Mercury contamination in spotted seatrout, Cynoscion nebulosus: An assessment of liver, kidney, blood, and nervous system health
Introduction
Marine and estuarine fishes in South Florida (Florida Keys–Florida Bay–Everglades region) often accumulate higher concentrations of methylmercury than do similar fishes from other areas of Florida and the southeastern U.S. (Adams and McMichael, 2001, Strom and Graves, 2001, Adams et al., 2003, Adams and Onorato, 2005, Evans and Crumley, 2005). Comparatively high available mercury in certain areas of South Florida is related to effective in situ benthic production of methylmercury within this region (Evans and Crumley, 2005) coupled with the elevated atmospheric deposition of mercury in South Florida relative to the rest of the continental United States (NADP, 2010). Although the very high mercury concentrations in some freshwater fish species in broad areas of the Everglades have decreased since the late 1980s, in other specific areas (e.g., Everglades National Park) concentrations have remained high or have increased in recent years (Axelrad et al., 2009), and concerns continue regarding the toxic effects of mercury exposure on fish and wildlife.
Most research on the effects of mercury on fishes has focused on laboratory experiments with methylmercury administered in water or food sources. The exposure levels used in these studies have frequently been at levels greater than those that occur now in natural systems. This limitation has been recognized, and in recent years several investigators have begun to characterize the physiological impacts of relevant exposures of fish to methylmercury and have found reproductive impairment (Matta et al., 2001, Hammerschmidt et al., 2002, Drevnick and Sandheinrich, 2003), oxidative stress (Gonzalez et al., 2005), morphological damage to gills and the olfactory epithelium (Jagoe et al., 1996; Oliveira Ribeiro et al., 1996, Oliveira Ribeiro et al., 2000), and histopathological effects on liver, kidney, spleen, and other tissues (Filenko et al., 1989, Skak and Baatrup, 1993, Handy and Penrice, 1993, Banerjee and Bhattacharya, 1994, Oliveira Ribeiro et al., 1995, Oliveira Ribeiro et al., 1996, Oliveira Ribeiro et al., 2002, Mela et al., 2007). Mercury-associated changes in neurochemical biomarkers have also been documented in other organisms (Basu et al., 2007, Scheuhammer et al., 2008) and there is a need to examine these specific biomarkers in fish. The laboratory evidence suggests that current levels of methylmercury in the environment may be affecting the health of wild fish populations, but few researchers have conducted such investigations in the field. Of those studies that have been done, most have been conducted in freshwater systems on wild fish from natural populations involving highly contaminated rivers, lakes, or reservoirs directly influenced by anthropogenic sources (Hontela et al., 1995, Adams et al., 1999, Raldúa et al., 2007) or in specific inland freshwater systems (Drevnick et al., 2008). Little attention has been devoted to studying the impacts of methylmercury on marine fishes.
The spotted seatrout (Cynoscion nebulosus) was selected as an effective model species for this study due to its broad distribution in coastal waters of the southeastern U.S., typically restricted patterns of movement within the confines of its natal estuaries, and the thorough documentation of its life-history parameters, throughout its range. The spotted seatrout is broadly distributed in estuarine and coastal marine waters along the eastern Atlantic and Gulf of Mexico (Chao, 2002). Genetic analyses (Gold et al., 1999, Wiley and Chapman, 2003), and conventional tagging studies (Music, 1981, Bryant et al., 1989, Baker and Matlock, 1993) indicate that spotted seatrout are relatively restricted to their natal estuaries with very limited interestuary movement. The objectives of this study were 1) to determine total mercury concentrations in multiple tissue types (dorsal muscle, liver [hepatopancreas], kidney, gonad, brain, and red blood cells) of spotted seatrout from an area of higher mercury contamination (South Florida) and an area of comparatively lower mercury contamination (Indian River Lagoon, Florida); 2) to interpret blood-plasma chemistry with regard to mercury concentration in spotted seatrout from each distinct study area; 3) to examine liver and kidney histology with regard to mercury concentrations; and 4) to investigate mercury-associated changes in neurochemical biomarkers that have been documented to be affected by mercury in other organisms.
Section snippets
Sample collection
Adult spotted seatrout were collected during April and May of 2008 from tropical waters of South Florida in the Florida Keys–Florida Bay–Everglades region, and from warm temperate–tropical waters of Indian River Lagoon system, located along the Florida Atlantic coast (Fig. 1). Mercury concentrations in marine, freshwater, and terrestrial biota from South Florida are typically higher than they are in those biota from other regions of Florida, including Indian River Lagoon, which served here as a
Spotted seatrout size, age, reproductive status
Twenty-eight spotted seatrout were collected from South Florida waters (n = 14 fish) and Indian River Lagoon, Florida (n = 14 fish) during April and May 2008. Most fish from both areas were female (South Florida: 12 females, 2 males; Indian River Lagoon: 10 females, 4 males). All fish were determined to be reproductively mature based on macroscopic examination of gonads and known size- and age-at-maturity for this species (Crabtree and Adams, 1998, Brown-Peterson, 2003). Gonad developmental stage
Mercury concentration
Total mercury concentration in all tissue types was higher in seatrout from South Florida than in those from Indian River Lagoon. Comparatively high mercury concentrations have also been recorded in muscle of estuarine and marine fishes from South Florida within the Florida Keys–Florida Bay–Everglades region (Adams and McMichael, 2001, Strom and Graves, 2001, Adams et al., 2003, Adams and Onorato, 2005, Evans and Crumley, 2005). Available mercury via direct importation from the Everglades
Conclusions
Total mercury concentration in all spotted seatrout tissues was significantly higher in fishes from South Florida than in those from Indian River Lagoon and was within the threshold range at which mercury exposure has been shown to affect tissues. Distribution among tissues followed the same trend in both study areas (kidney > liver > muscle > brain > gonad > red blood cells). Liver histology revealed that pyknosis/necrosis, interstitial inflammation, and bile duct hyperplasia were found only in seatrout
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