Fisheries Projects
Biologists are studying how freshwater fish community dynamics may be affected by varying water flows and levels.
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, Florida 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.
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
Sampling Freshwater Fish Communities with Gill Nets
Biologists with the Fish and Wildlife Research Institute (FWRI), as well as other FWC divisions, use a variety of methods to sample freshwater fish communities. The type of gear used depends on the target species, sampling location and habitat.
Gill nets are stationary nets used to sample offshore areas where pelagic (open-water) species, such as shads (Dorosoma spp.) and black crappie (Pomoxis nigromaculatus), are found from December to April.
Biologists initially used floating gill nets for statewide Long-Term Monitoring Project efforts, as it is the typical way the nets are fished; but they noticed most of the fish were captured in the lower part of the net. Following a 2008 gear study, researchers determined gill nets deployed on the lake bottom – rather than floating on top of the water – were more efficient.
From 2008 to 2012, biologists used approximately 20 gill nets to sample 26 Florida lakes. Each sinking gill net measures 275 feet long by 6 feet high, comprising 11 panels, each 25 feet long with a different mesh size. Mesh sizes vary to ensure a catch consisting of all sizes of fish. Biologists feed the nets into the water, where it settles on the lake bottom. Floats along the top line of the net keep it vertical in the water column, allowing it to capture fish as they swim. After positioning nets and allowing them to soak for two hours, researchers collect, identify, weigh and measure the fish caught.
Biologists recorded the following from 2008 to 2012 using gill-net sampling:
- Collected nearly 50,000 fish, representing 25 species.
- The smallest number of species (8) was collected in lakes Eloise (Polk County), Santa Fe (Alachua County) and Deer Point (Bay County).
- The largest number of species (20) was collected in Farm 13/Stick Marsh (Indian River County).
- Gizzard shad and threadfin shad were the most abundant species collected in the nets, accounting for approximately 80 percent of the total catch by number.
- Black crappie and bluegill were not as abundant as shad, but were observed in almost every lake sampled.
Currently, the use of gill nets has been suspended from the Long-Term Monitoring Project, in favor of other sampling methods such as electrofishing and fyke nets that tend to help researchers collect a wider variety of species. Following a recently published study using fish community data collected from 2008 to 2012, researchers found that gill nets, for the amount of effort expended, collect fewer species than other gears. However, biologists will continue to use gill nets to target certain pelagic species, including channel catfish (Ictalurus punctatus) and striped bass (Morone saxatilis).
Biologist use fyke nets to monitor shallow-water fish communities in Florida’s lakes.
Biologists with the Fish and Wildlife Research Institute (FWRI), as well as other FWC divisions, use a variety of methods to sample freshwater fish communities, depending on the target species, sampling location and habitat.
In summer 2007, FWRI researchers initiated a study to determine the most effective gear for collecting fish in shallow-water environments less than 2.5 feet deep. Gear tested included mini block nets, backpack electrofishing, throw traps and mini fyke nets. Among these, staff preferred fyke nets because they are easy to use and effective at capturing fishes in a variety of habitats.
Fyke nets are long, stationary nets with a lead extending to the lake’s edge designed to direct fish through two chambers and a funnel into a final collection chamber (Figure 1). Each summer, researchers sample approximately 10 to 15 Florida lakes with these nets as part of the Long-Term Monitoring Project. They split the lakes into three sections and select 10 sample sites within each section.
Each summer from 2008 to 2012, researchers sampled 29 lakes with fyke nets. The total catch rate and types of fishes collected varied widely by lake, but as a whole, the statewide community snapshot painted a similar picture each year. At some lakes, such as Minneola (Lake County), Jackson (Leon County), Trafford (Collier County), Eloise (Polk County) and Dorr (Lake County), researchers tend to collect a relatively low number of species compared to others, including lakes Monroe (Volusia County), Kissimmee (Osceola County), George (Volusia County), Istokpoga (Highlands) and Deerpoint (Bay County). Researchers collected between 12 and 35 fish species, with an overall average of 24 species. To date, the most commonly observed species collected were eastern mosquitofish (Gambusia holbrooki) and warmouth (Lepomis gulosus). Other common species included:
- bluefin killifish (Lucania goodei)
- bluegill (Lepomis macrochirus)
- brook silverside (Labidesthes sicculus)
- brown bullhead (Ameiurus nebulosus)
- dollar sunfish (Lepomis marginatus)
- Florida gar (Lepisosteus platyrhincus)
- golden topminnow (Fundulus chrysotus)
- least killifish (Heterandria formosa)
- redear sunfish (Lepomis microlophus)
- Seminole killifish (Fundulus seminolis)
- swamp darter (Etheostoma fusiforme)
As many of the species collected with fyke nets are food for sport fish and indicators of the overall health of a freshwater system, monitoring these shallow water communities is an important aspect of understanding how to best manage Florida’s freshwater resources.
Dissolved Oxygen Levels in Florida Freshwater Bodies: A Photo Set on Flickr
Biologists are studying dissolved oxygen’s (DO) impact on freshwater fish communities.