Joan A. Browder, NOAA, National Marine Fisheries Service, Miami, FL; Oren Bass, Everglades National Park, South Florida Natural Resource Center, Homestead, FL; Jennifer Gebelein, Steve Huang, Univ. Miami Rosenstiel School of Marine and Atmos. Sci., Miami, FL.

A spatial analysis of Florida Bay is being conducted that will examine biological distributions in relation to physical variables. The data will be viewed and analyzed as layers of geographical information. Layers of physical data in GIS format are being obtained from Everglades National Park and the Florida Department of Environmental Protection. These data include geographic shoreline configuration at the bay boundaries (mainland and Florida Keys), bottom topography, and bank delineations. Sea level data from which to estimate water depths throughout the bay for specific times of the year and times of the day are being computed initially based on ouput from John Wang's finite element numerical model. Data for overlaying salinity contours for representative days will be obtained from U.S. Geological Survey and others. Information on seagrass beds will be added when this is completed in another National Marine Fisheries Service study.

An aerial survey of the large scale and fine scale distribution of wading birds and other large water birds across the bay is providing the first biological layer of the analysis. The aerial survey is a joint environmental project of the National Marine Fisheries Service and the Miami Air Station of the U.S. Coast Guard. The USCG provides the platform, an HH65 Dolphin helicopter, and a pilot and flight mechanic. Everglades National Park is contributing personnel to the study.

Our first hypothesis is that Florida Bay is a significant feeding area for the same wading bird populations that also feed in the mangrove and freshwater areas of Everglades National Park. Our second hypothesis is that daily and seasonal variations in sea level have a major influence on the accessibility of prey to birds in different parts of the bay and in the bay as a whole.

A major objective of survey is to describe the use of the banks and shallow flats of the bay as foraging areas. We want to know how much the aquatic resources of the bay support wading bird populations, the extent to which daily, lunar, and annual cycles of the tide affect energy flow to the birds from the bay, and how the parts of the bay differ in their support value for the various wading and water bird species.

Wading and water birds, because they can be seen from the air, may be useful as indicators of the productivity of the bay. Information concerning bird distributions may help us better understand and compare the various parts of the bay. Determining how access to feeding sites is affected by periodic variation in sea level will improve the ability to interpret information on bird distributions in terms of bay productivity.

The periodic variation in water depth on the banks and flats caused by tide and seasonal changes in mean sea level must have a profound effect on aquatic communities. According to Smith and Pitts, tidal amplitude is as great as 35 cm, and the amplitude of annual variation in mean sea level is about 20 cm. Tidal amplitude is greatest in at the western edge of the bay and diminishes by about 2 cm/km moving eastward. By the time tidal waves reach the interior of the bay, amplitude has diminished so much that daily tidal processes have become inconsequential. Substantial changes in water levels in the interior of the bay do occur, but these are associated with processes that occur over an extended period of time. Because they are able to wade only when water depth is below a certain maximum threshold that roughly corresponds to the lengths of their legs, the activities of wading birds provide a means of viewing these dynamic conditions on a broad scale from a biological perspective.

The presence of birds feeding on the banks allows us to confirm or adjust the predictions of tidal stage at specific locations that we make for our flights on the basis of tidal predictions for Flamingo and Lignum Vitae Key (obtained from commercial computer software), adjusted for other locations by means of Smith and Pitts' graph showing the time lag across the bay of the semidiurnal (M2) component of the tide. With this information we intend to develop the capability to predict wading bird usage of banks in specific areas.

Obtaining monthly estimates of species abundance within the bay to compare with results of ongoing surveys in the coastal and freshwater wetlands of Everglades National Park has become another overall objective of our study, because there has been no recent systematic survey or census of wading and water birds in the bay. Published and unpublished information from past censuses of some wading bird species in the bay will allow us to make comparisons with bay abundances in previous years.

Our aerial survey is a census that covers the bay with 1-nautical-mile-spaced transects. We depart from transects to census birds on islands and to identify birds spotted from a distance. Then the transect is re-entered and along-transect flight continued. Every island within the part of the bay covered by the flight is circled. For logistical reasons, four parts of the bay are delineated. We attempt to cover one or more full quadrants on each flight day and to cover all quadrants each month. We cover the entire bay in 3 to 5 flight days, depending upon weather conditions. The transects are oriented north to south, and flights are initiated in the western part of each quadrant in order to be in phase as much as possible with the eastward progression of tidal stage across the bay.

