Carole C. McIvor

A Comparison of Four Enclosure Traps and Methods Used to Sample Fishes in Aquatic Macrophytes

ABSTRACT Sampling efficiency of four enclosure fish traps—a square, mesh-sided 1.0-m2 throw trap; a light weight, square, metal-sided 1.0-m2 throw trap; a heavier, square, metal-sided 1.0-m2 throw trap; and a circular 2.27-m2 drop trap—were compared in two vegetation types in Lake Okeechobee, Florida. Estimates of total fish/m2 for all species pooled, fish/m2 for each of the five most abundant species, and species/m2 were similar among traps. Clearing method (dip nets or bar seine) did not affect estimates of total fish/m2 for the heavier, metal-sided throw trap or the mesh-sided throw trap. Trap clearing and sample picking methods used with the light weight throw trap and 2.27-m2 drop trap required greater field effort. Samples from the 2.27-m2 drop trap required 18 times more laboratory processing time than the average sample taken with a throw trap. Although all traps yielded similar estimates of fish density and species density, a 1.0-m2 throw trap was the most efficient method with regard to field an...

Contrasting Patterns of Habitat Use by Prawns and Crayfish in a Headwater Marsh of the St. Johns River, Florida

Abstract We compared seasonal patterns of habitat use by the prawn Palaemonetes paludosus and the crayfish Procambarus alleni in Blue Cypress Marsh Conservation Area, Florida. Prawn densities were similar to those found in other oligotrophic wetlands of southern Florida, whereas crayfish densities were much greater than reported previously for other wetlands in the area. Prawns and crayfish had strikingly different patterns of habitat use. Prawn density and biomass were similar in wet prairies and sloughs, whereas crayfish density and biomass were significantly higher in wet prairies. Within habitats, the abundance of prawns and crayfish generally increased with increasing structural complexity and the abundance of crayfish generally decreased with increasing water depth. Differences in risk of predation, frequency of agonistic encounters, food availability, and other factors likely contributed to observed patterns of habitat use. Because of differences in their ability to burrow and avoid concentration into dry-season refugia, prawns and crayfish responded very differently to seasonal variation in hydrologic conditions. Prawn densities were initially low (following a severe drought) and then increased during much of the study period, whereas crayfish densities were relatively stable throughout the study period. Overall, it appears that prawns are more responsive to antecedent hydrologic conditions and crayfish are more responsive to the availability of suitable habitats such as wet prairies.

Spatial patterns of fish communities along two estuarine gradients in southern Florida

In tropical and subtropical estuaries, gradients of primary productivity and salinity are generally invoked to explain patterns in community structure and standing crops of fishes. We documented spatial and temporal patterns in fish community structure and standing crops along salinity and nutrient gradients in two subtropical drainages of Everglades National Park, USA. The Shark River drains into the Gulf of Mexico and experiences diurnal tides carrying relatively nutrient enriched waters, while Taylor River is more hydrologically isolated by the oligohaline Florida Bay and experiences no discernable lunar tides. We hypothesized that the more nutrient enriched system would support higher standing crops of fishes in its mangrove zone. We collected 50 species of fish from January 2000 to April 2004 at six sampling sites spanning fresh to brackish salinities in both the Shark and Taylor River drainages. Contrary to expectations, we observed lower standing crops and density of fishes in the more nutrient rich tidal mangrove forest of the Shark River than in the less nutrient rich mangrove habitats bordering the Taylor River. Tidal mangrove habitats in the Shark River were dominated by salt-tolerant fish and displayed lower species richness than mangrove communities in the Taylor River, which included more freshwater taxa and yielded relatively higher richness. These differences were maintained even after controlling for salinity at the time of sampling. Small-scale topographic relief differs between these two systems, possibly created by tidal action in the Shark River. We propose that this difference in topography limits movement of fishes from upstream marshes into the fringing mangrove forest in the Shark River system, but not the Taylor River system. Understanding the influence of habitat structure, including connectivity, on aquatic communities is important to anticipate effects of construction and operational alternatives associated with restoration of the Everglades ecosystem.