Manipulation of Sport Fish Growth to Reduce Mercury Bioaccumulation on a Whole-Lake Scale

Integration of mercury (Hg) into aquatic food webs results in contamination levels in fish that are a concern for human and ecosystem health.  Altering food web structure has been shown to influence Hg concentrations in sport fish.  Here, we describe a whole-system manipulation designed to assess the effectiveness of stocking relatively high quality, low Hg prey (rainbow trout, Oncorhynchus mykiss), as means of increasing northern pike (Esox lucius) growth to reduce Hg concentrations.  A replicated pond experiment served as a reference and to evaluate the relative importance of manipulating northern pike densities to increase growth rates and reduce Hg concentrations.  On the whole-system scale we performed repeated measures of Hg concentrations on individuals in a naturally reproducing fish population.  Results indicate that stocking relatively high quality, low Hg prey is a rapid and effective method to reduce sport fish Hg concentrations by up to 50%.  Stocking selectively targeted large fish, which pose the greatest threat to human health.  These findings highlight the benefits of conducting Hg studies at whole-system scales.