Mercury in fish in the Smallwood Reservoir,Labrador, twenty one years after impoundment

The elevated mercury (Hg) levels in fish flesh found after impoundment of a reservoir, are predicted to decline as the reservoir ages. The length of time required for a return to background levels is dependent on among other things, the trophic status of the fish. Predictions for omnivorous species range between 15 and 20 years while for piscivorous species they vary from 20 to 30 years. Fish in the Smallwood Reservoir, Labrador, Canada, were sampled 6 years after impoundment when hg levels were found to be elevated in most species. Selected of the sites were re-sampled after 16 years and again after 21 years. Mercury in the flesh of omnivorous species such as lake whitefish (Coregonus clupeaformis) had returned to background levels after 16 years as predicted. However, hg in the flesh of piscivores such as northern pike (Esox lucius) and lake trout (Salvelinus namaycush) remained elevated even 21 years after impoundment. While the levels in lake trout have declined somewhat in that time, there is no evidence of decline in the northern pike either within the reservoir or at downstream stations. Models predicting decline in hg levels in piscivorous fish in reservoirs must be re-evaluated in light of this extended data set.