Using Acoustic Telemetry to Expand Sonar Escapement Indices of Chinook Salmon to In-river Abundance Estimates

Acoustic telemetry was used to determine what proportion of adult Chinook salmon traveled within the sampling range of a sonar project at the Nushagak River, Alaska. These proportions were used to expand sonar indices of Chinook salmon to full-river escapement estimates. At this site, the river bottom topography does not allow full-river coverage by the dual-frequency identification sonars (DIDSON) located on either side of the river. Of the 300-m river width, 50 m is ensonfied along right bank (RB) and 30 m along left bank (LB). Chinook salmon are known to travel beyond the sonars’ detection range. Acoustic tags were inserted into Chinook salmon across a 4-year period (2011-2014). As tagged fish were detected at the sonar site by an array of acoustic receivers, they were classified as to whether they traveled through the sonar beam along RB, LB, or offshore of the sonars’ range (Os). Fish that traveled near the beam edges created uncertainty. To handle this uncertainty, we drew three probability regions around each beam footprint. The first extended from shore to 5 m short of the DIDSON end range. Tagged fish that traveled through this region were assigned a p of 1 or -1 depending on their direction of movement and a bank assignment, LB, RB, or Os. A tagged fish traveling upriver through the first RB region was assigned [0 LB, 1 RB, 0 Os], a potential DIDSON count of 1. The second region extended from the end of the first region to 5 m offshore of the DIDSON end range for a p assignment of 0.5. The assignment for a fish traveling along LB through this region would be [0.5 LB, 0 RB, 0.5 Os], an equal chance of passing inside or outside the beam footprint. The third region (p=0.25) extended from the end of the second region to 5 m farther offshore. Some fish traveled through both DIDSON beams during a single upriver trip and would have been counted as 2 fish, so their p value was 2. Many tagged fish made a single, upriver trip through the array (ST fish), but several made multiple up and downriver trips, (MT) fish. Each trip was assessed in the same manner as the ST fish except that downriver trips yielded negative probability counts. If a fish traveled upriver along LB, downriver along RB, and back up mid-river, the p value would be 1, [1 LB, -1 RB, 1 OS]. A special case was presented by implied trips. Tagged fish first detected moving downriver were assumed to have moved upriver first but were either not observed or the track was rejected by the filters. Also, a fish that made two consecutive upriver trips had to have made an unobserved (implied) downriver trip. We assumed these fish had an equal probability of traveling through each region so implied upriver trips were assigned [0.33 LB, 0.33 RB, 0.33 Os] and implied downriver trips were assigned [-0.33 LB, -0.33 RB, -0.33 Os]. Multiple assignments for a given fish were summed to produce a single p value. Assignment tables and fish tracks are included for each study year.

本研究采用声学遥测（acoustic telemetry）技术，旨在确定阿拉斯加州努沙加克河某声呐项目采样范围内成年奇努克鲑鱼（Chinook salmon）的通行比例，并以此将声呐监测得到的奇努克鲑鱼种群指数拓展为全河道洄游种群估计量。
该河段河床地形限制了河道两侧布设的双频识别声呐（dual-frequency identification sonars, DIDSON）实现全河道覆盖。在300米宽的河道中，右岸（RB）沿线有50米范围处于声束照射区内，左岸（LB）沿线则为30米。已有研究表明，奇努克鲑鱼会超出声呐的探测范围活动。

研究团队在2011至2014年的4年间，为奇努克鲑鱼植入声学标记（acoustic tags）。当标记鱼被声呐站点的声学接收器阵列检测到时，研究人员会对其通行路径进行分类：沿右岸声束、沿左岸声束，或是处于声呐探测范围外的河道中央区域（Os）。在声束边缘附近通行的鱼群会引入观测不确定性。

为处理该不确定性，我们在每个声呐波束覆盖区周边划定了三个概率区域。第一区域从岸线延伸至双频识别声呐探测终点前5米处；标记鱼若通过该区域，将根据其运动方向与岸别（左岸、右岸或河道中央）被赋予概率值1或-1。例如，沿右岸第一区域上溯的标记鱼会被赋值为[0 LB, 1 RB, 0 Os]，对应潜在声呐计数为1。

第二区域从第一区域的终点延伸至双频识别声呐探测终点外5米处，该区域内的鱼群概率赋值为0.5。例如，沿左岸通过该区域的标记鱼会被赋值为[0.5 LB, 0 RB, 0.5 Os]，即其处于声束内或声束外的概率均等。

第三区域（概率赋值为0.25）则从第二区域的终点延伸至再向外5米的范围。部分标记鱼在单次上溯行程中会穿过两道声呐波束，此时会被计为2尾鱼，对应概率值为2。

多数标记鱼仅完成单次上溯行程（单行程鱼，ST fish），但也有部分鱼完成了多次往返上、下行程（多行程鱼，MT fish）。单行程鱼的评估方式适用于每一次行程，而下溯行程会产生负的概率计数。例如，某标记鱼沿左岸上行、沿右岸下行，再沿河道中央上行，其概率值为1，对应赋值为[1 LB, -1 RB, 1 Os]。

存在一类隐含行程的特殊案例：首次被检测到顺河而下的标记鱼，研究人员默认其此前曾完成上溯行程，但该行程未被观测到，或是其轨迹被过滤规则剔除。此外，连续完成两次上溯行程的鱼群，必然存在一次未被观测到的（隐含）下溯行程。我们假设这类鱼群在各区域通行的概率均等，因此隐含上溯行程会被赋值为[0.33 LB, 0.33 RB, 0.33 Os]，隐含下溯行程则被赋值为[-0.33 LB, -0.33 RB, -0.33 Os]。

单尾鱼的多次路径赋值会被求和，以得到单一概率值。本研究各年度的赋值表格与鱼群轨迹数据均已附入数据集。