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Impacts of Hydrological Restoration on Three Estuarine Communities of the Southwest Florida Coast and on Associated Animal Inhabitants

Carole C. McIvor, Thomas J. Smith III, Michael B. Robblee, and Lynn Lefebvre

A primary goal of Everglades restoration is the recreation of water flows and water quality more closely approximating pre-drainage conditions in both freshwater and estuarine ecosystems within Everglades National Park. These estuarine systems include submerged aquatic vegetation, mangroves (tidal forests), and brackish marshes. Three primary groups of animals are closely associated with, and often dependent upon, one or more of these ecosystems: fish and decapod crustaceans (shrimp, crabs), manatees, and wading birds. Herein we focus on fish and decapod crustaceans, and manatees. This study focuses on how hydrological changes upstream affect: (1) the distribution, abundance and composition of submerged aquatic vegetation and selected animal inhabitants; and (2) the distribution and abundance of fish and decapod crustaceans associated with mangroves and brackish marshes.

Submerged aquatic vegetation (SAV) is an integral part of many shallow-water estuarine and coastal systems worldwide. Such vegetation (generally termed seagrass when it
Close-up photograph of a mangrove rivulus.
Figure 1. Close-up photograph of mangrove rivulus (actual size of fish is two inches). Click for larger image. Photo was taken by D. Scott Taylor.
occurs in near-marine salinities) provides many benefits to society including sediment stabilization, habitat for estuarine animals including manatees, and direct and indirect support of commercial and recreational fisheries. Very little is known of the submerged aquatic vegetation of the southwest coast of Florida and the associated rivers draining the Ten Thousand Islands where riverine water flows are projected to change as a result of Everglades Restoration. Importantly, the species composition and standing stocks of SAV (and macroalgae) appear to be quite sensitive to salinity variation such as that caused by seasonal and anthropogenic changes to freshwater inflow.

West Indian manatees are endangered aquatic mammals that inhabit the coastal rivers, canals, and estuaries on both coasts of South Florida year-round. Because manatees are reliant on submerged aquatic vegetation for feeding, it is likely that manatee distribution, relative abundance, habitat use, and movement patterns will change as a result of altered water management regimes and resulting changes in nearshore salinity. These issues are being addressed through (1) the development of a spatially explicit, individual-based model that will predict manatee response to different restoration scenarios, and (2) comprehensive field studies in southwest Florida that provide data for the model and that document the current distribution and status of the manatee population prior to implementation of restoration activities.

Concern about the fate of mangrove ecosystems derives from their known use as habitat for a wide range of both terrestrial and aquatic animal species, especially fishes and decapod crustaceans of forage as well as of commercial and recreational importance. Additionally, mangroves at the mangrove/marsh boundary in southwestern Everglades National Park were historically the sites of extensive colonial wading bird nesting areas. Birds apparently foraged and fed their young primarily from adjacent brackish marshes. One of the goals of Everglades Restoration is return of extensive wading bird rookeries to these headwater mangroves. It is therefore essential that we understand the dynamics of fish (and decapod crustaceans) in mangrove and adjacent marsh habitats in order to judge the effectiveness of restoration of the forage base necessary to support wading bird rookeries.

Since project initiation in April 2000, study sites have been chosen and instrumented in all Shark River locations and in one-third of the mangrove locations in Lostmans River. Brackish marshes at mid- and upstream locations are being sampled. Submerged aquatic vegetation surveys and preliminary mapping are underway. Aerial surveys for manatees are being conducted, and an individual-based population model is being developed. Analogous sampling is underway in Lost-mans River; however, Everglades restoration is likely to change the quantity and timing of freshwater inflows to this river far less than it will change freshwater inflows to Shark River.

Bar chart showing the verage number of mangrove rivulus in fringing forests along the Shark River.
Figure 2. Average number of mangrove rivulus in fringing forests along the Shark River. Click for larger image.

In the mangrove forests of Shark River, we have performed pilot studies were performed to determine the best design for capturing fishes and decapod crustaceans using flooded forests. A two-pronged approach was applied to this technical challenge. We are using 2X3 m2 pull-up nets buried in the forest floor are being used to quantify the density, biomass and population dynamics of a unique mangrove fish, mangrove rivulus, that remains in the forest even during low tide events. The remainder of the fish and decapod assemblage is being targeted with 1.5X1.0 m block nets placed over the mouths of intertidal rivulets at slack flood tide. These nets capture fishes and crustaceans leaving the forest on the ebb tide. Results are expressed as catch per unit effort (CPUE). Future plans call for development of a curve of stage height versus area inundated for each block net location so that results may be expressed as density and biomass per m2 . Currently, there are nine nets of each type arrayed along the estuarine salinity gradient as follows: three of each type near the freshwater/oligohaline interface in Tarpon Bay, three each midway along the salinity gradient on the Harney River, and three each about 3 km up from the mouth of the Shark River near Ponce de Leon Bay, our site of highest (near marine) salinity (fig. 1). Nets are being sampled bi-monthly to capture patterns of juvenile recruitment and changes in relative abundance of species influenced by wet and dry season changes.

