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Fish Population Responses to Floodplain Inundation

Biologists are sampling the St. Johns River and the Ocklawaha River in Florida to assess the benefits of floodplain inundation at a fish population and fish community level.

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 Fluvial, or water-based, connections between rivers and adjacent floodplain habitat play an important role in the life history of riverine fishes. Biologists believe the timing, frequency, magnitude, rate of change, and duration of flood events affect species differently. This study assesses the benefits of floodplain inundation at a fish population and fish community level, by comparing fish population responses to flood events and fish population responses to low water events.

Two low gradient, warm water rivers in north central Florida were selected for this study: the St. Johns and Ocklawaha rivers. Three sampling stations were established in each river. Each station was selected because it allowed a connection between the floodplain and the river at high water (this type of connection between the floodplain and the river is known as floodplain connectivity). Four 10-minunte transects were established at each station along alternating banks of the river. Fish were collected using electrofishing techniques. All fish encountered during the electrofishing transects were collected, measured, and weighed. Samples were collected from the rivers at “bankfull” river stage after a significant low water event (pre-inundation) and at “bankfull” river stage after a significant flooding event (post-inundation). Samples were collected only at bankfull river stage in order to standardize the efficiency of biologists’ sampling gear. Sampling took place in 2000 (pre-inundation), 2002 (post-inundation), and 2004 (post-inundation) at the St. Johns River and in 2001 (pre-inundation) and 2003 (post-inundation) at the Ocklawaha River. Researchers expect to collect one additional sample (pre-inundation) from the St. Johns River and two additional samples (one pre-inundation and one post-inundation) from the Ocklawaha River.

As the study progresses, population characteristics for the fish species collected from the two rivers will be compared between pre-inundation and post-inundation years to determine the importance of floodplain inundation events. At this point in the study, some preliminary comparisons have been examined for sport fish species. Condition is a population metric used to assess the quality of a fish population relative to other populations. Relative weight (Wr) is a value that can be calculated as a measure of condition. Relative weight is the ratio of the measured weight to a pre-calculated standard weight (Ws) for a given fish length: Wr = 100 * (fish weight/Ws). Relative weights were calculated for four species collected from the St. Johns River in 2000 (pre-inundation) and 2002 (post-inundation): bluegill sunfish, largemouth bass, redbreast sunfish, and redear sunfish. Relative weights for each species were then averaged by year and compared using an analysis of variance (ANOVA) in order to identify significant variations in the relative weight among years.

Figure 1. Relative weight comparisons (Wr) for bluegill sunfish, largemouth bass, redear sunfish, and redbreast sunfish collected from the St. Johns river in 2000 (pre-inundation) and 2002 (post-inundation). Analysis of variance results show a significant increase in the Wr of bluegill sunfish (p < 0.001) and redear sunfish (p = 0.001) from the pre-inundation period to the post-inundation years.

Results of the relative weight comparison show an overall increase from pre-inundation to post-inundation years for all four species. This increase was statistically significant (p < 0.05) for bluegill and redear sunfish only. These results may indicate that floodplain inundation positively influences the condition of bluegill and redear sunfish. However, further investigations of additional population characteristics and additional sampling events should improve researchers’ understanding of how fish populations are responding to floodplain inundation.