Figure 1. (above) Average Number of Bathygobius soporator in Rivulet Nets, and Average Salinities at Sites in Shark River, ENP - 2001 and 2002. [larger image]

We have been sampling fishes that directly use tidally flooded mangrove forest habitat along Shark River for three years. We sample every other month using two passive sampling methods: 2 X 3 m² bottomless lift nets and 1 X 1.5 m block nets across the mouths of intertidal rivulets. Lift nets are located within the first 16 meters of fringing forests and yield density data; intertidal rivulet nets are located at the forest/river bank interface and yield CPUE (catch per unit effort) data as they drain an unknown and variable area. Sites are fixed, and located off Tarpon Bay upriver (S2), midway along the Harney River (S4), and downriver near the mouth of Shark River about 3 km upstream of Ponce de Leon Bay (S3).

The 18-month period January 2001 - June 30, 2002 is representative. We captured 25 fish species from 16 families on 8 sampling dates. Average catch per rivulet net was 13.8 fish; average density per lift net was 3.4 fish /6 m². Five families dominated the mangrove forest assemblage. Gobies (frillfin, crested), mojarras (silver jenny, tidewater mojarra), rivulins (mangrove rivulus), anchovies (bay anchovy) and killifishes made up 92 percent of the catch. Introduced species (walking catfishes, pike killifish, cichlids) were rare, as were juveniles of estuarine transient species that spawn offshore, e.g., gray snapper, pinfish, and mullets.

Figure 2. (above) Average Number of Rivulus marmoratus in Lift Nets - Shark River, ENP - 2001 and 2002. [larger image]

Frillfin goby (Bathygobius soporator) and mangrove rivulus (Rivulus marmoratus) were the most abundant species. Frillfin gobies appear to be best suited to higher salinities, showing a monotonic decline with decreasing salinity upstream in Shark River (fig. 1). Mangrove rivulus, once considered a species of special concern in Florida, is common along the entire salinity gradient sampled (fig. 2). This species is the most frequently captured species in our bottomless lift nets. Rather than being rare, it is actually a habitat specialist poorly captured by traditional sampling methods. Unlike other fish species, mangrove rivulus remains in the forest over low tide by a combination of aerial respiration and use of damp refuges, e.g., crab burrows, under wet leaves and wood. Although data analysis is incomplete, three trends are apparent in rivulus distribution along Shark River:

• Individuals tend to be smaller upriver.
• The greatest density occurs at the mid-river location.
• Rivulus have the highest condition factors downriver.

The relative abundance and distribution of these two common species (frillfin goby, mangrove rivulus) can be used as part of a larger monitoring effort to judge the effects of hydrological modifications made upstream in the catchment as part of restoration activities.

Contact: McIvor, Carole C., U.S. Geological Survey Center for Coastal and Watershed Studies, 600 Fourth St. S, St Petersburg, FL 33715, Phone: 727-803-8747, Fax: 727-803-2032, carole_mcivor@usgs.gov

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

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