Only very preliminary results are available. Despite the fact that mangrove rivulus is being considered for listing, it is quite abundant in fringing mangroves along the entire Shark River gradient investigated. Decapod crustaceans routinely using flooded mangrove forests include three species of caridean shrimp and less commonly, blue crabs. A range of estuarine fishes has been captured including schooling silversides and anchovies, two species of killifishes, and gerrids of the genus Eucinostomus. Less common are predaceous toadfishes and needlefishes, and the exotic Mayan cichlid. Two species of gobies are present, one of which (Bathygobius soporator) is sufficiently abundant and widespread that it might prove a useful integrator of broad scale salinity patterns. Interestingly, young-of-year gray snapper are present although in low numbers. This finding contrasts with fringing mangroves in the Taylor Slough drainage that contain only subadult and adult gray snapper. Continued sampling will elucidate the extent to which these forests are an important nursery area for this recreationally important species.

Table 1. Fish species captured in fringing mangrove forests along Shark and Harney Rivers. Lift net data for March - July 2000; rivulet net data for May - July 2000. S2 is on Tarpon Bay and is <5 ppt salinity; S4 is 5.1 - 18 ppt midway along the Harney River; S3 is ca 3 km upstream from Ponce de Leon Bay on Shark River and is near marine salinity.
  Site Number: S2 S2 S4 S4 S3 S3  
  Type of net used: Lift Rivulet Lift Rivulet Lift Rivulet  
   Genus Species
Common Name             Total
   Anchoa mitchilli (Valenciennes, 1848) bay anchovy 0 0 0 0 0 30 30
Clariidae-labyrinth catfishes
    Clarius batrachus (Linnaeus) walking catfish 0 0 0 2 0 0 2
    Opsanus beta (Goodes and Bean) gulf toadfish 0 0 0 0 0 1 1
    Strongylura timucu (Walbaum, 1792) timucu 0 0 0 0 1 0 1
    Strongylura spp.   0 1 0 1 0 1 3
    Rivulus marmoratus (Poey, 1880) mangrove rivulus 23 0 53 0 41 0 117
    Fundulus confluentus (Goode and Bean 1879) marsh killifish 2 22 0 1 0 2 27
    Fundulus grandis (Baird and Girard, 1853) gulf killifish 0 2 1 5 0 0 8
    Fundulus spp.   0 32 0 1 0 1 34
    Lucania parva (Baird and Girard, 1855) rainwater killifish 1 9 0 11 0 5 26
    Belonesox belizanus (Kner, 1860) pike killifish 0 3 0 1 0 0 4
    Gambusia holbrooki (Girard, 1859) mosquitofish 0 1 0 2 0 13 16
    Poecilia latipinna (Lesueur, 1821) sailfin molly 6 0 5 0 0 0 11
    Menidia beryllina (Cope, 1866) inland silverside 0 113 0 2 0 0 115
    Lutjanus griseus (Linnaeus, 1758) gray snapper 0 0 0 2 0 1 3
    Eucinostomus harengulus (Goode and Bean, 1879) tidewater mojarra 0 75 0 0 0 7 82
    Eucinostomus spp.   2 37 0 147 9 81 276
    Lagodon rhombodies (Linnaeus, 1766) pinfish 0 0 0 2 0 0 2
    Cichlasoma urophthalmus (Gunther, 1862) Mayan cichlid 2 3 2 7 0 0 14
    Bathygobius soporator (Valenciennes, 1937) frillfin goby 0 0 2 6 6 136 150
    Lophogobius cyprinoides (Pallas, 1770) crested goby 0 0 0 0 4 31 35
TOTAL NUMBER OF FISH COLLECTED    36  298  63  190  61  309  957
NUMBER OF NETS SAMPLED    12  9  12  9  18  9  69
NUMBER OF DATES SAMPLED    4  3  4  3  6  3  

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report (PDF, 8.7 MB))

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