Effects of copper-zinc mining pollution on a spawning migration of atlantic salmon

Pollution from a base metal mine on a tributary of the Northwest Miramichi River caused many adult Atlantic salmon, which were on their normal upstream spawning migration, to return prematurely downstream through a counting fence on that river during summer and early autumn. These observations gave an opportunity to document avoidance reactions of salmon to pollution, which has seldom been done in the fishes' natural environment. Downstream returns of salmon rose from between 1 and 3 per cent during 6 years before pollution to between 10 and 22 per cent during 4 years of pollution. Early runs (June–July) of salmon to the headwaters were delayed and reduced in number. Chemical analyses of river water showed levels of Cu2+ and Zn2+ which varied with rates of river discharge. During some periods Cu2+ + Zn2+ concentrations exceeded lethal levels for immature salmon, as established in another (laboratory) study. The threshold concentration for 50 per cent survival of fish under specified temperature conditions is designated as 1·0 toxic unit. Adult salmon in nature showed avoidance reactions at about 0·35–0·43 toxic unit of Cu2+ + Zn2+. A level of 0·8 toxic unit may have blocked all upstream movement. Of the salmon returning downstream because of pollution, about 31 per cent reascended, 62 per cent were not seen again and 7 per cent were taken by angling and commercial fishing below the counting fence. Estimated losses from the stock available in the upper part of the river from 1960 to 1963 varied from 8 to 15 per cent of the total run. There is no evidence that successive year-classes of salmon are growing accustomed to the pollution.