Tidal stages occur at different times in different parts of the bay because tidal wave propagation is slowed by the shallow banks and islands. Low and high tides occur earliest at the western edge of the bay and progress eastward. Tidal patterns in the bay near the Middle Florida Keys are complicated by the influence of Atlantic tides, which differ from those of the Gulf of Mexico. John Wang's computer model takes into account the bottom friction and extreme constriction resulting from drag due to islands and banks, which cause a phase lag in tidal stage from the southwest boundary northeastward across the bay. The model simulates sea level over the entire bay throughout the lunar cycle and will be very useful in examining our bird data in relation to conditions in the bay at the time of our flights.

In our survey flights, large water birds, including herons, egrets, pelicans, cormorants, and frigate birds, are counted, by species, and their behavior noted. Behaviors include wading or floating in the water, perching on posts or snags over water, roosting in trees, nesting, standing on the ground, standing on the shore at the water's edge, and flying. Birds flying over islands are distinguished from those flying over water.

In summarizing the data or preparing it for spatial analysis, birds loafing or nesting on islands (in trees or on the ground or shore) and on posts, are compiled separately from those wading (and presumably feeding) on interior ponds of the islands or wading (or floating, in the case of cormorants and pelicans) in the bay. The pond and baywater birds also are separated from each other. By compiling the data in this manner, we can examine how the different parts of the bay are being used by the birds at the time of our flights. Nesting birds also will be distinguished separately in recording data so that the temporal and spatial distribution of nesting can be analyzed for each nesting species.

At the present time, 6 months of flights have been made. We hope to continue the study for at least 8 more months to cover the entire year and allow for repeated coverage of the first 2 months, which was a learning and gearing up period. A complete year of observations is important to determine how differences in mean sea level and reproductive activity of the birds affects their use of the banks for feeding. Seasonal changes in mean sea level or nesting activity may also affect the principal islands selected by the birds. We are up to date in entering the data and have developed programs to summarize the data. Thus we are in the process of obtaining quantitative results from the first 6 months of flights.

Our cursory observations suggest that, at least during the summer months, Great White Herons and, to a lesser extent, Great Blue Herons, wade (and presumably feed) on the banks more than any other species. Reddish Egrets, in both red and white phase, have a major presence in the eastermost island chains of the northcentral part of the bay and feed on the shallow banks associated with these islands and also along the shoreline at Snake Bite. Brown Pelicans, Double Crested Cormorants, and, to a lesser extent, Magnificent Frigate Birds dominate the avian fauna in the southcentral part of the bay, roosting in number on the Arsenicker, Buchanan, and Barnes Keys. The cormorants sometimes form large circular rafts of densely packed birds floating on the water. By far the greatest concentration of feeding wading birds that we have seen is on the shallow flats that extend some distance south from the shoreline at Snake Bite. At various times this aggregation has included such species as Great White Herons, Great Blue Herons, Great Egrets, Reddish Egrets, White Ibis, Little Blue Herons, and Roseate Spoonbills. Wading birds are usually seen at low tide on First National Bank in the vicinity of Carl Ross and Sandy Keys, on Nine Mile Bank in the vicinity of the Arsenicker Keys, and on Petersen Bank near Lignum Vitae Key. We have seen Great White and Great Blue Herons feeding far out on banks well removed from islands.

In the future, we hope to acquire information on fish and invertebrate community composition and densities to include as GIS data layer. These data would be examined in relation to physical factors, including the overlap of salinity bands with habitat features such as banks and seagrass beds. Fish data also would be examined in relation to bird densities and feeding, nesting, and roosting patterns.

Frank J. Mazzotti, Department of Wildlife Ecology and Conservation, University of Florida, 3245 College Avenue, Davie, FL 33314; Laura A. Brandt, National Biological Service Cooperative Research Unit, Department of Wildlife Ecology and Conservation, University of Florida, 117 Newins-Ziegler Hall, Gainesville, FL 32611.

The American crocodile is a federally listed endangered species, whose main population center is in an area Florida Bay likely to be affected by C-111/Taylor Slough and other restoration projects. Although the status of the American crocodile has long been a matter of concern it now appears that the population has stabilized in this region. However, as for other species of wildlife in southern Florida, the survival of crocodiles has been linked with regional hydrological conditions, especially water levels and salinities. Alternatives for improving water delivery into Florida Bay via Taylor Slough and the C-111 system may change salinities and water levels in the receiving water bodies. The purpose of the American crocodile research and monitoring program is to insure the continued survival of an endangered species in a changing environment.

