“Hotter Than a $2 Pistol”: Changes in Mercury Dynamics of an Impacted Largemouth Bass Population Since Discovery

Length specific evaluation of fish mercury levels have not been performed in the Lower Ouachita River System of Arkansas since mercury was discovered in these waters in the 1990s.  Due to their popularity for sport angling and consumption in Arkansas, largemouth bass are commonly used as an indicator species for mercury consumption levels.  DC Electrofishing was used to collect 4-6 largemouth bass per 25.4-mm (1 inch) length group from each of 6 geographically distinct regions in the Lower Ouachita River System from Camden, Arkansas to Felsenthal Lock and Dam.  Largemouth bass were weighed, measured, aged, and sexed for species specific mercury level assessment with the intent of developing models to better communicate which fish are safe for consumption.  Data collection was performed in the summer of 2010.  These data were compared to data collected in 1995 to assess temporal changes in mercury levels.  Analysis of variance coupled with Tukey’s post hoc analysis indicated that age and geographic region were the only variables associated with mercury levels of largemouth bass in the sampled area (P = 0.0002 and P < 0.0001, respectively).  The analysis also indicated that largemouth bass in Felsenthal Reservoir, 2 years and older, exceeded the mercury action level of 1.0 ppm, designated by the Arkansas Department of Health and U.S. Food and Drug Administration.  Furthermore, all samples collected from the Lower Saline River tested well above the action level.   While largemouth bass from the Camden Pool tested below the action level.  Analyses between 2010 and 1995 data sets indicated that mercury levels in 2010 were statistically different than mercury levels in 1995 (P = 0.0014).  Length group mean mercury level comparisons indicated that largemouth bass 305-355 mm (12 and 13 inches) in Felsenthal Reservoir exceeded mercury action levels in 2010.  Whereas, 1995 data indicated that largemouth bass less than 356 mm (14 inches) did not exceed the action level.  Additional periodic, intensive sampling will be required to determine if a long term shift in the dynamics of the largemouth bass population has occurred.