Fig. 3. Recovery of the apex predator from mercury loading.
Comparison of changes in body burdens of lake spike MeHg during the recovery phase for the northern pike population (annual mean, black circles) to that of individual northern pike (grey lines and triangles). Individual northern pike were sampled at the end of the addition phase (in 2007; n = 16) and subsequently recaptured during the recovery phase (each line represents an individual fish). Population data are based on all fish captured each autumn (n = 280). All northern pike were sampled using a non-lethal biopsy (represented in images) in the autumn of each year and returned to the lake. Fish body burdens of lake spike MeHg (body burden = lake spike MeHg (ng g−1) × fish mass (g)) were normalized to concentrations in the autumn of 2007 (t0; the final time isotope-enriched Hg was added to Lake 658 and the beginning of the recovery period). Exponential decay regression starting in the second year of recovery estimated a 50% reduction in lake spike MeHg burden in the population in 4.2 years (data are mean (black circle) ± 95% confidence interval (shaded band); line fit: y = 1.7439 × e−0.2928x, R2 = 0.95, F1,6 = 95.5, P = 0.0002).