The Effects of Harvest on Pig Frog
Populations from the Everglades

Cristina A. Ugarte¹ and Kenneth G. Rice²

Abstract

The pig frog, Rana grylio, is a fully aquatic frog found in freshwater marshes and forested wetlands throughout the Everglades. In Florida, people harvest the frog commercially and recreationally for its edible legs. We collected preliminary data from harvested frogs from Water Conservation Areas (WCA) 3A and 3 B in March and May of 1999.  We visited sites within Everglades National Park's Shark River Slough (SRS) and revisited WCA in August and September of 2000. We compared sex ratios and size distributions of randomly captured frogs using time constrained searches in both harvested (WCA 3A and 3B) and protected (SRS) areas. Between harvested areas, females and males were smaller in WCA 3A than in 3B. Females were also smaller in 3B in May than in March 1999. There were more juveniles in 3A than in 3B. Between harvested and protected areas, we found no differences in sizes of adults but there were differences in proportions of males, females and juveniles. 3A (harvested) had the highest proportion of juveniles again in 2000, 3B (harvested) had the highest proportion of males, and SRS (protected) had more adult females.  In contrast to initial predictions, Site 3A had the highest catch per effort compared to SRS. Intense harvesting of adults may be partially responsible for these differences.

Introduction

The long-term goals of this study are:

• To examine harvest impacts on abundance, sex-size distribution, sex ratios, recruitment, and fecundity of R. grylio in the Everglades.
• To assess relationships between abundance of this frog with hydrology and vegetation; and to predict responses to hydrological change. Given its biphasic lifestyle, and aquatic nature this species should be an excellent indicator to changes in hydrology.
  Here we present data collected from 1999 and 2000 in harvested sites (WCA 3A and 3B) and in 2000 from Shark River Slough (SRS), a site protected from frog harvest in Everglades National Park.

The objectives are:

• Compare sizes and sex ratios of adult and juvenile Rana grylio to assess initial spatial and temporal variation between harvested sites 3A and 3B.
• Compare abundances, sizes and sex ratios of adult and juvenile Rana grylio between harvested and protected sites.

Methods

I: HARVESTED SITES

• Water Conservation Areas 3A and 3B are the harvested sites.  Frogs were sampled at WCA 3A on March 19th, 1999, and WCA 3B on March 26th, and May 7th, 1999
• Sites were sampled at night by airboat. We sampled airboat trails, wet prairie dominated by Eleocharis cellulosa, and slough habitats dominated by Nymphea odorata.
• Frogs were sexed and their snout vent length (SVL) measured in mm.
• A two-way anova was used to determine differences in size of each sex/age class of frog by harvested site: adult male, adult female, and juvenile.  Juveniles are classified as < 90 mm (males<75 mm) (Ligas 1960) (Figure 3).  A Chi square test was used to detect differences in the proportion of adult male, female and juveniles by harvested site (Figure 4).

II HARVESTED VERSUS PROTECTED SITES

• Sites were surveyed at night in airboat trails within SRS and WCA 3B during August and September 2000.
• Frogs were captured by hand during set time intervals (time constraint searches) and abundance was calculated by number of frogs captured per hour. (Figure 2)
• A Kruskall Wallis nonparametric test was run to compare SVL by Site/Sex and Date.
• A Chi square test was used to detect differences in the proportion of adult male, female and juveniles between harvested and protected sites. (Figure 5)

 Results

Figure 2

Figure 3

Figure 4

Conclusions

• More females are found in SRS than in 3A and 3B. We believe that many froggers select larger frogs, since females tend to be larger than males. We also believe that harvest pressure is more intense in WCA 3A than in 3B.  This could explain the higher number of juveniles and smaller females in 3A than in 3B. Within the same site 3B, there were smaller females later in the season than in March. This could also suggest a removal of the larger females from the population.
   
• There was large variation in numbers and size/sex of frogs within harvested sites. Differences may be attributable to harvest or local site differences.  Water depth, temperature and precipitation are  known to affect these frogs behavior and may also be responsible (Ligas 1960).
   
• Overall Monthly sampling of these sites over consecutive nights will provide more conclusive results.
   
• High abundance of juveniles in A may be a result of younger females producing more eggs (Begon and Mortimer 1981; Pitcher and Hart 1982). Other exploited populations respond to harvest by producing more yet smaller eggs. The differences in fecundities between the harvested and protected areas is going to be assessed.
   

Literature Cited

Begon, M and M. Mortimer. 1981. Population Ecology- A unified study of animals and plants. Blackwell Scientific Publications. Boston, Massachussets.

Ligas, F.J. 1960. The everglades bullfrog: Life history and management. Florida Game and Freshwater Fish Commission. Tallahassee, Florida.

Pitcher, T.J. and P. Hart. 1982. Fisheries Ecology. Croon Helm. Ltd. Australia.
 

Acknowledgements

We wish to thank Franklin Percival and Bill Loftus for logistic support. Troy Brannen, Hardin Waddle, Tori Foster, Travis Tuten, Kevin Whelan, Aileen Soto, Joe O'Brien, Kirsten Hines, James Watling, Mo Donnelly, Carlo Calandriello, Kirsten Nicholson, Ann Lindholm, Alison Brevold, and Pere Morera for assistance in the field. This project is partially supported by the Florida Integrated Science Center USGS, the Tropical Biology Program at Florida International University,  and an EPA STAR fellowship # U-91579701-0