USE OF ASSEMBLAGE STRUCTURE, ABUNDANCE, AND YEAR-CLASS STRENGTH TO ASSESS FISH ASSEMBLAGE RESPONSES TO WATER LEVEL FLUCTUATIONS
Date of Award
Master of Science
Water level fluctuations can influence lateral connectivity of a water body; during high water levels, fish gain access to off-channel and floodplain habitats that can provide refuge for smaller fish and provide access to abundant aquatic and terrestrial prey. Water level dynamics play a critical role in ecosystem productivity and can have varying effects on biota. Modifications to water flow in Buttonland Swamp, located near the headwaters of the Lower Cache River in southern Illinois, were put in place to improve drainage and restore historical water levels; however, these alterations have disrupted the natural hydrologic dynamics and flood pulse processes, which could influence fish assemblage dynamics. The fish assemblage in Buttonland Swamp was sampled using electrofishing, fyke nets, and mini fyke nets during 2020 and 2021 to evaluate effects of water level dynamics on fish abundance, assemblage structure, and year-class strength. Random stratified sampling was used to select sampling sites each month. Four macrohabitats (Cache River channel, Buttonland Swamp main channel, side channels, and Eagle Pond) within the swamp were surveyed monthly. Open water, nearshore vegetated, and offshore vegetated habitats within each of the macrohabitats were also surveyed. The first chapter of this study evaluated whether assemblage structure and abundance of fishes in Buttonland Swamp differed across seasons, years (using historic data), and habitats over varying water levels and assessed associations between particular species (especially species of conservation concern) and habitats across seasons, years, and water levels. Each gear was analyzed separately, although electrofishing most effectively sampled all habitats, so analyses of fish assemblage structure across seasons, habitats, and years only used electrofishing data. Non-metric multidimensional scaling (NMDS) and analysis of similarities (ANOSIM) were used to evaluate spatiotemporal patterns in assemblage structure in relation to water level. Indicator Species Analysis (ISA) was used to identify prominent species among habitats, seasons, years (using historical data from Illinois Department of Natural Resources (IDNR) sampling in Buttonland Swamp), and datasets (from IDNR and SIU sampling). Repeated measures using mixed models were used to distinguish significant differences in catch per unit effort (CPUE) (catch per minute using electrofishing; catch per net night for fyke nets and mini fyke nets) among macrohabitats, microhabitats, and seasons. There was a significant difference in assemblage structure among macrohabitats, microhabitats, years, and seasons; however, the R-value was so low among macrohabitats and years that the significant P-values were disregarded since this could have been by chance since the sample size was large. Microhabitats had significantly different assemblage structures, with Silver Carp, Shortnose Gar, Threadfin Shad, and Gizzard Shad being associated with open water, whereas Taillight Shiner, Orangespotted Sunfish, Golden Shiner, Largemouth Bass, Redear Sunfish, Bluegill, Warmouth, Black Crappie, and Smallmouth Buffalo were associated with vegetated habitats. Using IDNR data, there were no significant differences in assemblage structure among years, although there was a significant difference in assemblage structure between the IDNR and SIU data; species that were associated with vegetated habitat were associated with IDNR data, whereas SIU data were more associated with species that were associated with open water. Additionally, some microhabitats and macrohabitats had significant differences in assemblage structure among seasons. ISA had similar results to NMDS and ANOSIM, but also showed prominent species for macrohabitats and years. Additionally, depending on the gear used, CPUE was significantly different among macrohabitats, microhabitats, and seasons. Species that were associated with vegetated habitats may be more impacted by water level fluctuations because at very low water levels these species may have less shallow water vegetated habitat to utilize or may be left stranded as the water level recedes. The second chapter assessed associations between historic water level data, air temperature data, and historical catch per unit effort (CPUE) data of predator and competitor fish species from Buttonland Swamp with year-class strength indices of Silver Carp, Bluegill, and Gizzard Shad to evaluate relationships between hydrology, temperature, and other species interactions with fish recruitment. Random stratified sampling was used to select sixteen electrofishing transects each month during September through November 2020 and 2021. Among fish collected, a subsample of Bluegill, Gizzard Shad, and Silver Carp were aged by otolith and cleithra annuli counts. An age-length key was developed to assign ages to unaged fish. Year-class strength indices were determined using the residual method from catch curve regressions. Bluegill and Gizzard Shad had weak year-class strength in 2017, which coincided with a low average water level that year. Water level during fall and spawning was positively associated with year-class strength for Bluegill and Gizzard Shad. Gizzard Shad and Bluegill year-class strength was positively associated with minimum winter temperature. Bluegill and Gizzard Shad year-class strength had a negative association with catch per unit effort (CPUE) of Common Carp and Largemouth Bass. Other predator and competitor fish species CPUE were also significantly associated with year-class strength of Bluegill, Gizzard Shad, and Silver Carp. Only a few age-classes of Silver Carp were caught, representing a boom and bust pattern of recruitment. Since Silver Carp could have migrated into the study area, Buttonland Swamp water level may not be an important factor influencing year-class strength of this species. Water level, along with other abiotic and biotic factors, appear to be influencing year-class strength of fishes within Buttonland Swamp. High water level throughout the year, especially from May-June and October-December, was associated with strong year-class strength of Bluegill and Gizzard Shad. Additionally, maintaining high water level in the winter could allow for deep water refuges for cold intolerant fishes to avoid winter mortality and maintain their abundance. This information contributes to both understand and management of hydrologically impaired systems and can also facilitate future assessments of how current and future hydrologic management regimes will affect the fish community.
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