Mercury Concentrations and Dynamics in Fish Species of Different Trophic Levels from the Ohio River

Mercury (Hg) can be efficiently bioaccumulated and transferred in aquatic food webs thereby posing a greater risk of contamination to higher trophic-level organisms. The occurrence, concentrations, and relationship of total recoverable (TrHg) and methyl Hg (MeHg) mercury in the muscle tissue of large trophic-level 3 (TL₃) and TL₄ fish species in the Ohio River is relatively unknown and warrants investigation.  In this study, the relationship between TrHg and MeHg concentrations in 4 fish species was examined along an extensive longitudinal portion of the mainstem Ohio River.  Species examined include two TL₃ species [freshwater drum (Aplodinotus grunniens) and channel catfish (Ictalurus punctatus)] as well as two TL₄ species [hybrid striped bass (Morone saxatilis x M. chrysops) and largemouth bass (Micropterus salmoides)].  Many reports assume that 95-100% of the TrHg concentrations in fish tissue of higher trophic levels is in the methylated form. However, these species exhibited mean MeHg:TrHg ratios ranging between 74% and 78%.  Despite the similar ratios among all species, variations in TrHg concentrations were observed. Although ORSANCO has historically reported numerous TrHg concentrations violating the 0.3 ppm human health criterion, these are the first reported violations for MeHg concentrations on the Ohio River.  This study highlights the necessity for further investigations focusing on the Hg concentrations in large TL₃ and TL₄ species collected from large Midwestern rivers.