MERCURY CONCENTRATIONS IN PREY FISH INDICATE ENHANCED BIOACCUMULATIONIN MESOPELAGIC ENVIRONMENTS

The detection of high concentrations of methylmercury in the sub-therm ocline low oxygen seawater indicates a potential for enhanced bioaccum ulation of mercury in such environments not yet explored. Here we pres ent mercury concentrations in 8 fish species of low trophic level in r elation to their vertical distribution. Fish species were selected to cover a wide range of vertical distributions, from epipelagic (< 200 m ) to mesopelagic (> 300 m) environments in the sub-tropical mid-North Atlantic. Mean mercury concentrations in the study species ranged from 57 to 377 ppb dry wt and were significantly and positively correlated with median daytime depth. Concentrations increased by 4-fold from ep ipelagic to mesopelagic species with no further increases with depth a mong mesopelagic species down to about 1200 m. Such enhanced mercury b ioaccumulation in the marine mesopelagic compartment seems to be deter mined proximately by levels in food and ultimately by water chemistry that controls mercury speciation and uptake at the base of the food ch ain. We conclude that this is the best explanation for high and yet po orly understood mercury concentrations found in deep-sea predators.