Figure 1. Big Sable Creek complex. Intertidal mudflats show up in B&W photo as light gray openings in an otherwise continuous dark forest . [larger image]

Hurricanes routinely cause damage to mangrove forests, generally by breaking and toppling trees. Normally forests recover through growth of new plants from seedling germination. For reasons that are not completely understood, the passage of two category 4-5 hurricanes across the Cape Sable peninsula in southwestern Florida (1935, 1960) resulted in permanent damage to some mangrove forests: adult trees were killed, sediments eroded, and no seedlings germinated. The net result was localized conversion of mangrove forests to unvegetated mudflats. Mangroves are generally considered to be critical nursery habitat to the juveniles of many species of estuarine transient fishes whose adults spawn offshore and whose young life history stages use mangrove environments. This project asks the question "What is the consequence of the conversion of mangrove to mudflat habitat on intertidal assemblages of fish and decapod crustaceans within the creeks in the Big Sable Creek complex?"

The Big Cape Sable Creek complex is located at the far downstream end of the Everglades restoration area, but is of interest because it naturally receives little freshwater inflow. The creek complex consists of six tidal creeks that are a mosaic of mangrove forest and mudflats (fig. 1): both habitats are inundated at high tide. Intertidal rivulets, i.e., drainage features smaller than first order creeks, also drain both habitats. Rivulets are depressions in the substrate up to 1 m deeper than the forest floor or mudflat around them. Rivulets fill earlier on flood tides and retain water later on ebb tides. Rivulets are "hotspots" for the entry and egress of fish and decapod crustaceans (shrimp, crabs) from intertidal habitats, and are a convenient location for sampling these animals with block nets (fig. 2) to compare the fish and decapod fauna leaving replicate forest and mudflat habitats.

We hypothesize that forested sites will be dominated by small benthic forage fishes (e.g., gobies, killifishes) that experience a lower risk of predation within complex intertidal vegetation. Alternatively, we expect deeper mudflat sites lacking vegetation to be dominated by two groups of fishes: water column schooling fishes (e.g., anchovies, silversides), and large roving predators (e.g., subadult snappers, catfishes), both of whose movements will be unimpeded by the structural complexity of stems and roots of mangrove trees.

The statistical design is a repeated measures ANOVA. The dependent variable is catch per unit effort (CPUE), the independent variable is habitat type: catch will be quantified as both numbers and biomass. We are sampling three replicate creeks, each with a forested and a mudflat site. The rivulet sites are fixed and drain an unknown area that varies with both tidal height and with location. Sampling will occur every 2 months for 12-18 months. A major challenge is defining either the area drained by each net, or the volume of water flowing through each net to refine our measurement of catch.

Species composition will be compared between habitat types using an ordination technique, multidimensional scaling (MDS), followed by analysis of similarity (ANOSIM) to ascertain statistical significance of species groupings. Very preliminary analysis of the first data collected in fall 2002 indicates compositional differences in the fish faunas of the two types of intertidal habitats.

Figure 2. Permanent end posts either side of a routinely sampled intertidal rivulet: posts accommodate block net for capturing fish and crustaceans leaving the mangrove forest on an ebb tide. [larger image]

Contact: Silverman, Noah. U.S.Geological Survey; 600 Fourth Street South, St Petersburg, FL 33701, Phone: 727-803-8747, Fax: 727-803-2032, nsilverman@usgs.gov

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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