Monitoring an apex predator: Alligator gar (Atractosteus spatula) reintroduced into Merwin Preserve, Illinois

Background/Question/Methods

Many studies have demonstrated that apex predators provide stability across trophic levels in ecosystems. Despite their importance, populations and geographic ranges of apex predators continue to decline due to anthropogenic impacts. Alligator gar (Atractosteus spatula) are apex predators that once occupied major Illinois rivers until extirpated in 1994. The Illinois Department of Natural Resources joined efforts with ten other states to restore populations in 2009. Benefits of reintroduction may include stabilizing native fish communities, invasive species control, and providing a trophy fishery. There is a dearth of information on alligator gar life-history and ecology. The purpose of this study was to measure growth rates, fitness, diets and recapture success of stock-sized alligator gar (n = 100) tagged with passive integrated transponders and released in The Nature Conservancy’s Merwin Preserve wetland September 29, 2011. Gars averaged 538 mm and 886 g upon release. Monthly surveys were conducted May - October 2012 to recapture alligator gar and sample the fish community using trap nets,  mini fyke nets, DC electrofishing, experimental gill nets, and modified  gill nets (3” bar mesh, dyed black).

Results/Conclusions

Seventeen alligator gar were recaptured and growth rate averaged 1.6 mm and 7.2 g per day during the sample period.  The largest individual grew from egg to 959 mm and 6.4 kg in only 17 months. The gar grew at the same rate and fitness as populations in Louisiana, despite being at the northern extreme of their historic geographic range. Diet analysis resulted in 53% empty stomachs, 29% gizzard shad (Dorosoma cepedianum), and 18% unidentifiable fish. Shortnose gar (Lepisosteus platostomus) diets were also collected, resulting in 86% empty and 14% gizzard shad. Feeding behavior of alligator gar was opportunistic based on the high abundance of gizzard shad in the environment. The largest prey item recovered was 41% of the alligator gars total length, supporting previous findings of their ability to consume large prey. All sampling gears used except mini fyke nets caught alligator gar; however, trap nets and modified gill nets produced the most recaptures at eight and six respectively. This study provides baseline data on life-history and ecology of alligator gar in Illinois to aid reintroduction and conservation efforts.