In the Taylor Slough/C-111/Florida Bay system a successful endangered species research and monitoring program should investigate population parameters likely to be affected by alternatives proposed for ecosystem restoration. For crocodiles population parameters most responsive to hydrological conditions are distribution, growth, survival, and nesting effort and success.

The objectives of this project are:

1. To monitor nesting effort (number of crocodiles that attempt to nest) and success (number of nests that hatch) of the American crocodile in Florida Bay.

2. To determine the patterns of growth and survival of crocodiles from nests from different locations and habitats.

3. To evaluate the relationship between salinity and distribution of non-hatchling crocodiles.

Crocodile nesting effort and success will be determined during 1994 and 1995 by searching known and potential nesting habitat in Florida Bay during April and May (effort) and July and August (success) for activity ( tail drags, digging or scraping) or the presence of eggs or hatchlings. When nests are located and their vegetation, substrate, distance from shore, dimensions (lxwxh) and salinity of adjacent waters are recorded. Hatched eggshells or hatchling crocodiles are evidence of successful nests. The number and causes of egg failure are noted whenever possible.

Growth and survival of crocodiles will be assessed by capturing and marking them during nest surveys, followed up with periodic efforts at recapturing them. Over 700 crocodiles have been marked in Everglades National Park (over 2000 in south Florida). Recapture of crocodiles tagged from previous studies will yield valuable information on long term growth and survival of crocodiles.

A salinity based habitat model was developed based on data on salinity and habitat relations of crocodiles in Everglades National Park. Overall crocodile habitat in northeastern Florida Bay was defined as the mangrove and coastal prairie fringe and offshore islands. Within this area suitability of the habitat was based on salinity levels with the most suitable habitat defined as between 0-10 ppt, intermediate suitability as 11-30 ppt, and the least suitable areas as over 30 ppt. Seasonal isohalines were obtained from monthly summaries of water quality data provided by the South Florida Water Management District.

An Everglades National Park record twenty nests were located in 1994 and new record of 21 nests were located in 1995 in an area between Cape Sable in western Florida Bay and Trout Cove in northeastern Florida Bay. Fourteen nests were successful in 1994, with raccoon predation responsible for the loss of three nests and early embryonic mortality indicated in the failure of four nests. Nine nests were successful in 1995. Nine nests were depredated by raccoons, two were lost to flooding and the cause of failure undetermined in one nest. The 1994 and 1995 nesting seasons were notable not only for the record number of nests in both years, but also in the discovery of crocodile nests on Cape Sable by Everglades National Park Ranger Lora Peppers. The last (and only) record for crocodile nesting on Cape Sable was by Willoughby in 1897.

Ninety-two hatchlings were marked in 1994 and 53 were marked in 1995. Recaptures of these animals during the 1995-96 dry season will form the basis for the assessment of growth and survival.

The following results can be interpreted from the salinity based habitat model:

1. More freshwater (lower salinity) in northeastern Florida Bay increases the amount and suitability of crocodile habitat.

2. Flows through Taylor Slough (rather than C-111) provide more and better crocodile habitat.

3. Under current conditions most suitable crocodile habitat occurs closer to C-111 drainage area than Taylor Slough.

Two predictions based on the results of this model are important.

First, we predict that crocodiles should occur most frequently in areas with the most suitable habitat. Seventy-seven percent (n=22) of the crocodiles captured in northeastern Florida Bay during 1987-1988 were captured in an area in and around to Joe Bay. Under most hydrological conditions this area contains the most suitable habitat in proximity to known nesting areas.

The second prediction, or really recommendation, is that freshwater flows directed down Taylor Slough will have the greatest effect in improving the amount and suitability of crocodile habitat. An associated prediction is that the frequency of crocodile sightings in the Little Madeira/Mud Bay area should increase.

We hypothesize that increased freshwater flow into Florida Bay through Taylor Slough (rather than C-111) would have beneficial effects on American crocodiles. Our analyses suggest that unless hydrological conditions are changed dramatically nesting effort and success will not be adversely affected, while the amount and suitability of habitat for crocodiles would be increased. Our habitat model predicts that Taylor Slough flows also would provide more suitable habitat in closer proximity to known nesting areas. We hypothesize that this would increase the growth and survival of hatchling crocodiles. This hypothesis can be tested by continuing to assess the relationship between growth and survival of crocodiles and hydrological conditions.

