Results
Fish species diversity was highest at Wadley (H=2.88) and lowest at
Horseshoe Bend (H=2.49), although values were generally similar across
sites (range among sites was 0.39; Table 1). Centrarchids made up the
largest proportion of collected fish across all sites with their highest
contribution being in the tailrace and at Horseshoe Bend (Figure 2).
Cyprinids/leuciscids contributed the second largest proportion of fish
collected from all sites except the tailrace (where they were third
highest), where the second largest proportion was from percids (Figure
2). The proportion of the assemblage composed of percids was also
relatively high at Wadley, with much lower values at Lee’s Bridge and
Horseshoe Bend (Figure 2). Catastomids made up larger proportions of the
collected families at Lee’s Bridge, Wadley, and Horseshoe Bend and a
much lower proportion in the tailrace (Figure 2). At Lee’s Bridge, 3 of
the 10 most abundant species were centrarchids and at the downstream
sites it was between 3 of 10 in the tailrace and 4 of 10 at Horseshoe
Bend (Table 2).
NMDS yielded stable, two-dimensional ordinations (Figure 3a, final
stress = 0.09) in 40 iterations. Ordination axis MDS-1 correlated most
positively with Snail Bullhead (T= 0.98), Bandfin Shiner
(T= 0.80), Black Madtom (T =0.80), and Rough Shiner
(T= 0.79; Figure 4) and most negatively with White Bass
(T=- 0.96), Yellow Perch (T =-0.92), River Redhorse
(T =-0.92), and Bullhead Minnow (T =-0.92). Axis MDS-2
correlated most positively with Blueback Herring (T =0.63),
Skipjack Herring (T =0.63), Golden Shiner (T =0.63), and
Black Bullhead (T =0.63); and most negatively with Grass Carp
(T =-0.62), Pretty Shiner (T =-0.62), Snail Bullhead
(T =-0.34), and Bandfin Shiner (T= -0.31; Figure 4). At the
family level, MDS-1 correlated most positively with ictalurids and
percids and most negatively with clupeids, catastomids, and moronids
(Table 3).
Ordinations showed clear separation between sites as well as seasonal
variation in the assemblages within sites (Figure 3a). Separation across
sites in MDS-1 showed longitudinal variation in fish assemblage
downstream of Harris Dam; in addition, as distance downstream of the
tailrace increased, MDS-1 of the fish assemblage approached that of the
upstream reference site, identifying a potential recovery gradient
(Figure 3a). Differences between seasons were driven by generally
negative summer and positive spring values for MDS-2 (Figure 3a). MDS-2
appeared to vary seasonally, being higher in the spring (March-May) and
lowest in the summer (June-August; Figure 3a).
Multiresponse Permutation Procedure and Indicator Species Analysis
MRPP indicated a significant difference among site-specific fish
assemblages (p = 0.001, A =0.298, δ=0.290; Figure 3b). Indicator
species analysis identified 1 - 5 significant indicator species
depending on site (Table 4). Lee’s Bridge had the highest number of
indicator species which included five separate families (Table 4). In
the tailrace three indicator species were identified including one
percid (an Alabama listed Priority 2 species; Mirarchi et al. 2004), one
ictalurid, and one centrarchid. Wadley had three indicator species
including one percid and two campostomids (Table 4). The single
indicator species identified at Horseshoe Bend was Black Redhorse, a
campostomid (Table 4). Average between- and within-group dissimilarities
grouped Horseshoe Bend and Wadley as the site block with the least
dissimilarity and Lee’s Bridge as the most dissimilar (Figure 5). The
Tailrace fell within a block containing the downstream sites although
separated from Wadley and Horseshoe Bend (Figure 5).