## 3 Results
### Survey Missions  
Seven survey missions were conducted from 19 May to 26 May 2016 to track the movements of the juvenile salmon, including six missions (M20-M25) in Big Port Walter and one mission (M26) in Port Walter (Table 1). A total of 19 tagged fish were released, ranging from two to four individuals per mission. The fish began to disperse from the immediate release site within 10-15 min. Several fish remained in the study area over several days and were present during subsequent missions, increasing the total number of potential targets during the study to 31 fish (Table 1). The number of fish present (including those released during a previous mission) ranged from two to nine individuals per mission. All of the fish ultimately left the study area. Of the 31 potential targets present during the study, 28 (90%) were detected by the AUV (Table 1). A comparable number of fish (29, 94%) were detected during the vessel-based surveys. In most cases, the same fish were detected by both methods (Table 2). However, in interpreting these data it is important to understand that the AUV and vessel-based surveys were spatially and temporally disjunct, and that the similar detection rates belie considerable differences in the number and pattern of detections per fish. We had considerable difficulty detecting, locating, and maintaining contact with the fish during the vessel-based surveys, particularly when greater numbers were present within the area. Due to these difficulties, we initially attempted to locate and track the movements of a particular fish,but would often lose contact as it moved through the area. To address this problem, we serendipitously used real-time output from the AUV to facilitate the vessel-based surveys. We were able to monitor the movements of the AUV on a laptop computer and could see when it was performing a maneuver. By moving the boat to that location, we were often able to detect the fish and confirm what the AUV was targeting. The number of fish detected and the number of location estimates per fish would have been substantially less if we had relied solely on the vessel-based tracking. There were no false positives (i.e., detections for a transmitter not released) in the 6760 detections recorded by the AUV. A total of 107 best location estimates were obtained during the study. The movements of the fish based on these estimates are illustrated in Figure 4. Five fish were located during Mission 24 (left \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & & & & & **Fish Detected** \\
**Mission** & **Date** & **Location** & **Legs** & **Fish Present 1** & **AUV** & **Boat** \\ \hline M20 & 19 May & BPW & 8 & 2 (2, 0) & 2 & 2 \\ M21 & 21 May & BPW & 4 & 3 (3, 0) & 2 & 3 \\ M22 & 24 May & BPW & 6 & 4 (3, 1) & 4 & 4 \\ M23 & 24 May & BPW & 6 & 5 (2, 3) & 4 & 5 \\ M24 & 25 May & BPW & 5 & 5 (3, 2) & 5 & 4 \\ M25 & 25 May & BPW & 3 & 9 (4, 5) & 8 & 8 \\ M26 & 26 May & PW & 8 & 3 (2, 1) & 3 & 3 \\ Total & & & & 31 (19, 12) & 28 & 29 \\ \hline \end{tabular} 1 Number of fish released during the current and previous missions, respectively, in parentheses.  
\end{table} 
Table 1: Numbers of acoustically tagged Chinook salmon located during survey missions conducted concurrently with an autonomous underwater vehicle and boat in marine waters of Big Port Walter (BPW) and Port Walter (PW), Alaska, during 2016. The total number of fish present and detected in the area (including those released as part of the mission and those remaining from previous releases) are indicated.