Current studies are scheduled to terminate November 1996.

William B. Robertson, Jr. , Everglades National Park/National Biological Service; and, Oron L. Bass, Jr., Everglades National Park.

Data on the Bald Eagles (Haliaeetus leucocephalus) and Ospreys (Pandion haliaetus) inhabiting Florida Bay, initially collected because of concerns about chlorinated pesticides, may also provide insights on present ecological perturbations in the Bay. Bald Eagles and Ospreys are major avian predators in the Florida Bay system. Both have substantial populations, both nest in the winter months, and the breeding adults of both appear to be essentially resident in the Bay. Both species are predominantly fish-eaters, but Eagles, in addition, prey to lesser extent on water-birds and diamondback terrapins (Malaclemmys), and consume carrion of whatever origin when it's available.

The population database for Bald Eagles consists of annual records of nesting activity and young produced by the entire Florida Bay population. These records are derived from monthly aerial surveys of the 30 or so known nesting territories in Florida Bay (beginning in October/November) continued until the results of nesting in a given season are known with certainty. Nests are visited on the ground whenever aerial observations are ambiguous. This record of the Florida Bay population is continuous from the nesting season of 1959-60 through that of 1994-95, except for the years 1984-85 and 1985-86 when funding failed utterly. The population database for Ospreys consists of whole-Bay air and ground counts of active nests made in eight seasons from 1968-69 to 1994-95.

Based on the above data, the breeding population and productivity of Bald Eagles in Florida Bay exhibited remarkable stability over a 32-year period of record that embraced many of the ecological extremes to which the Bay is subject. Conversely, the Bay count of active Osprey nests declined by 72.5 percent over the period of record, most of the decline apparently occurring in the 1970s. Other aspects and implications of these databases will be discussed.

Figure 1. Comparative productivity rates of primary producer components in Florida Bay. All values are the mean of at least five replicates, standard error is indicated by error bars.

Barbara A. Schroeder, Blair E. Witherington, Allen M. Foley, Florida Department of Environmental Protection, Florida Marine Research Institute.

Introduction

Distribution, abundance, and seasonality of marine turtles on nesting beaches in Florida are well documented and long-term standardized surveys for nesting population monitoring are in place. In contrast, relatively little is known about the ecology and migrations of turtles in Florida waters. Information on the occurrence of marine turtles in Florida Bay is sparse. Dr. Archie Carr described Florida Bay as the center of abundance for Kemp's ridleys in the Gulf of Mexico and provided an account of the capture (for shark bait) of three species - the loggerhead, green turtle, and Kemp's ridley at Sand (Sandy) Key in 1941. Although much attention has been focused recently on the Florida Bay ecosystem, marine turtles have been essentially overlooked, despite their endangered status and their position as upper-trophic level consumers. Subsequent to Carr's account, no published accounts of marine turtles in Florida Bay can be found prior to that of Schroeder and Foley (1992) describing the initiation of marine turtle studies in Florida Bay in 1990. The Science Plan for Florida Bay recognizes living resources as a major research area and outlines six key research needs for marine turtles. In addition to the relevance of our study to the Florida Bay Science Plan, this research also responds directly to specific recommendations made by The National Research Council in their report entitled "Decline of the Sea Turtles: Causes and Preventions." The report addresses the critical need for long-term in-water studies such as this one. Likewise, the Federal Recovery Plans for the loggerhead, green turtle, Kemp's ridley, and hawksbill clearly emphasize the need for studies of developmental, foraging, and migratory habitats, all of which characterize the significance of Florida Bay to marine turtles. The primary objectives of our study are 1) to determine the species composition, population structure, sex ratio, genetic identity, and seasonal and spatial distribution of marine turtles in Florida Bay; 2) to describe daily and seasonal activity patterns and habitat use for loggerhead and green turtles by conducting visual tracking and VHF radio, sonic, and satellite telemetry; 3) to evaluate health status of marine turtles in Florida Bay by establishing blood chemistry profiles; and 4) to describe the occurrence of fibropapilloma disease and the species composition, population structure, and spatial distribution of afflicted turtles.

