Physicochemical Effects of the Flood Pulse on Fishes in the Atchafalaya River Basin, Louisiana

Abstract

We examined the relationships between physicochemistry and the distribution and abundance of fishes at 36 sites 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., standing timber, submerged macrophytes, and brushy cover) and water quality (e.g., dissolved oxygen (DO), pH, and specific conductance) variables. To further examine the relationship between physicochemistry and species abundance and distribution, we regressed the habitat and water quality variables on the abundance of common ARB fishes. Regression analysis indicated that DO concentration, DO differential (surface less bottom), specific conductance, and 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 distributaries of the Atchafalaya River and Intracoastal Waterway), and black-water (hypoxic (DO < 2.0 mg/L) interior bayous and canals) habitats. Analyses of variance of rank fish abundance showed that 15 of the 29 most common fish species had significant differences in mean abundance (P < 0.05) among black-, brown-, and green-water habitats. Bluegill Lepomis macrochirus, gizzard shad Dorosoma cepedianum, threadfin shad D. petenense, white bass Morone chrysops, largemouth bass Micropterus salmoides, redear sunfish L. microlophus, and black crappie Pomoxis nigromaculatus had higher abundances in green-water than in black-water or brown-water habitats. The bullhead minnow Pimephales vigilax was significantly more abundant in brown-water than in black-water or green-water areas, and warmouth L. gulosus had higher abundance in black water than in other water types. Four species (smallmouth buffalo Ictiobus bubalus, longear sunfish L. megalotis, freshwater drum Aplodinotus grunniens, and spotted bass Micropterus punctulatus) showed similar abundance patterns in both green and brown water and were collected at higher abundances there than in black-water habitats. The results of this study indicate that water quality (particularly DO) is more important than physical habitat characteristics in determining fish distribution and abundance in the lower ARB during the flood pulse. The ARB has historically been managed for flood control and navigation, and flow management has resulted in substantial sedimentation and widespread water quality problems. Thus, improved water circulation in the ARB would probably increase production of commercially valuable and sport fish species.