 93% of the fish during this study (\(\leq\)0.25 ms\({}^{-1}\), 1.1 BLs\({}^{-1}\)) was typically \(\leq\) the optimal swimming speed, likely due to the milling behavior exhibited by the fish. Although not egregious, the maximum movement rate observed (6.54 BLs\({}^{-1}\)) was unrealistic and more closely approximates burst swimming speeds exhibited by adult salmonids [57; 58].