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Mercury (Hg) bioaccumulation in deep-sea fauna (> 200 m depth) of the northern Mexican Pacific is unknown. We measured the total Hg concentration (THg µg/g wet weight) in 18 fish species, caught between 90 and 1100 m depth, in two ecoregions of the Mexican Pacific: The Gulf of California (GC) and the western coast of Baja California (BC), in the Southern Californian Pacific. We assessed spatial (ecoregion and depth) and biological (species, tissues, trophic position, sex and size) patterns of bioaccumulation. The highest THg concentrations were observed in liver (geometric mean: 1.28 ± 2.31 µg/g, range: < Method Detection Limit (MDL)–16.17 µg/g), followed by muscle (geometric mean: 0.34 ± 0.53 µg/g, range: < MDL–6.12 µg/g) and gonad (geometric mean: 0.29 ± 0.49 µg/g, range: < MDL–2.18 µg/g). The maximum limit established for human consumption (1 µg/g wet weight) was exceeded by 7.8% of the muscle samples and 31% exceeded the protection threshold for possible harmful effects in fish (0.2 µg/g wet weight). We observed that THg concentrations tended to increase at higher trophic levels, total lengths and condition factors. Our results indicate a clear differentiation of the mercury bioaccumulation patterns between GC and BC, where GC species (200–500 m) had consistently higher liver/muscle concentration ratios than those from BC (600–1100 m). Altogether, the findings presented in this study are possible evidence of differential scenarios of environmental contamination.

Supralittoral and high intertidal coastal zones are exposed to pollution from both marine and terrestrial sources and undergo higher deposition rates than the subtidal zone. It is therefore important to identify organisms for this section of the coastal area that can be tolerant to contaminants. The aim of this study was to determine if supralittoral isopods of the genus Ligia can be used as biomonitors, since they are abundant and widely distributed. For this purpose, concentrations of trace elements were determined in Ligia isopods in toto from 26 locations across the Gulf of California and Pacific coast of the Baja California peninsula, which were collected during the summers of 2009 and 2010. The concentrations of trace elements followed the order of; Zn≥Cu>As>Cd>Pb>Hg. Elevated concentrations of copper (up to 1010 μg/g) were detected in Ligia from Santa Rosalia (SRo), a locality where industrial mining of copper has historically occurred. Industrial and municipal sewage discharges appear to have contributed to the high concentrations of zinc (326 μg/g) and lead (144 μg/g) found in organisms from Guaymas location. The high mercury concentration in organisms from Mazatlan (M) (2.01 μg/g) was associated with a thermoelectric plant. Natural sources of metals were also detected; coastal upwelling appears to be associated with high cadmium concentrations in Ligia from Punta Baja (PB) (256 μg/g) in the Pacific coast, whereas hydrothermal vents may have contributed to high concentrations of arsenic at Ensenada (E) (61 μg/g). Our results suggest that Ligia isopods reflect the natural and anthropogenic inputs of trace metals in the environment and could potentially be used as biomonitor organisms of the intertidal rocky shores of the Gulf of California and Pacific coast.