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.

Fluvial, or water-based connections between rivers and adjacent floodplain habitats, play an important role in the life history of riverine, or river-based, 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, or flooding, at a fish population and fish community level, by comparing fish population responses to flood events and 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 transects were established at each station along alternating banks of the river and were sampled for 10 minutes each.

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 a "bank full" river stage, or when a river's water level is as high as the banks, after a significant low water event (pre-inundation), and at a "bank full" river stage after a significant flooding event (post-inundation). Samples were collected only at bank full 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.

Population characteristics for the fish species collected from the two rivers were compared between pre-inundation and post-inundation years to determine the importance of inundation events in a floodplain. The most revealing observationwas a shift in sportfish condition. Condition is a population metric, or parameter, 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 then compared using an analysis of variance (ANOVA) in order to identify significant variations in the relative weight among years.

Weight comparison graphs, caption belowFigure 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 is less than 0.05) for bluegill and redear sunfish only. These results may indicate that floodplain inundation positively influences the condition of bluegill and redear sunfish.

Another interesting finding was a change in the species composition between the pre-inundation and post-inundation periods. In the Ocklawaha River, 87 percent of the pre-inundation fishes and 89 percent of the post-inundation fishes were sunfishes Lepomis sp. However, during the pre-inundation collection, the majority of the catch was composed of bluegill L. macrochirus and redbreast sunfish L. microlophus, typically considered lentic (still-water) sunfishes. In contrast, the majority of the sunfish catch in the post-inundation period was composed of redbreast sunfish L. auritus and spotted sunfish L. punctatus, typically considered a lotic (flowing water) species.

In the St. Johns River collections, differences in species composition were also observed. During the pre-inundation collection, lentic species and salt water migrants occupied a large portion of the total species composition. Gizzard shad D. cepedianum and threadfin shad D. petenense composed 16 percent of the fish caught, and five percent of the total catch included striped mullet Mugil cephalus and ladyfish Elops saurus. During the post inundation collections, these species were relatively absent, and the species compositions were more characterized by lotic species.

This study has assisted researchers' understanding of how fish populations and communities respond to floodplain inundation. Improvements in sportfish condition and a fish community more characteristic of lotic conditions are important findings. Despite the potential for extended periods of low or high flow to dictate the short-term composition of the fish community, the plasticity of the fish communities appears to provide some resiliency to permanent changes.

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