RECOVER:
2009 System Status Report

Lake Okeechobee
Native Fish Results

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Typical fish sampling locations.
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Typical sampling locations for fish in Lake Okeechobee.

Summary

The abundance and diversity of Lake Okeechobee’s native fish population is dependent on a variety of related factors. These factors include submerged aquatic vegetation (SAV) as habitat and the macroinvertebrate community as the primary food source. Water and basin management directly affect lake stage and water quality, which in turn affect SAV coverage and algal bloom severity and duration. The fish population varies from year-to-year, with very good years interspersed with years less so. Factors related to lake stage and nutrient loading may coincide with changes in the fish community. Extremely high or low lake stage may negatively affect emergent and SAV coverage reducing the amount of available fish habitat. If lake stages are more frequently within the desirable range of 12.5 to 15.5 feet, emergent plant and SAV coverage might be maximized, thereby increasing the amount of habitat and prey. However, continuous excessive nutrient loading may indirectly negatively impact the fish communities by shifting their macroinvertebrate prey base from preferred taxa such as chironomids (non-biting midges) to one dominated by less utilized oligochaete (annelid worm) taxa. Recent downturns in fish community health following three hurricanes in 2004 and 2005 are worrisome, but additional data will be necessary to determine if these are cyclic events.

Electrofishing

Lakewide electrofishing did not take place in 2007 due to low lake levels. With the increasing lake level due to Tropical Storm Fay, depths were sufficient to complete lakewide electrofishing at all sites during fall 2008. Electrofishing resulted in the capture of 4,974 fish with a combined biomass of 361 kilograms (kg). Thirty fish species were represented in the catch.

Four species collectively comprised 82% of the catch by number and were, in order of abundance:

  • Threadfin shad (Dorosoma petenense)
  • Gizzard shad (Dorosoma cepedianum)
  • Eastern mosquitofish (Gambusia holbrooki)
  • Bluegill (Lepomis macrochirus)

Six species collectively comprised 78% of the catch by weight and were, in order of biomass:

  • Florida gar (Lepisosteus platyrhincus)
  • Gizzard shad (Dorosoma cepedianum)
  • Bluegill (Lepomis macrochirus)
  • Largemouth bass (Micropterus salmoides)
  • Bowfin (Amia calva)
  • Redear sunfish (Lepomis microlophus)

Comparison of electrofishing data.
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Comparison of lakewide electrofishing data from 2005, 2006 and 2008 shows a change in abundance of prey species while piscivorous species have either declined or remained steady.

Trawl Sampling

In 2008, trawl sampling resulted in the capture of 2,816 fish with a combined biomass of 221 kilograms (kg). Seventeen fish species were represented in the catch.

Three species collectively comprised 84% of the catch by number and were, in order of abundance:

  • Threadfin shad (Dorosoma petenense)
  • White catfish (Ameiurus catus)
  • Bluegill (Lepomis macrochirus)

Three species collectively comprised 79% of the catch by weight and were, in order of biomass:

  • White catfish (Ameiurus catus)
  • Florida gar (Lepisosteus platyrhincus)
  • Bluegill (Lepomis macrochirus)

Comparison of lakewide trawl sampling data for selected dominant species shows an increase in abundance of threadfin shad and white catfish, while black crappie (Pomaxis nigromaculatus) shows a continued decline in abundance. The decline in black crappie relative abundance is due to extremely poor recruitment since 2002 and the short-lived nature of the species. The decline in crappie was anticipated and was the reason the Florida Fish and Wildlife Conservation Commission (FWC) implemented the ten-inch minimum in July 2008. The high abundance of white catfish in the 2008 lakewide trawl sampling was skewed by an abnormally large number captured at one site during sampling.

Comparison of trawl sampling data.
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A majority of both threadfin and gizzard shad (Dorosoma cepedianum) captured in 2008 were young-of-year fish. Threadfin shad abundances have increased since 2005 but remain well below levels observed during 1988 to 1991, a period when black crappie abundances were high. Food habitat analyses have shown that young-of-year shad are primary forage of adult black crappie in Lake Okeechobee. Low shad numbers are a major contributing factor to extremely low relative abundance of crappie; thus, the increase in shad observed in 2008 is an important indicator for the potential to rebuild crappie stock levels. This, along with the large number of eastern mosquito fish (Gambusia holbrooki), is key in the recovery of the largemouth bass (Micropterus salmoides) populations of Lake Okeechobee because without prey, predatory fish populations would continue to decline.

Black Crappie

Counts of chironomidae.
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Chironomid larvae are the primary food source of juvenile black crappie (Pomaxis nigromaculatus) and the decline in the former is another causative factor, along with the decline in threadfin shad (Dorosoma petenense), explaining the decline of black crappie. Bluegill (Lepomis macrochirus), also known as bream or brim, feed on very small fish and invertebrates. In 2005 and 2006, bluegill abundance decreased in comparison to the 1987 to 1991 data by 94 and 92%, respectively, which mirrors the decline in invertebrates as their direct prey and that of many of the smaller fish upon which they feed. However, concern regarding these precipitous declines in black crappie population must be tempered by observing 1986 to 1990 was an unusually productive period and, accordingly, the 1987 to 1991 dataset was biased high.

Black crappie catch rate 1973-2008.
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Although the 2005 to 2007 timeframe denotes the lowest catch rate on record, other periods of time have been similarly poor. Nevertheless, the preceding clearly illustrates the intertwined relationships among all the lake health attributes (e.g., SAV, water quality, lake stage, macroinvertebrates) and demonstrates the necessity to assess and manage Lake Okeechobee from the widest holistic perspective of balanced ecosystem function

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