Distribution and Growth of American Alligators in a Texas Coastal Marsh

Authors: M. Todd Merendino, G. Matt Nelson, Kevin. H. Kriegel, and Justin. P. Hurst, Texas Parks and Wildlife Department

American alligators (Alligator mississippiensis) are common throughout the coastal marshes and prairies ecological region of Texas, being most abundant on the upper Texas coast.  Populations have also spread inland to freshwater reservoirs along the numerous river systems.  The alligator has recovered from extremely low population levels in the mid-1900s and is now recreationally and commercially hunted in many southeastern states.  The alligator is no longer endangered.  However, it is listed as 'threatened by similarity of appearance', and as such, harvest and management is governed by the CITES Treaty (Convention in International Trade in Endangered Species).

The management plan for the American alligator in Texas involves various harvest and management techniques for the species, most of which are based on intensive research conducted in Louisiana.  Most research activity to date in Texas has focused on harvest techniques and harvest rates.  Little detailed biological data has been collected, especially from the various habitats occupied by alligators in Texas.  Determination of growth rates is needed to verify the applicability of population model assumptions.  The mark/recapture study we propose, herein, will provide information on growth rates that will improve our understanding of alligator population dynamics in Texas coastal marsh habitats.

There is likely variation in growth rates among the geographic clines, which will subsequently affect the time it takes alligators to enter the breeding population.  Given that such differences in populations exist, research is needed in Texas to improve our harvest management and habitat management for American alligators.  Without such critical information such as growth rates, habitat use, and movements, attempts at alligator management will be a guessing game, at best.

We propose to test the hypotheses that:

  1. growth rates of American alligators do not differ along a salinity gradient and
  2. distribution and movements of American alligators are not affected by water salinity.

Study objectives are to:

  1. determine growth rates for American alligators in a Texas coastal marsh,
  2. assess movements and habitat use along a salinity gradient in a Texas coastal marsh,
  3. provide habitat management recommendations for coastal Texas.

This long-term capture/recapture study has been ongoing at Mad Island Wildlife Management Area since 1994.  Alligators were captured at night from airboats.  Searches/surveys were conducted twice monthly.  Spotlights were used to search for alligators throughout the marsh ponds along the salinity gradient.  Smaller alligators (<4', or <1.2 m, in length) were usually hand-grabbed, whereas alligators >5' (1.5 m) in length were captured with snares.  Alligators >5' (1.5 m) were slid into a piece of PVC pipe (10" x 5' or 25 cm x 1.5 m) to allow for safe handling.  Captured alligators were tagged with electronic PIT tags that were inserted near the base of the tail.  Numbered plastic ear tags, similar to those used to mark cattle, were placed in the tail to provide a secondary tag and to prevent unnecessary re-captures of large alligators.  Total length was measured to the nearest millimeter.  Sex was determined via cloacal examination. Subsequent capture efforts were used to determine movements and growth of marked alligators.

From 1994 through summer 2000, 425 alligators ranging from 15" to 11' (38 cm to 3.4 m) in size have been captured.  Of those, 93 have been recaptured.  Overall growth rate of all recaptured alligators was 5.68 inches (14 cm) per year.  Using data from 58 alligators re-captured >3 growing seasons from initial capture, growth is about 6.29 inches (16 cm) per year.  This growth rate is considerably less than the 8" to 12" (20 cm to 30 cm) a year reported in other regions of the United States (South Carolina, Florida, and Louisiana).  The importance of this information is that alligators may require longer to reach sexual maturity, thus making it extremely important to understand how harvest and other survival parameters may affect recruitment into the breeding population.

Most alligators are captured at the fresher sites along the salinity gradient, and in fact, most alligators are captured within 1 or 2 lakes on the study area.  During times of heavy rainfall or run-off, alligators are seen throughout most of the study area.  Similarly, most alligators are re-captured within close proximity to the initial capture site; 1 alligator recaptured in 1999 was recaptured within about 50 yards of the initial 1994 capture site.  The importance of this information is that alligators may be somewhat limited to fresher sites along the Texas coast.  As degradation of coastal marshes continues, most notably due to saltwater intrusion, alligators may be forced into more inland habitats such as creeks and ditches and become further concentrated into shrinking habitats.  These high concentrations of alligators may then in turn begin to affect population levels of other organisms such as furbearers, breeding ducks, etc.

We propose to continue this study through summer 2001.