Introduction

Nest monitoring studies at the Everglades National Park have shown that nest production and reproductive efficiency of the alligators are closely correlated with dry-season water levels in the park(1). It is not known what the effects of water level are on prey availability, nutrient value of the prey or the resulting nutritional effect on alligator egg quality and growth potential of hatchlings. Previous work has shown that critical nutrients in the diet of the alligator effect egg quality, hatchling growth performance and specific reproductive parameters(2,3,4). A first step in evaluating an alligator population is the determination of egg quality because good egg quality is necessary for high reproductive potential. Growth performance of the hatchlings is also necessary in order to fully evaluate egg quality.

Methods

A random sample of 119 eggs from 31 different clutches was collected in the Southern Everglades (SEV-eggs). We collected an estimated 10% of the eggs in a clutch (projected clutch size was 30 eggs) from a random sample of 15% of the approximately 200 clutches projected for the study area for 1995. Eleven (11) eggs from the sample were determined to be infertile or had very early embryonic death. Thirteen embryos were either dead on arrival or died during incubation. The remaining 95 eggs were placed in trays of moist sphagnum moss and the trays incubated at 31 C in a low temperature incubator. A control sample of 3 infertile and 12 fertile eggs from each of 12 clutches was collected from the Rockefeller Refuge in Southwest Louisiana. Conditions of incubation, egg treatments and hatching techniques have been described(2,3,4,5). The following measurements and calculations were made on the infertile eggs: 1) egg length; 2) egg width; 3) egg weight; 4) egg volume; 5) calculated egg volume 6) volume error (% deviation of calculated volume from the measured volume); 7) length/width ratio; 8) shell weight; 9) shell thickness; 10) shell density; 11) yolk weight; 12) membrane weight; 13) % yolk; 14) % white; 15) % shell; 16) % membrane. The stage of development at the time of embryonic death was determined for embryos that were dead on arrival or died during incubation by the use of a developmental stage chart showing normal embryo development for incubation techniques used in our laboratory (6). Growth potential of the hatchlings was determined by their growth performance at low stocking densities in a temperature controlled house with daily feeding to satiation of a high performance diet (2,3,4). The following measurements or calculations were made on hatchlings: 1) day-1 length (all lengths are total lengths); 2) day-56 length; 3) day-99 length; 4) day 1 weight; 5) day 56 weight; 6) day 99 weight; 7) hatchling yield from the egg (% of the egg represented by hatchling weight at day-l); 8) day-1 condition factor; 9) day-56 condition factor; 10) day-99 condition factor; 11) 56 day growth rate (inches/day); 12) 56 to 99 day growth rate; 13) 99 day growth rate; 14) day 56 performance; 15) day 99 performance. Descriptive statistics including a stem-and-leaf plot (or a horizontal bar chart), a box plot and a normal quantile plot were made using the SAS system for OS2.

Results

Hatch rate for the total SEV-egg sample (119 eggs) was 79.8 % and 88% for fertile eggs (108 eggs). Hatch rate for the 144 fertile control eggs from Louisiana was 88%. Hatch rate for fertile eggs from Florida lakes and the Rockefeller refuge usually range from 75% to 92% in our laboratory. Separate normal quartile plots of egg size and other parameters for SEV eggs and control eggs were approximately normal. Median egg size of the SEV-sample was 60 cc and the mean value 60.2 cc. The SEV-eggs were significantly smaller than the Louisiana control eggs which had a median value of 75 cc and a mean value of 75.1 cc. The shape of the SEV-eggs based on egg-length, egg width and ratios of length/width (1.71) was significantly different from the Louisiana eggs (1.69) (Table I). All parameters used to measure egg quality for the SEV-eggs were significantly different from the control eggs with the exception of "developmental stage at embryonic death" and "hatchling yield from the egg" (Table I). Mean hatchling yield from the egg for SEV-eggs was 75.2% and 69.0% for Louisiana eggs. Mean length at hatch was 22.7 cm for SEV-eggs and 24.2 for controls. The mean length for SEV-hatchlings after 56 days of culture was 46.8 cm (an increase of 24.1 cm) compared with 46.5 cm (an increase of 22.3 cm) for the Louisiana control hatchlings. The 99-day length for SEV-hatchlings was 54.3 cm (an increase in length from day-1 of 31.6 cm) which was significantly smaller (5.2%) than controls which were 57.3 cm in length (an increase of 33.1 cm from day-1). There were no significant differences in growth performance parameters for SEV-hatchlings and controls except for hatchling length at day 99. (Table 2)

