3.1 | Detection Models
Our most supported detection probability model while assessing taxa
effects indicated differences in bigheaded carp and freshwater drum
detections while shad and percids had similar detection
(ΔAICc = 0.00, wi = 0.69;
Table 1). The second ranked model with each taxonomic group estimated
separately also received support (ΔAICc = 1.62,wi = 0.31; Table 1). Next, we assessed potential
variation in detection probability for taxa among habitats. Our most
supported model indicated detection of bigheaded carp and freshwater
drum was similar between thalweg and channel border habitats combined
that had different detection compared to backwater habitats
(ΔAICc = 0.00, wi = 0.33;
Table 1). In contrast, detection probability was similar among habitats
for shad and percids (Table 1). Other model combinations receiving
support include thalweg and channel border habitats combined for all
taxa (ΔAICc = 1.20, wi =
0.18) and all habitats separate for carp and freshwater drum
(ΔAICc = 1.29, wi = 0.17;
Table 1). The final step of detection model selection was assessing the
effect of water volume filtered (i.e., sampling effort) to the top taxa
model. The addition of water volume filtered to all taxa
(ΔAICc =0.00,wi = 0.35; Table 1) was the most supported model,
whereas water volume filtered applied to only bigheaded carp and shad
and percids (ΔAICc = 1.48,wi = 0.17) or only to drum and shad/percids
(ΔAICc = 2.00, wi = 0.33;
Table 1) were also considered competitive models.
The most supported model estimated bigheaded carp detection
probabilities higher in the thalweg and channel border habitat (p= 0.15, 95% CI = 0.09 to 0.22) than in the backwater (p = 0.09,
95% CI = 0.05 to 0.17), but estimates had overlapping confidence
intervals (Figure 2). Drum detection probability was also higher in the
thalweg and channel border (p = 0.40, 95% CI = 0.35 to 0.45)
compared to the backwater (p = 0.20, 95% CI = 0.15 to 0.26)
while shad and percid detection probability was similar across all three
habitats (p = 0.27, 95% CI = 0.23 to 0.31; Figure 2). Detection
of bigheaded carp within each habitat tended to be lower than native
fishes (Figure 2). Water volume filtered had a positive effect on
detection estimates for all taxa/habitat combinations (β = 0.12, 95% CI
= 0.01 to 0.23) where detection probability of bigheaded carp increased
from 0.15 to 0.48, detection of freshwater drum increased from 0.34 to
0.77, and detection of shad and percids increased from 0.24 to 0.34 as
water volume filtered increased from 30 m3 to 50
m3 (Figure 3).
Differences in detection probabilities among taxa resulted in
differences in cumulative detection curves and the number of
ichthyoplankton samples needed to detect each taxonomic group. Under the
average water volume filtered (30.12 m3), bigheaded
carp required the most ichthyoplankton samples (25 backwater or 14
thalweg/channel border) to reach a 90% detection probability, 1.4 to 5
times as many samples as native taxa (Figure 4). Freshwater drum
required 5 to 10 samples and shad/percids required 8 samples to reach a
90% probability of detection. By increasing the water volume filtered
to 40 m3 we observed a 60% to 108% decrease in the
samples required to achieve a 90% detection probability compared to the
average sample volume (9 to 15 samples for bigheaded carp, 4 to 8
samples for freshwater drum, and 7 samples for shad and percids (Figure
4).