Abundance estimation.—Only targets clearly identified as
sturgeon (Flowers and Hightower 2013) were used in our analyses.
For the N-mixture model, side-scan observer data were
compiled into a data matrix containing river, site, and the three
daily counts of individual sturgeon. Sites were arranged in
rows (N D 179), while river index and daily counts were listed
in columns (N D 4). A separate matrix for distance sampling
data incorporated corrected horizontal distance from a sidescan
transect’s centerline to each sturgeon identified in a given
site for each survey day. The number of rows in this matrix
was equal to the number of distance observations by site (N D
1,436) and had columns representing river, site, sample day,and distance (N D 4). A “slant range correction” was used to
account for positioning errors because the cross-track coordinates
of a side-scan image are a function of range to the sonar
rather than horizontal distance on the bottom (Cobra et al.
1992). Although our side-scan swath was 25 m/side, based on
slant-range, the maximum horizontal range was actually 24 m.