Discussion

We conclude that in 1995 SEV-eggs were significantly different in shape and smaller than the Louisiana control eggs. Differences in shape and composition were minor but the SEV-eggs appear to be smaller than any comparable sample of eggs from Florida or Louisiana that have been examined by the laboratory over the past decade. However, hatchling yield and hatchling quality appear to be similar to the Louisiana control eggs and eggs from Florida lakes for the first 99 days of culture. Although there are statistically significant differences in the 99 day lengths of SEV- and control hatchlings the magnitude of the differences was small (the SEV hatchlings were 5.2% smaller than controls). The culture conditions used (density, temperature control and high performance diet) produced a fast-growing, uniform-sized hatchling. Low variance in growth rate permits the detection of relatively small differences in hatchling length between groups. The length of SEV-hatchlings at day-1 was 6.2% smaller than the control hatchlings and there was no significant difference in growth rate parameters. The significant difference in length at day-l resulted from SEV-hatchlings coming from significantly smaller eggs.

The 99-day growth rate of SEV-hatchlings under intensive culture was not significantly different from the Louisiana controls; however, it was 8.5 times greater than rates reported for wild hatchlings from the Southern Everglades (6,7). Increase in total length is reported to be linear at 13.6 cm per year for the first 4 years for different areas in the Southern Everglades (6). The SEV-hatchlings had increased 24.1 cm in length after 56 days of intensive culture which is equal to the 1.77 years (24.1/13.6 = 1.77) of wild-growth in the Southern Everglades. The increase in length for the SEV-hatchlings intensively cultured for 99 days was 31.6 cm or equivalent to over 2 years of wild-growth in the Southern Everglades.

We conclude from our results that the 1995-hatchlings in the Southern Everglades had normal growth-potential; although they may never show this potential because of "resource limitation" reported for the area (6). Everglades alligators are reported to "show slow growth and mature at a smaller size, at a greater age" (6). The Southern Everglades is reported to have low egg survival because of many years of natural and man-made nest flooding and is not considered to be a stable population (1,7). Resource-limitation may be affecting the apparent changes in egg size and shape we found in the present study.

Table 1.

1.  egg volume = calculated egg volume in cc.

2.  embryo death = embryonic death as a % of fertile eggs collected.

3.  death stage = estimated developmental stage in days at death based on a chart standard (4). % of the egg represented by hatchling based on egg weight and day 1 hatchling weight.

4.  day 1 length = hatchling length in cm on day 1 (hatch day).

5.  day 1 weight = hatchling weight in grams on day 1.

6.  day 1 condition = hatchling condition factor on day 1 (5).

7.  S = statistical significance at p< 0.05 or NS = not significant.

Table 2.

1.  length = total body length in cm.

2.  weight = body weight in grams.

3.  condition = body condition factor (5).

4.  growth rate = linear increases in length in inches per day X 1,000.

5.  performance = (growth rate + condition)/2.

6.  S = statistical significance at P< 0.05 or NS = not significant.

References

1.  Fleming, D. M. Wildlife Ecology Studies. South Florida Research Center Ann. Reports. Everglades National Park. Homestead F1. Vol 1 Sec V, v-10-1 v-10-53, 1991.

2.  Cardeilhac P. T. Diagnosis and Treatment of Infertility in Captive Alligators. Final Reports for the Aquaculture Market Development Aid Program. Vols I - III. 1987 - l991. Florida Dept. of Agriculture, Tallahassee, FL 32399-0810.

3.  Cardeilhac, P. T. Annual Reports on Husbandry Research, 1991- 92 Florida Game & Fresh Water Fish Commission, Tallahassee, FL 32399 - 1600.

4.  Millstein S. R., Vander Meer R. K., Schoenagle E. M. and Cardeilhac P. T. Dietary Therapy for Egg Fertility in the American Alligator: An Evaluation by Determining Fatty Acid Profiles of Egg Yolk. Proceedings International Association for Aquatic Animal Medicine. vol 25 pp 10 - 15, 1994.

5.  Cardeilhac, P. T. and Hoffingberg, M. L. Development of the Alligator. A Color Poster to Grossly Identify Developmental Stages of Normal Embryos Incubated at 88 F (31 C). 1985 Aquatic Animal Laboratory, College Of Veterinary Medicine Box 100126 HSC University of Florida, Gainesville FL 32610.

6.  Dalrymple G. H. Growth of American Alligators in the Shark Valley Region of Everglades National Park. Copeia 1996 212-216.

7.  Kushlan, J. A. and Jacobson, J. Environmental Variability and the Reproductive Success of Everglades Alligators J. Herp, 24 (2); 176-184. 1990.