The Influence of Habitat Modifications on the Fish Community in the Weaver Bottoms Backwater of Mississippi River Navigational Pool 5
Metz, Michael T.
University of Wisconsin-Stevens Point, College of Natural Resources
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The influence of secondary channel modification and artificial island and levee construction on the fish community was investigated in Weaver Bottoms, a 1,620 hectare backwater lake of Mississippi River Navigational Pool 5. Experimental gill nets, trap nets, and electrofishing were used to sample the fish community at 11 treatment stations within, and 6 reference stations outside the backwater during July and September 1984-1994. Analyses of pre-project (1984-1986) and post-project (1988-1994) data generally indicate that the rehabilitation project did not affect species diversity nor did it increase or decrease overall relative abundance of fishes in Weaver Bottoms. No change in species richness relative to project modification was evident. Overall, species richness was significantly greater at stations located within the modified backwater compared to reference stations (Wilcoxon rank sum, p < 0.05) but differences may have been due to the greater number of treatment stations rather than differences between treatment and reference stations. Trends in species diversity (Shannon-Wiener Index) were similar at treatment and reference stations and changes in diversity appeared to be related to variation in year-class strength of predominant species. Similarly, no significant change (MANCOV AR, P < 0.05) in relative abundance occurred for most species (e.g., black crappies, bluegills, common carp, and northern pike) because of habitat modification. However, for many of these species (e.g., black crappie, bluegill, common carp, gizzard shad, largmouth bass, and shorthead redhorse) relative abundance or relative biomass differed significantly among years at both modified and reference sites (MANCOV AR, P < 0.05), suggesting that some changes resulted from system-wide influences. Some of the changes in relative abundance or relative biomass were related to variation in year-class strength that occurred system-wide. For example, strong black crappie, bluegill, and common carp year classes appeared in 1986 and 1987, and persisted for several years. Length frequency distributions for most of the predominant species were also significantly different (Chi-square, p < 0.001) between pre-project and post-project periods at both treatment and reference stations. For some species (e.g., black crappie, bluegill, common carp) changes in length frequency distributions appeared to influenced by strong year classes persisting in subsequent years, but for other species ( e.g., northern pike) changes in length frequencies were not clearly associated with strong or weak year classes. Habitat modification did appear to increase relative abundance of smallmouth bass in select areas. Smallmouth bass catch-per-effort increased significantly (MANCOV AR, P < 0.05) at stations in secondary channels entering the backwater following partial closure and placement of rock rip-rap, and these areas appeared to be used by all size/age classes of smallmouth bass recruitable to electrofishing gear. Therefore, this analysis indicates that project modifications produced few measurable changes in the fish community and suggests that systemic factors affecting year-class strength and persistence may have had the greatest influence on the fish community in Weaver Bottoms.