Marine Endangered Species

1996 Abstracts

A Monitoring Program for the Endangered American Crocodile in Florida Bay

Frank J. Mazzotti, University of Florida, Department of Wildlife Ecology and Conservation, Belle Glade, FL; and Oron (Sonny) Bass, Everglades National Park, Homestead, FL.

The American crocodile is a federally listed endangered species, whose main population center is in an area of northeastern Florida Bay likely to be affected by C-111 and Taylor Slough 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. To insure the continued survival of an endangered species in a changing environment it is important to monitor the population.

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

The objectives of the crocodile monitoring program were:

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

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

Crocodile nesting effort and success was determined by searching known and potential nesting habitat in Everglades national Park 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 were located and their vegetation, substrate, distance from shore, dimensions (lxwxh) and salinity of adjacent waters were recorded. Hatched eggshells or hatchling crocodiles are evidence of successful nests. The number and causes of egg failure was noted whenever possible.

Growth and survival of crocodiles was assessed by capturing and marking them during nest surveys, followed up with periodic efforts at recapturing them. Over 800 crocodiles have been marked in Everglades National Park since 1978 (over 2500 in south Florida). Recapture of crocodiles tagged from previous studies will yield valuable information on long term growth and survival of crocodiles. Twenty nests were located in 1994 (70% successful, 20% depredated by raccoons and 10% failed for unknown reasons) and 21-23 in 1995 (43-47% successful, 30-33% depredated, 17% flooded and 10% failed for unknown reasons).

Ninety-three hatchlings were marked in 1994 and fifty-four were marked in 1995. Six non- hatchling crocodiles were captured.

It is important not to over interpret two years worth of nesting data. The number of nests continues to increase, but nesting success (proportion of nests that produce at least one hatchling) took a notable plunge in 1995. Is this natural variation or the beginning of a trend that we should be concerned about? Increased nest failure due to flooding is not surprising during a wet year. Increased nest predation could be due to an unrelated reason or perhaps, could be the result of a concentration of raccoons avoiding flood conditions by moving to higher ground where crocodiles nest.

It is worthwhile to review aspects of crocodile nesting biology as they relate to the moisture content of nests. The moisture content of nests, including catastrophic flooding events is most dependent on local rainfall. However, high ground water levels near creek nest sites have been correlated with upstream water discharges into C-111. Although the sudden rise in water level that floods a nest is caused by rainfall, the frequency at which rain events flood nests may be increased because of high water levels caused by upstream discharges. This may have happened in 1995. Whatever the causes, the dramatic increase in nest failure was concentrated in northeastern Florida Bay in an area likely to be influenced by C-111/Florida Bay restoration plans.

Although useful as a general indicator of the health of a crocodile population, monitoring nesting success alone is not adequate to evaluate effects of hydrological changes. Adding nesting effort (locations of all nests, and hence failed nests) to nesting success is more indicative of the effects of alternative water delivery plans, and is the minimum level of monitoring that should be undertaken. The shortcoming of using the number and location of nests as the only measure of crocodile population responses is that some effects may not become evident for several years. Growth and survival of hatchling crocodiles, and the distribution of all size classes of crocodiles would be more sensitive measures of population responses to experimental water deliveries.

This is particularly relevant here. We hypothesize that increased freshwater flows into northeastern Florida Bay could increase growth and survival of hatchling crocodiles. It is possible that an increase in survival could more than compensate for any loss of successful nest nests (high water related or not). There are little field data to support this hypothesis. We will continue to capture crocodiles to address this issue.

Last updated: 2/26/98
by: Monika Gurnée
gurnee@aoml.noaa.gov