Relationship between trophic level and total mercury concentrations in 5 Steller sea lion prey species

Abstract

Total mercury concentrations [THg] were measured in 5 Steller sea lion finfish prey species collected in the eastern Aleutian Islands to determine if the amount and/or variation in mercury in select prey could explain the wide range of [THg] in sea lion pup hair and blood (Castellini et al. 2012, Rea et al. 2013). Atka mackerel (ATMA; Pleurogrammus monopterygius), Pacific cod (PACO; Gadus macrocephalus), walleye pollock (WAPO; Theragra chalcogramma), arrowtooth flounder (ARFL; Atheresthes stomias), and Kamchatka flounder (KAFL; Atheresthes evermanni) are known or suspected Steller sea lion diet items (Sinclair and Zeppelin 2002) and thus were chosen as the first focal prey species for this preliminary study. Fish samples (20 individuals per species) were collected and donated by Ocean Peace Inc. from winter 2013 commercial operations in fisheries management area 541. Fish were frozen whole at sea and subsampled at the UAF Wildlife Toxicology Lab for mercury and stable isotope (δ13C and δ15N) analyses. The [THg] increased with fork length (fish length) and mass in PACO, KAFL and ARFL (p<0.05) suggesting mercury bioaccumulates with age. PACO and KAFL showed significantly higher [THg] than WAPO, ATMA, and ARFL (p<0.05) although no concentrations exceeded 0.18 μg/g, ww. Thresholds of concern for human consumption of fish are 1 μg/g, ww. More enriched stable nitrogen isotope values in PACO and KAFL (12.9±0.9 and 12.2±0.3 respectively) suggest that these fish were feeding at a higher trophic levels than the ATMA, ARFL, WAPO (10.5±0.4, 11.5±0.5 and 10.5±0.8 respectively) which could explain the slightly higher mercury levels in these two species.