Mercury in Brazos River Alligator Gar

Baylor researcher Zach Moran with a 6-foot 7-inch Alligator Gar. (Credit Ryan King)

Being an apex predator, Alligator Gar can bioaccumulate harmful chemicals like mercury over the course of their entire lives by inhabiting polluted areas and consuming contaminated prey. This transfer of mercury through the food web–also known as biomagnification–is a key driver of bioaccumulation within individuals. 

The effects of such long-term exposure are still being studied, but they could play a role in the decline of the species’ population and impact communities that rely on the fish for food.

Comparing Inland and Coastal Alligator Gar

In 2023, Zach Moran, Assistant Professor of Fish Science, investigated trends and contributors to bioaccumulation in Alligator Gar in Texas. He compared inland and coastal populations to see which demographic contained larger concentrations of mercury. 

A 6-foot 7-inch Alligator Gar. (Credit Ryan King)

The Brazos River was selected as the research site due to the coastal and inland regions experiencing different volumes of aerial mercury deposition. The coastal waters were exposed to high rates, while the inland water specimens were exposed to moderate deposition. 

A total of 93 Alligator Gar were captured, measured (length), and then biopsied (nonlethally) for a muscle sample. All of the specimens analyzed were within 1000 to 1700 mm to remove potential size bias, as larger gar were collected from the inland population. 

Due to the deposition differences, Moran hypothesized that gar in the coastal, high deposition region would have higher concentrations, as previous literature has suggested that aerial deposition is a top driver of mercury concentrations. However, the study revealed the opposite—gar in the moderate deposition zone contained higher concentrations.

A muscle biopsy from an Alligator Gar. (Credit Zach Moran)

Drivers of Mercury Concentrations in Alligator Gar

Moran proposes a few theories as to why the results of this study differed from previous studies, ranging from conditions in the Brazos River specifically, complications with sampling, and feeding behaviors of the two different groups.

First, the coastal waters can have an abundance of sulfate, which, when reduced to sulfide, can remove mercury “from methylation pathways in sulfate reducing bacteria and consequently limit [mercury] and other cation availability.” 

While it could be that the coastal region is receiving more mercury, it may not be entering the biological pathways due to biogeochemical processes. 

Second, while the gar evaluated over the course of the study were all similar lengths, the age of the fish could vary drastically. Because gar grow quickly but then slow down in growth as they get older (a trait known as indeterminate growth), fish within that size range could be 5 years old or all the way up to 25 years old. 

Baylor researcher Zach Moran (right) and TPWD biologist Trevor Troxel (left) hold a 7 ½ foot Alligator Gar. (Credit Zach Moran)

Older individuals have more time to accumulate mercury, which could have skewed the results of the study if the coastal specimens were younger than the inland representatives.

Finally, according to Moran, a primary driver of mercury uptake in fish is diet-related. “We speculate the reason for dissimilarity of [mercury] concentrations in Alligator Gar between our two regions is likely linked to different [mercury] levels in their prey species.”

The inland alligator gar feed primarily upon gizzard shad and buffalo fish which tend to have higher mercury concentrations—0.28-1.04 mg/kg and 0.08-0.22 mg/kg, respectively—compared to the preferred prey for the coastal gar: striped mullet (0.049–0.057 mg/kg) and gulf menhaden (0.023–0.03 mg/kg). 

Moran believes that this biomagnification and the trophic feeding level of the fish were the top contributors to mercury concentrations in the two populations studied, which ultimately led to the inland gar having higher concentrations.   

Baylor researchers Zach Moran (left) and Bianca Possamai (right) with an Alligator Gar. (Credit: Zach Moran)

Future Research

While the study proposes a few theories as to why concentrations differed from what was expected based on previous research, more work needs to be done in order to confirm these drivers of bioaccumulation and biomagnification. 

In Texas, Alligator Gar are a popular trophy fish and are often eaten for meals. As a harvest-oriented fishery, it’s important to understand drivers of high concentrations as well as how high levels of mercury or other chemicals like PFAS impact fish. 

“We really want to conserve these fish because they’re slow to mature and have to have just the right conditions to spawn,” Moran explains. “It could be that mercury is one of these contributing factors to low recruitment.”

Read the full study here.