Documenting en route mortality and carcass sampling
With the help of local residents with deep-time knowledge of the Koyukuk River, we surveyed approximately 250 km via jet boat, beginning at the village of Hughes (66.05 N, -154.28 W) and traveling downstream to the community of Huslia (65.69 N, -156.38 W). While traveling we counted the number of carcasses floating in the river and those that were clearly visible along sandbars and shore banks. On an ad hoc basis and as time allowed, we stopped and surveyed along the river banks where we,i ) counted the number of carcasses along approximately 0.5 km upstream and downstream of the landing site and noted characteristics of carcasses including external signs of scavenging or disease (e.g. fungal growth on fins or gills), ii ) recorded the sex and maturation status of dead fish confirmed through internal examination, iii ) recorded water temperature and dissolved oxygen (mg/L) using a handheld YSI PRO 2030 model multimeter, and iv ) measured the body length of carcasses to the nearest millimeter between the middle of the eye and fork of the tail. Due to decomposition of the tail on 36 individuals, length was measured between the middle of the eye and the hypural flexure and adjusted to mid-eye fork length using the equation derived from the data (r2= 0.92)\(:\)
\begin{equation} \ {\text{\ \ }\ \text{MEF}}_{\text{est}}=1.00*\text{MEH}_{\text{obs}}+52.5\nonumber \\ \end{equation}
Where MEF­est is the estimated middle eye to fork (mm) and MEHobs is the observed middle eye to hypural measurement (mm).