Trophic Positions and Mercury Bioaccumulation in Rainbow Smelt (Osmerus mordax) and Native Forage Fishes in Northwestern Ontario Lakes

Abstract

Rainbow smelt (Osmerus mordax) is a recent invader to the lakes of the Hudson Bay drainage in northwestern Ontario, Canada. In some systems, the invasion has been linked to an increase in mercury (Hg) concentration in native predatory fish. This increase may be due to the fact that rainbow smelt are trophically elevated and thus accumulate more Hg than native forage fish species. To test this hypothesis, we compared the trophic positions and Hg concentrations of rainbow smelt and native forage fish in a series of smelt-invaded and reference lakes in northwestern Ontario. A comparison of forage fish 15 N (an index of trophic position) between the smelt-invaded and reference lakes indicated that rainbow smelt moved into a trophic niche that was unoccupied prior to their arrival. Relationships between 15 N and body size and between Hg concentration and body size differed among the forage species. This indicates that the response of predator Hg concentrations to smelt invasion depends on both the species and size composition of their preversus post-invasion diet. At a standardized body mass of 10 g, rainbow smelt were significantly trophically elevated relative to most native forage species, but they did not have significantly higher muscle Hg concentrations. Relationships between Hg concentration and 15 N were weak, both within and among forage fish species. This study shows that trophic elevation on a fine scale (within the forage fish community) may not result in increased contaminant bioaccumulation. It further challenges the general assumptions of food web theory and contaminant bioaccumulation.