Changes in mercury levels in lake whitefish (Coregonus clupeaformis) and northern pike (Esox lucius) in the LG-2 reservoir since flooding

Abstract

After flooding of the LG-2 reservoir in 1978–1979, it was noticed that Hg levels in fish rose dramatically. In this study the Hg data have been examined on the basis of fish age for lake whitefish (Coregonus clupeaformis) and northern pike (Esox lucius), representing two different trophic levels. Data were fit to Von Bertalanffy functions using non-linear regression analyses. Reductions in error sums of squares indicated that differences among years were the major sources of variation, but that there were differences among stations within years. For age profiles both species showed changes after flooding, whitefish increasing immediately and remaining similar until 8 years after flooding, with decreases afterward, while pike continued to increase until 8 years after flooding, remaining somewhat stable afterwards. For whitefish, cohorts (year classes) from before flooding had similar concentrations, with maxima reached by around 1982, concentrations afterward remaining stable. Subsequent cohorts reached plateaus at around 2–4 years of age, each successive cohort having a lower plateau. For pike, Hg kept increasing in an almost linear fashion, cohorts from before flooding having similar concentrations. Analyses indicated the very gradual return of whitefish to conditions present before flooding, while pike were remaining at much higher levels. Inputs to older pike were therefore not decreasing, but younger pike may have been improving.