Acoustic Tagging Research on Coastal Alligator Gar

Alligator gar, Atractosteus spatula, is the largest freshwater fish species in Texas and one of the largest in North America. It is not uncommon for them to grow to 6 feet or greater and live past 50 years, as a matter-of-fact individuals that survived to be around 100 years old have been recorded. Fossil evidence has shown that modern-day gar look very similar to their ancestors that evolved over 200 million years ago. In fact, fossilized gar have been found in the Permian deposits within Texas, so the alligator gar is truly a native Texas species. Gar were often targeted for removal until somewhat recently because they were misunderstood by anglers and management agencies and mistakenly considered a threat to sport fish abundance or even people due to their fierce appearance. Their total population size and geographical distribution has decreased greatly over the past century due to culling and habitat destruction, though high concentrations can still be found in southeast Texas such as the Trinity River and the Sabine Lake system.

The Texas Parks & Wildlife Department (TPWD) is studying the movement and habitat utilization patterns of alligator gar using acoustic tagging with the goal of best management and long-term protection of the species. In the Sabine Lake area, beginning in 2022 and continuing in 2023-24, alligator gar caught during TPWD routine sampling have been released with surgically implanted acoustic tags that record that fish’s presence when it swims close (within 1 km) to receivers located throughout the Texas coast. These gar were tagged mainly within two general areas: the first was the Salt Bayou marsh located to the west of Sabine Lake; and the second was the Louisiana side of Sabine Lake (Figure 1). Ninety alligator gar have been tagged thus far with sizes ranging from 28” to 66” representing a mix of adults and juveniles.

Results from this study suggest that the Sabine Lake area has at least two groups of gar with relatively distinct home territories outside of which few individuals travel. For example, 89% of the gar tagged within the Salt Bayou area never left that marsh. There were however two exceptions of note including one individual that left Salt Bayou and travelled down the intracoastal waterway to Galveston Bay where it remained for 312 days before returning to Salt Bayou, which is a round-trip distance of at least 250 km. In another instance the gar left Salt Bayou for 130 days and traveled the extent of Sabine Lake including up the Sabine and Neches Rivers. Similarly, 96% of gar tagged in Sabine Lake never entered Salt Bayou but were recorded spending time along the Louisiana side of the Lake near and within its many bayous emptying the Sabine National Wildlife Refuge, and also venturing into the Neches and Sabine Rivers that flow into the north end of Sabine Lake. A few gar traveled down to Sabine Pass, which opens up to the Gulf of Mexico, but that area was used infrequently, likely due to higher salinities.

The results of this study reaffirm similar findings from previous gar research that has found that most gar don’t move very far, however, there seem to be a few that will swim a much greater distance than the rest. This suggests that local populations of alligator gar consist of a mix of individuals where most will remain within a smaller territory while a few individuals will travel to new areas. The desire of a few gar to travel further than most may be evolutionarily advantageous by promoting gene flow of individuals inclined to explore new territory for better prey resources, environmental conditions, or improved spawning habitat.

Alligator gar spawning opportunities may occur just a few times over many years because of their unique needs for reproduction that include flooding across terrestrial vegetation, such as marsh grass or swamp understory, combined with water temperatures above 68̊ F. In addition, individuals are not able to reproduce until they have grown several feet in length, and because of this the overharvest of reproductively capable fish can result in long-acting, negative consequences for gar populations. As a result, management agencies are now trying to reduce the take of alligator gar, which has grown more popular over recent years not only for eating but also as a trophy fish. Texas regulations now include a one fish daily bag limit with potential closures to fishing during certain conditions when spawning is likely, as well as other area-specific regulations in Falcon Lake and the Trinity River. For information on limits please review the TPWD Outdoor Annual at https://tpwd.texas.gov/regulations/outdoor-annual/fishing/freshwater-fishing/gar-closure

TPWD’s gar research is ongoing and has been expanded to include other coastal areas like East Matagorda Bay and Cedar Lakes. The internal acoustic tags that we’ve used will last several years, so if you happen to catch a gar with an external tag on their dorsal fin, please release that fish and call the number on the tag to report the capture location, as additional data will help us better understand their movement patterns over time with the aim of better protecting and preserving this misunderstood species.