The detection of high concentrations of methylmercury in the sub-thermocline low oxygen seawater indicates a potential for enhanced bioaccumulation of mercury in such environments not yet explored. Here we present mercury concentrations in 8 fish species of low trophic level in relation to their vertical distribution. Fish species were selected to cover a wide range of vertical distributions, from epipelayic (<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 \vt and were significantly and positively correlated with median daytime depth. Concentrations increased by 4-fold from epipelagic to mesopelagic species with no further increases with depth among mesopelagic species down to about 1200 m. Such enhanced mercury bioaccumulation in the marine mesopelagic compartment seems to be determined proximately by levels in food and ultimately by water chemistry that controls mercury speciation and uptake at the base of the food chain. We conclude that this is the best explanation for high and yet poorly understood mercury concentrations found in deep-sea predators.

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To obtain important baseline information about population dynamics and to provide valuable insights about the possible effects of artisanal fishing on the demersal fish community, this study had three specific objectives: (1) to update the annotated list of demersal teleost species occurring in the Azores (mid-North Atlantic); (2) to describe their depth distribution and related fishery information; and (3) to evaluate annual changes in their observed abundance and length composition. To do this, a time series with about 25 years of scientific data from the commercial fishery and surveys was analyzed. The demersal teleost community was represented by 213 species, predominantly deep-water species, reflecting the main marine habitat in the Azores region. Fishery takes place mainly in the intermediate strata (200–600 m), where the most commercially important species occur. No changing in the fork length (LF) distribution toward small individuals caused by size-selective fishing was detected in this study. The high variability observed in the LF data indicates the need for more accurate studies considering alternative statistical analyses (e.g., generalised linear models) to examine the possible factors (e.g. depth coverage, gear configuration, soak time, and fishing area) that cause this variability. On the other hand, the abundance indices of some commercially important species appear to have declined (Phycis phycis, Pontinus kuhlii, Conger conger, Beryx splendens and B. decadactylus), while others appear to be more stable (Helicolenus dactylopterus) or even increasing (Mora moro). Although the available data are insufficient for a formal assessment on the status of exploited populations, the long-term analysis of commercial landings and survey data was used as a basis to assess deep sea demersal teleost fisheries of the Azores, under a precautionary approach.