Methodology

Sampling is conducted monthly from May through September and quarterly from November through April. Our primary study sites are located in the western portion of Florida Bay. Turtles are captured by several methods depending upon water clarity, tidal conditions, and type of habitat. Capture methodologies include large mesh stationary tangle nets, large mesh tangle nets drifted with the current, and hand capture by snorkelers diving from boats near turtles or swimming in the water behind turtles. Captured turtles are brought on board and measured, weighed, flipper tagged, externally examined, and photographed. Blood samples are collected from the dorsal cervical sinus and are used to determine the sex of immature turtles through radioimmunoassay, to determine genetic identities through mtDNA analysis, and to establish blood biochemical profiles as a measure of physiological status. Detailed data on number, size, and location of external tumors are collected from turtles afflicted with fibropapilloma disease. Tumor samples are collected from selected individuals. Prior to release, an individual carapace scute is spray painted with quick-dry enamel to enable in-water identification to prevent recapture by hand during the sampling period. Capture locations are determined using GPS. Environmental data collected at each capture location include water temperature, salinity, water depth, and bottom characteristics. Sightings of turtles that are not captured are recorded and geo-referenced.

To study the movements and habitat use of a subsample of turtles in Florida Bay we use VHF radio, sonic, and/or satellite transmitters. VHF radio and sonic transmitters are attached to the posterior marginals of the carapace and are designed to operate for a period of 12-18 months. Satellite transmitters are a back-pack design and are attached to the carapace using fiberglass cloth and polyester resin. The expected battery life of the satellite transmitters is 5-11 months.

Results

We captured 187 turtles between June 1990 and August 1995. The species composition is as follows: 142 (76%) loggerheads (Caretta caretta), 44 (24%) green turtles (Chelonia mydas), and one hawksbill (Eretmochelys imbricata). Seventy-nine percent of the turtles were captured by hand and 21% were captured in large-mesh tangle nets. Most turtles (68%) have been captured in Rabbit Key Basin and in the channels crossing Nine Mile Bank. Fifteen percent of our turtles were captured in Man of War Channel and Iron Pipe Channel and 5% of total captures were made in Twin Key Basin. Water clarity is a determining factor in visually searching for and hand capturing turtles and, as such, our capture locations have shifted to areas where clear water is most commonly encountered. Exploratory sampling in the Shark River area and near East and West Bahia Honda Keys has resulted in turtle sightings and some captures.

Loggerheads ranged in size from 48.9cm to 98.7cm standard carapace length straight line (SCL), with a mean length of 80.4cm. Thirty percent of the loggerheads were males and 5% were females based on the externally observable characteristic of tail length. Because tail length may not be a reliable indicator of sex in maturing animals, we do not assign sex unless the SCL is greater than 90.0cm or the tail is clearly elongated beyond the posterior tip of the carapace, indicating a male. Of the males identified by tail length, 52% were less than 90.0cm (SCL). Green turtles ranged in size from 25.5cm to 62.9cm; no adults were captured or sighted. Mean SCL was 46.9cm. The only hawksbill captured was also immature and measured 38.2cm SCL. Sex ratios determined through radioimmunoassay will enable a calculation of the sex ratio for all life history stages represented in Florida Bay.

The prevalence of green turtle fibropapilloma disease (GTFP) in Florida Bay is alarming. Sixty percent of captured green turtles are afflicted. Of urgent concern is the 11% GTFP prevalence in loggerhead turtles, an unprecedented rate of occurrence in this species.

Our recapture rate in Florida Bay is 7%. Recapture intervals range 20 - 1,013 days. Adult male, adult female, and immature turtles have been recaptured. Loggerhead recapture locations have been in close proximity to the original capture locations. Only one green turtle has been recaptured in Florida Bay. The recapture interval was 357 days and the turtle was captured in the same fork of the same channel crossing Nine Mile Bank. We have received notification of three long-distance recaptures of turtles we tagged in Florida Bay. All three recaptures, two green turtles and one loggerhead, were captured in Cuban waters,several years after their initial capture in Florida Bay.

Three loggerheads, captured in Rabbit Key Basin, are currently outfitted with radio and sonic transmitters to facilitate the study of local movements and habitat use. Location data are collected at least once each month and all three turtles have remained in the vicinity of Rabbit Key Basin.

In collaboration with the National Marine Fisheries Service Charleston Laboratory, blood profile assessments are underway and genetic analyses will begin in early 1996. Satellite telemetry of adult male loggerheads will be initiated this winter. A long-term approach to most aspects of our research is essential due to the complex life history and longevity of these species.

Last updated: 07/16/98
by: Monika Gurnée
gurnee@aoml.noaa.gov