Physicochemical effects of the flood pulse on fishes in the Atchafalaya River basin, Louisiana

We examined the relationships between physicochemistry and the distribution and abundance of fishes at 36 sires in the lower Atchafalaya River basin ( ARB), Louisiana, a bottomland hardwood swamp fed by waters (distributaries) of the Mississippi and Red rivers. We used principal components analysis ( PCA) of the 29 most common ARB fishes to examine fish assemblage structure and then related the first two components to physical habitat (e.g., standi ng timber, submerged macrophytes, and brushy cover) and water quality (e.g. , dissolved oxygen [DO], pH, and specific conductance) variables. To furthe r examine the relationship between physicochemistry and species abundance a nd distribution, we regressed the habitat and water quality variables on th e abundance of common ARB fishes. Regression analysis indicated that DO con centration, DO differential (surface less bottom), specific conductance, an d current velocity were most strongly associated with fish distribution and abundance. A PCA of these four variables separated sample sites into green -water (lakes, lentic bayous, and canals), brown-water (lotic distributarie s of the Atchafalaya River and Intracoastal Waterway), and black-water (hyp oxic [DO < 2.0 mg/L] interior bayous and canals) habitats. Analyses of vari ance of rank fish abundance showed that 15 of the 29 most common fish speci es had significant differences in mean abundance (P < 0.05) among black-, b rown-, and green-water habitats. Bluegill Lepomis macrochirus, gizzard shad Dorosoma cepedianum, threadfin shad D, petenense, white bass Morone chryso ps, Largemouth bass Micropterus salmoides, redear sunfish L. microlophus, a nd black crappie Pomoxis nigromaculatus had higher abundances in green-wate r than in black-water or brown-water habitats. The bullhead minnow Pimephal es vigilax was significantly more abundant in brown-water than in black-wat er or green-water areas, and warmouth L. gulosus had higher abundance in bl ack water than in other water types. Four species (smallmouth buffalo Ictio bus bubalus, longear sunfish L. megalotis, freshwater drum Aplodinotus grun niens, and spotted bass Micropterus punctulatus) showed similar abundance p atterns in both green and brown water and were collected at higher abundanc es there than in black-water habitats. The results of this study indicate t hat water quality (particularly DO) is more important than physical habitat characteristics in determining fish distribution and abundance in the lowe r ARB during the hood pulse. The ARB has historically been managed for floo d control and navigation, and dow management has resulted in substantial se dimentation and widespread water quality problems. Thus, improved water cir culation in the ARB would probably increase production of commercially valu able and sport fish species.