Interspecific and intraspecific variation in selenium:mercury molar ratios in saltwater fish from the Aleutians: potential protection on mercury toxicity by selenium

Abstract

A number of factors affect the consumption risk from mercury in fish, including mercury levels, seasonal patterns of mercury concentrations, human consumption patterns, and sensitive populations (e.g. pregnant women, fetuses, young children, and yet unknown genetic factors). Recently the protective effects of selenium on methylmercury toxicity have been publicized, particularly for saltwater fish. We examine levels of mercury and selenium in several species of fish and seabirds from the Aleutians (Alaska), determine selenium:mercury molar ratios, and examine species-specific and individual variation in the ratios as a means of exploring the use of the ratio in risk assessment and risk management. Variation among species was similar for mercury and selenium. There was significant interspecific and intraspecific variation in selenium:mercury molar ratios for fish, and for birds. The mean selenium:mercury molar ratios for all fish and bird species were above 1, meaning there was an excess of selenium relative to mercury. It has been suggested that an excess of selenium confers some protective advantage for salt water fish, although the degree of excess necessary is unclear. The selenium:mercury molar ratio was significantly correlated negatively with total length for most fish species, but not for dolly varden. Some individuals of Pacific cod, yellow irish lord, rock greenling, Pacific halibut, dolly varden, and to a lesser extent, flathead sole, had selenium:mercury ratios below 1. No bird muscle had an excess of mercury (ratio below 1), and only glaucous-winged gull and pigeon guillemot had ratios between 1 and 5. There was a great deal of variation in selenium:mercury molar ratios within fish species, and within bird species, making it difficult and impractical to use these ratios in risk assessment or management, for fish advisories, or for consumers, particularly given the difficulty of interpreting the ratios.

Fig. 1

Map showing sampling locations for fish collected in the Aleutian Islands, Alaska, from 2003 to 2005.

Fig. 2

Relationship between mean selenium:mercury molar ratios and mean mercury levels for fish from the Aleutians. Kendall tau= −0.6 for the relationship between selenium:mercury molar ratio and mean mercury levels.

Fig. 3

Relationship between mean selenium:mercury molar ratios and mean total fish length for fish from the Aleutians. There was not a significant relationship between the ratio and mean fish length among fish species.

Fig. 4

Individual selenium:mercury molar ratios for Pacific cod, yellow irish lord, flathead sole and great sculpin collected from the Aleutians. Kendall tau correlations indicate whether there was a significant relationship between the ratio and length for each species.

Fig. 5

Individual selenium:mercury molar ratios for rock greenling, halibut, dolly verden and black rockfish collected from the Aleutians. Kendall tau correlations indicate whether there was a significant relationship between the ratio and length for each species.

Fig. 6

Individual selenium:mercury molar ratios for red irish lord, rock sole, walleye Pollock and northern rock sole collected from the Aleutians. Kendall tau correlations indicate whether there was a significant relationship between the ratio and length for each species.

Fig. 7

Individual selenium:mercury molar ratios for Pacific ocean perch and Atka mackerel collected from the Aleutians. Kendall tau correlations indicate whether there was a significant relationship between the ratio and length for each species.

Fig. 8

Individual selenium:mercury molar ratios for muscle of common eider, glaucous-winged gull, pigeon guillemot and tufted puffin collected from the Aleutians. Kendall tau correlations indicate whether there was a significant relationship between the ratio and mercury levels for each species.

Fig. 9

Individual selenium:mercury molar ratios for eggs of common eider and glaucous-winged gull collected from the Aleutians. Kendall tau correlations indicate whether there was a significant relationship between the ratio and mercury levels for each species.