Dispersal and successional patterns of the fish community of the Rocky Glades of southern Florida

William F. Loftus¹, Joel C. Trexler², and Sue Perry<sup>3

1</sup> USGS, 40001 SR 9336, Homestead, FL 33034
² FIU, University Park, Miami, FL 33199
³ Everglades NP, 40001 SR 9336, Homestead, FL 33034

Presented at the 2001 Annual Meeting of the American Society of Ichthyologists
and Herpetologists at Penn State University in June, 2001.

ABSTRACT

The Rocky Glades, or Rockland, of southern Florida is a landscape threatened by drainage and land conversion.  It remains structurally intact in Everglades National Park. These short-hydroperiod wetlands maintain persistent fish communities because the highly eroded karst offers dry-season refuges in solution holes that access groundwater. Fishes survive beneath the surface for months until rains re-flood the area, but the degree that local refuges (solution holes) versus distant refuges (sloughs and canals) contribute to providing recruits for recolonization is unclear.  Past observations of directed mass movements of fish days after the Rocky Glades re-flooded supports a local-refuge hypothesis.

In the wet season of 2000, we performed a pilot study to test use drift fence/funnel traps, combined with visual surveys, to study fish dispersal, recruitment, and successional patterns. Fishes made mass directional movements on the surface in the wet season that appeared strongly related to water flow. The appearance of fishes in traps within a day of marsh flooding indicated presence of local refuges. Only adults were taken at re-flooding.  These reproduced within days. The majority of the 24 fish species collected appeared during the first week of reflooding.  Larger-bodied species and non-native fishes appeared to immigrate later. The study will be expanded spatially in summer 2001.

INTRODUCTION

• Karst wetlands occur around the world, yet little is known about the complex interactions of hydrology, geology, and ecology in those landscapes.
• Solution holes directly connect surface and ground waters and provide critical refuges for aquatic organisms throughout the dry season.
• Half of South Florida Rockland wetlands have been lost to development and the remainder has suffered drainage.

SAMPLING AREA – ROCKLAND (Fig 1)

• The Rockland is a transitional wetland lying between Shark Slough and the Atlantic Coastal Ridge of southern Florida.
• The highly eroded, karstic topography offers many shallow to deep solution holes that serve as aquatic refugia (Fig. 2).
• The Rockland is inundated for 3 to 7 months of the year, significantly less than under historical conditions.

OBJECTIVES

• Quantify fish community succession and dispersal patterns by use of drift fences and funnel traps in the Everglades Rockland.
• Test a visual-survey method for sampling fish communities in open, rugged terrain, where other methods fare poorly.
• Predict the response of Rockland aquatic fauna to system restoration.

METHODS

• Array Traps

In May 2000, we built four drift-fence arrays (Fig 3a) to measure dispersal and relative abundance of fishes with the arrival of the wet season (Fig. 4).  Three minnow traps, with 3cm-bar wire mesh, and one end of the trap blocked, were placed to capture animals moving from the west, north, and (Fig. 3b).  The road shoulder served as a border to the south end of each array. Once marshes flooded, minnow traps were set out once per day for two weeks (Fig. 3c).  Traps were fished for 24 hours and the samples brought back to the lab for analysis.  Sampling was reduced to two trap-nights per week for the next two weeks, then to one night per week for the duration of the sampling period.

• Visual Surveys

Because array traps are activity traps, we tested another method that did not depend on activity to assess community patterns.  We established 24 visual-survey plot; 6 plots flanking each array.  Each plot covered a 4-m² area that was marked at the corners by flagging.  The survey consisted of a 2-minute visual search of each plot using binoculars. Generally, visual surveys were conducted every week, in conjunction with checking the array traps.

RESULTS

HYDROPERIOD

• A staff gauge was permanently affixed to each array to measure depth and flooding.
• Western arrays 3 & 4, lower in elevation, flooded a month and a half earlier than eastern
  arrays 1 & 2 (Fig. 4).
• Several sites dried and reflooded during the wet season.

SPECIES COMPOSITION

• Twenty-four species were collected during the six months of the pilot study. Gambusia holbrooki was the most numerous fish taken in the traps, followed by Jordanella floridae, Lepomis marginatus, and Poecilia latipinna (Table 1).  There was an inverse correlation between flooding period of the array and the number of fishes caught per trap-day (Table 2).  Of the 24 species collected, from 15 to 19 were shared between arrays (Table 3).

DRIFT FENCE ARRAY TRAPS

• Animals reappeared in the traps on the same day that the wetlands reflooded, indicating existence of local subterranean refuges (Figs. 6a, 7a).
• Most species were present within the first week, and exhibited directional movements mainly with the flow of water (Figs. 6b, 7b).
• Larger species appeared later in the sample period, possibly recruiting from distant refuges
  (Figs. 6c, 7c).

ARRAY 2

ARRAY 4

VISUAL SAMPLING PLOTS

• Eastern mosquitofish (Gambusia holbrooki) were most active and, therefore, most visible.
• Sedentary, cryptic species were more difficult to observe.
• The number of fish seen in visual plots showed an almost immediate response to periodic reversals and wettings (Fig. 8a).
• All fishes emerging onto the surface were adults that began reproducing within one or two weeks. Small juveniles appeared in the wetlands within a month of reflooding (Fig. 8b).

ARRAY 4

CONCLUSIONS

• Drift-fence arrays provided data on fish movement, orientation to water flow, species composition, and recruitment.
• There was evidence for local and distant refuges providing recruits to the Rockland sites.
• Fishes reproduced quickly on the marsh surface and colonized the landscape quickly.
• Visual sampling of arrays did not provide accurate data on species presence or number of fish.

NEW IN 2001

• Nine more arrays have been added (13 total), with traps facing all four cardinal directions (N,E,S,W).
• Processing of trap catch is done in the field.
• Visual sampling is discontinued, replaced by electrofishing.

ACKNOWLEDGEMENTS

Special thanks to technicians Victoria Foster and Andrew Martin. Phil George, Diane Riggs, Hardin Waddle, and Angela Griffith assisted in array construction and lab processing. Victoria Foster also was instrumental in preparing this poster. This study is supported by DOI's Critical Ecosystem Studies Initiative (CESI), and by the USGS Florida Integrated Science Center.