Mercury Bioaccumulation Patterns in Two Estuarine Sportfish Populations

Abstract

Mercury (Hg) is a harmful bioaccumulative heavy metal to which humans are exposed primarily through fish consumption. Several consumption advisories for mercury have been issued for fishes along the US Atlantic and Gulf coasts, including the Mobile-Tensaw River Delta in coastal Alabama. Mercury cycling in estuaries is complex and little is known about the extent of mercury bioaccumulation in the ecologically diverse fishes in coastal areas such as the Mobile-Tensaw Delta, Alabama. Using traditional tissue analysis techniques combined with otolith microchemistry, diet an age and growth analysis, and bioenergetics modeling, I investigated seasonal and spatial trends of mercury accumulation in largemouth bass, Micropterus salmoides, and southern flounder, Paralichthys lethostigma, inhabiting the Mobile-Tensaw Delta.

?Age-normalized largemouth bass mercury tissue concentrations increased significantly from downstream to upstream locations with little seasonal variation. Largemouth bass collected at both upstream and downstream locations exceeded common minimum consumption and no consumption advisory levels. Microchemistry of largemouth bass otoliths indicated that individuals did not migrate between upstream and downstream regions in the Mobile-Tensaw Delta. Diet analysis showed that largemouth bass at downstream locations foraged on lower trophic levels than those upstream and bioenergetics analysis confirmed that this difference was the primary factor responsible for lower largemouth bass Hg accumulation downstream. Southern flounder mercury tissue concentrations were uniform across the sample area and were lower than those of largemouth bass. No southern flounder had Hg tissue concentrations that exceeded common minimum consumption advisory levels. Microchemistry analysis of southern flounder otoliths indicated a highly variable migratory life history across salinity gradients. This migratory life history was likely responsible for similar Hg tissue concentrations across the Mobile Delta for southern flounder. Differences in Hg bioaccumulation between largemouth bass and southern flounder were not the result of trophic differences between species but rather the result of faster growth rates in southern flounder. For both largemouth bass and southern flounder a negative relationship existed between salinity exposure and Hg tissue concentration, although there was considerable variability in this relationship. Initial efforts to detect mercury directly in the otoliths of both species as a way to measure lifetime Hg accumulation trends were not successful.