3.2 | Occupancy Models
We retained the most supported detection probability model structure for
each taxa when assessing various larval fish occupancy model
combinations. Similar to detection models, the combination of shad and
percids, with carp and freshwater drum occupancy estimated separately,
received the most support (ΔAICc = 0.00,wi = 0.44; Table 2). Other likely combinations
include all taxa estimated separately (ΔAICc =
1.63, wi = 0.20) and freshwater drum estimated
separately from bigheaded carp, shad, and percids
(ΔAICc = 1.78, wi = 0.18;
Table 2). Through the addition of habitat to this taxa model structure,
the most supported model included backwater estimated separately and
thalweg combined with channel border habitats for shad and percids
(ΔAICc = 0.00, wi = 0.21;
Table 2). Two of the six top and competitive model structures
incorporated this habitat grouping among different taxa (Table 2).
Finally, we added environmental covariates to determine the combination
that best improved the taxa × habitat model. The most supported taxa ×
habitat × environmental covariate model (ΔAICc =
0.00, wi = 0.69) incorporated Julian date and CV
temperature for all taxa with the added effect of temperature on
bigheaded carp and freshwater drum (Table 2). Bigheaded carp were the
only taxonomic group to retain both discharge and CV discharge in the
top model (Table 2).
Freshwater drum occupancy among all habitats (Ψ = 0.84, 95% CI = 0.73
to 0.91) was higher than bigheaded carp (Ψ = 0.29, 95% CI = 0.15 to
0.47) and shad and percids in backwaters (Ψ = 0.53, 95% CI = 0.41 to
0.66) or thalweg/channel border (Ψ = 0.42, 95% CI = 0.34 to 0.51).
Julian day had a quadratic effect on occupancy probability for all taxa:
shad and percids occupancy was highest on day 144 (May
24th) followed by bigheaded carp on day 170 (June
19th) and freshwater drum on day 184 (July
3rd; Figure 5). Bigheaded carp (β = -0.63, 95% CI =
-1.03 to -0.23) and freshwater drum (β = -0.89, 95% CI = -1.24 to
-0.55) occupancy declined with increased variation in weekly water
temperature while shad and percids occupancy increased with increasing
water temperature CV (β = 0.33, 95% CI = 0.16 to 0.51; Figure 5).
Weekly water temperatures also had a quadratic effect on occupancy of
bigheaded carp and freshwater drum: bigheaded carp occupancy was highest
at 20.7°C and drum was highest at 23.9°C (Figure 5). Bigheaded carp
occupancy was also positively related to mean weekly discharge
(β = 1.29, 95% CI = 0.19 to
2.39) whereas the slope estimate of CV discharge incorporated zero (β =
-0.01, 95% CI = -0.35 to 0.36; Figure 5).