Mercury Concentrations in Fish from the Guadalupe River, Texas: Relationships with Body Length and Trophic Position

Gold Quiros, Taylor Rene
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Mercury (Hg) is known to bioaccumulate over time in freshwater fish and biomagnify up freshwater food webs, so top predatory fish have the highest Hg body burden. Within Texas, Hg studies in freshwater fish have primarily focused on the northern half of the state and south Texas is relatively understudied. This study investigated the concentration of Hg in muscle tissue from 41 trophically diverse species (n = 1,772) in relation to body length and trophic position at five sites on the Guadalupe River in South Central Texas using a direct mercury analyzer and stable isotope analysis (δ13C, δ15N). The majority of fishes showed a positive relationship between body length and Hg concentration, indicating that Hg was bioaccumulating over time. Striped mullet was the only species that displayed an inverse relationship suggesting growth dilution is occurring. Mercury concentrations were higher in top predators including longnose gar, flathead catfish, and striped bass, and lower in moderate and low trophic level fishes, including Mexican tetra, threadfin shad, and suckermouth catfish. Within the five sites examined, the average Hg concentration in each species was higher in reservoir sites than riverine sites. There was a positive relationship (p < 0.05) between δ15N and Hg concentration at 4 of the 5 sites, indicating that Hg biomagnification is occurring at these sites. The biomagnification factor differed between sites, however it was not positively correlated with food chain length. Among species, the estimated trophic level was the strongest predictor of Hg concentration and within species, total length was the strongest predictor of Hg concentration. These findings provide valuable insight into bioaccumulation and biomagnification of Hg in a relatively understudied freshwater system in South Central Texas. Four species (flathead catfish, white bass, striped bass and longnose gar) had at least one individual that exceeded the Texas Department of State Health Services (TDSHS) human health criterion for Hg (0.7 µg/g wet weight), with at least one species at Canyon Lake, Lake Dunlap, and Victoria exceeding the guideline. Based on this data, the current Hg advisory for Canyon Lake needs to be reevaluated and Victoria and Lake Dunlap may need to have Hg advisories issued.
Mercury, Fish, Texas, Trophic level, Reservoir, Risk assessment
Gold Quiros, T. R. (2018). <i>Mercury concentrations in fish from the Guadalupe River, Texas: Relationships with body length and trophic position</i> (Unpublished thesis). Texas State University, San Marcos, Texas.