Proposed Experiment on The Effects of Microtopography and Deep-water Refugia on Fish Concentrations in the Loxahatchee Impoundment Landscape Assessment (LILA) Research Facility
Geoffrey B. West, and Dale Gawlik
South Florida Water Management District, West Palm Beach, FL
One of the leading explanations for population declines of wading birds in the Everglades is the change in prey availability as a result of changes in hydropatterns. There is more to prey availability than the density of prey. Prey availability is defined as a composite variable consisting of prey density and the vulnerability of the prey to capture. Prey availability is linked to the seasonal drydown in water levels, but there is very little understanding of the specific conditions that produce patches of highly available prey. We hypothesize that microtopography (i.e. ridge and slough) plays a key role in determining the spatial and temporal arrangement of high quality prey patches. If true, then any changes in the historic microtopography of the Everglades would have had serious implications for wading bird foraging. The objective of this study is to determine the effects of microtopography and deep-water refugia on small-scale fish densities. To accomplish this, we plan to investigate the formation of these highly available food patches by evaluating prey response to seasonal drydown. By manipulating the water depth and measuring fish movements and fish density, we will be able to determine the extent of movement among fish, the degree of exchange between ridge and slough communities, and the concentrations of high-density patches formed during the drydown. With these studies, we will test our hypotheses that fish community composition, physical features of the marsh, and hydrologic factors have an effect on maximum prey density (i.e., the magnitude of prey concentration). In addition, we hypothesize that during drydown events, the exchange of fish from areas of higher elevations such as ridges to areas of deeper water found in the sloughs will create small-scale high-density patches. The alternative is to remain on the ridges and be subject to desiccation and/or capture.
These experiments will be conducted in the Loxahatchee Impoundment Landscape Assessment (LILA) research facility located on the premises of the Arthur R. Marshall Loxahatchee National Wildlife Refuge in Palm Beach County, Florida. The facility consists of four 7-hectare macrocosms constructed to mimic the key physical features of the Everglades landscape.
Each macrocosm will receive the same hydrologic treatment (flow rate and water depths). The microtopography treatment will come from elevation differences in ridges and sloughs and in alligator holes. Each macrocosm will contain a shallow and deep slough. Each slough will contain a shallow and deep hole 6-m in diameter designed to simulate alligator holes and provide deep-water refuges for fishes. In addition to the holes, the deep slough will contain two tree islands 14 m x 49 m in size and 0.91 m higher than the slough bottom.
Macrocosms will be randomly selected to receive either a shallow or deep refugia treatment. Holes at the depth that was not selected as a treatment for a given macrocosm will be encircled with impervious plastic material to keep aquatic animals from reaching them. Each macrocosm will be further divided with mesh fence into two subplots for the microtopography factor.
The plastic fence with a mesh size of 1 cm2 will be erected so that it bisects lengthwise the widest ridge in each macrocosm, thus providing two microtopography treatments within each macrocosm. Fish that are too small to be preferred prey for wading birds (<4 cm in length) will pass through the mesh and will be monitored but will not be the focus of statistical analyses. One microtopography treatment will be a slough and ridge elevation difference of 20 cm and the other will be a slough and ridge elevation difference of 35 cm. Initial fish population densities in each plot will be controlled through removal or stocking, and final fish densities will be adjusted for the area of slough and ridges within plots. Fish densities will be measured in each slough weekly using throw-traps (Kushlan 1974b, Jordan et al. 1997). Sloughs will be delineated with flagging into 20 m segments. Initially, one throw trap sample will be taken randomly within each segment during each sampling period. The number of samples may be increased depending on the variability in samples.
Fish movements will be measured using radio telemetry and individual fishes implanted with radio transmitters. This will answer the question of how individual fishes move to and from the high-density prey patches preferred by birds. Radios have been used successfully to track movements of fishes in the southern Everglades (J. Trexler, Florida International University, pers. Comm.). Fifty sunfishes (Lepomis sp.) will be implanted with radio transmitter and released into randomly selected plots. Additional radios will be added as transmitters fail or are lost. The transmitters weigh approximately 1.5 g and will be inserted in the body cavity of fishes approximately 30 g in mass (100-150 mm in length) using established techniques that conform to accepted guidelines for the use of fishes in field research (Nickum 1988). Transmitter weight will be 5% of body weight. Longevity of transmitter battery life and transmitter range is inversely related. Using a target battery longevity of two months, transmitter range is expected to be approximately 100 m, accounting for heavy vegetation and 50 cm of water depth. Actual range may be greater because of the open structure of sloughs and the shallower water depths for much of the dry season. The location of all telemetered fishes will be monitored weekly from the perimeter levees using a hand-held Yagi antenna. Semi permanent antennas may be established later depending on the range of transmitters. It also may be necessary to supplement readings from the levee with those taken in the marsh. To determine short-term movement patterns and ensure that daily locations are indicative of diurnal movements, a random telemetered fish from each plot will be monitored hourly for one 24-h period each month.
The experiment is designed as a standard three-factor split-plot with impoundment as the replicate factor, refugia depth as the main-factor, and microtopography as the subplot factor. The response variable will be maximum fish density during the dry season. The experiment will be repeated in subsequent years.
Thus far we have tested the transmitter range to an acceptable distance of approximately 100-m. We are currently conducting a laboratory experiment to determine if the transmitters affect the behavior of the fish. To achieve this, we will use time activity budgets to analyze behavioral differences in the sunfish before and after implantation of the transmitters. We will begin the fish concentration studies during the 2004 dry season.
West, Geoffrey B., South Florida Water Management District, 3301 Gun Club Road, West Palm Beach, FL, 33406, Phone: (561) 681-2500, Ext. 4543, Fax: (561) 681-6310,
Gawlik, Dale, South Florida Water Management District, 3301 Gun Club Road, West Palm Beach, FL, 33406, Phone: (561) 681-2500, Ext. 4539, Fax: (561) 681-6310, .