At more than eight feet in length and exceeding 60 years of age, alligator gar are the largest and longest-living freshwater fish species in Texas. They can be found in the fossil record from 100 million years ago and have changed relatively little during that time - which is why they are sometimes referred to as a 'living fossil.' Although alligator gar managed to outlive the dinosaurs, current populations are in trouble. Throughout most of their native distribution in the lower Mississippi River basin and Gulf of Mexico coastal drainages, alligator gar populations have declined substantially over the past several decades, likely due to flood control and overharvest.
"As human populations grow along rivers, we try to inhibit flooding, such as with levees on the Trinity River," says UNT Associate Professor of Biological Sciences David Hoeinghaus, adding that alligator gar require inundated floodplain habitats for spawning during the late spring and early summer.
"We're learning that conditions have to be just right. It can't be too shallow. It can't be too deep. It has to be this Goldilocks scenario of ideally two- to three-feet deep, plus flooded at the right time and long enough for spawning and larval development to occur and with the right kind of vegetation for eggs and larvae to attach to," he says. "Without those conditions, alligator gar do not successfully reproduce. During a female's long lifespan, she may only successfully spawn a few times, so every opportunity counts."
Hoeinghaus, whose area of expertise is aquatic ecology, has been working with Texas Parks and Wildlife and colleagues at other institutions to better understand exactly what set of conditions benefit alligator gar spawning. A recent collaborative project led by Hoeinghaus combined vegetation maps, very high-resolution remote sensing data, and floodplain inundation models to quantify availability of suitable spawning habitat through time for the lower Trinity River. Alligator gar were collected and aged by counting annual rings on otoliths (ear stones), analogous to counting rings when ageing trees, to identify the year each individual was spawned. The oldest fish collected were approximately 60 years old, and since the mid-1950's only five exceptional year classes were identified. Those year classes were associated with sustained spawning habitat availability, especially late in the spawning season. High floods that were not sustained, or river levels frequently increasing and decreasing, negatively affected year class strength.
"Although most populations in Texas are considered healthy, declining alligator gar populations are very difficult to restore due to their long lifespans and infrequent spawning, so it makes sense to take actions to prevent the kinds of declines we've seen in other parts of their native range," Hoeinghaus says.
For this reason, recent management efforts have been implemented to protect alligator gar populations in Texas, including the Trinity River, which has a world-recognized recreational fishery for alligator gar, as featured on Animal Planet's River Monsters and National Geographic's Monster Fish.
"Historically, gar were treated as trash or nuisance fish. The management strategy was basically to kill them because they were thought to compete with or eat the trophy sport fish that the managers were focused on," Hoeinghaus says. "There's been a really dramatic shift in terms of how managers and the public view gar. Now, they're stars of TV shows, and people travel from all over to try to catch a river monster."
Television shows play up the prehistoric appearance, but alligator gar are more than just an exciting angling target or 'monster' navigating our waterways - they play a fundamental role in river ecosystems. Alligator gar are top predators that help to control populations of other species in the food web. Plus, if areas are protected or restored to benefit alligator gar populations, it will certainly benefit other floodplain-dependent species as well.
"In many ways, alligator gar can be considered an umbrella species," Hoeinghaus says. "When they are protected, entire ecosystems are protected."
Hoeinghaus is planning a new multi-year project with Texas Parks and Wildlife and in collaboration with U.S. Fish and Wildlife biologists at the Trinity River National Wildlife Refuge. Hoeinghaus and his team will be tagging juvenile alligator gar with passive integrator transponder tags and monitoring their movement in the refuge and adjacent areas.
"We want to understand how juvenile alligator gar use the floodplain landscape of the refuge," he says. "Rising water levels are opportunities to disperse, whereas low water levels can leave fish isolated in backwater habitats such as oxbows and sloughs."
Part of Hoeinghaus' research is looking at whether remaining confined in these locations is beneficial or detrimental for alligator gar growth and survival. His research will address the questions: Is being in the backwaters a bad thing - are they trapped in poor quality habitats? Or is it good because there are fewer predators and competitors so the growth rates are higher? Like his previous research, findings from the new project will inform management.
"Our findings will have direct application to alligator gar management. State and federal agencies tasked with managing landscapes and populations need to make decisions backed by really sound ecological data," Hoeinghaus says. "Any information that we generate can lead to restoration actions, such as improving connectivity, on these federally owned lands. There is almost nothing known about how juvenile alligator gar use floodplain landscapes, so it will be very interesting to see what we find over the next couple of years."