Rapid, Efficient Growth Reduces Mercury Concentrations in Stream-Dwelling Atlantic Salmon

Mercury (Hg) is a potent toxin that biomagnifies in aquatic food webs. Large fish generally have higher Hg concentrations than small fish of the same species. However, models predict that fish that grow large faster will have lower Hg concentrations than small, slow-growing fish owing to somatic growth dilution (SGD). We examined the relationship between Hg concentration and growth rate in fish by means of a large-scale field experiment. Atlantic salmon Salmo salar fry hatched under uniform initial conditions were released at 18 sites in natural streams, collected after one growing season, and measured with respect to Hg concentration and growth. As expected for Hg accumulation from food, the Hg concentrations in the fish tracked those in their prey. Nonetheless, large, fast-growing fish had lower Hg concentrations than small, slow-growing ones, consistent with SGD. While the Hg concentration of the prey accounted for 59% of the explained variation in the Hg concentration in the Atlantic salmon across sites, the salmon growth rate accounted for 38%. A mass-balance Hg accumulation model shows that such SGD occurs when fast growth is associated with high growth efficiency. Fish growth is tremendously variable and sensitive to anthropogenic impacts, so the SGD of Hg has important implications for fisheries management.