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Bleached kraft mill effluent (BKME) seems to contain at least two different toxic mechanisms or materials which affect aquatic animals. BKME stored under clean and quiescent conditions, lost much of its toxicity for salmon parr ( Salmo salar L.) in 1 week, and all in 2 weeks. By contrast, it lost little toxicity to lobster larvae ( Homarus americanus Milne-Edwards) in 2 weeks. Bio-oxidation of BKME for 1 week eliminated toxicity for salmon parr, but did not greatly change toxicity for lobster larvae. Biooxidation for 2 weeks reduced but did not eliminate toxicity to lobster larvae. A waste treatment process which reduces toxicity of BKME for one species may not protect other species.

It is now possible to predict toxicity of mixtures of two or more pollutants on the basis of chemical measurements. Toxicities of individual components are added up as fractions of the incipientlc50 (lethal threshold concentration). Total toxicity of the mixture is expressed as a single number. A standard terminology is given for the effects of two or more toxicants acting simultaneously. Modifying conditions (temperature, water hardness, etc.) greatly affect toxicity. Much fragmentary information should be brought together and examined for underlying patterns. Concise summaries for estimating modifying effects have been published for rainbow trout. Computerized multivariable analysis is a promising tool, especially for studies of sublethal toxicity. Effects of fluctuating concentrations of toxicants may be evaluated by four approximate techniques; the theoretical basis for understanding effects still seems unproven. Acclimation to toxicants should be studied in detail for representative pollutants. There is a great deal of information on relative resistance of different species of fish but it needs summarizing. A trend towards use of ‘standard’ fish species in research is good. With invertebrates and algae, as much as with fish, tests of sublethal effects such as growth and development are often more meaningful than tests of acute toxicity. Chemical autopsy methods seem promising, more so than histopathological approaches, which suffer from practical difficulties in field situations.

The lethal threshold concentration of neutralized bleached kraft pulp mill effluent for juvenile Atlantic salmon (Salmo salar L.) is about 12–15% in fresh water. This agrees in general with work on other salmonid fish. Larval lobsters (Homarus americanus Milne-Edwards) are reasonably resistant to this waste, and concentrations below 10% did not greatly affect their survival in sea water. Adult lobsters in sea water are more resistant than lobster larvae or salmon parr.

The review describes profitable methods for measuring lethal levels of pollutants for aquatic organisms. Methods for research in the laboratory are emphasized but the same principles could be applied in field work. Greater use of standard toxicological methods and terminology is urged. For 211 out of 375 toxicity tests reviewed, acute lethal action apparently ceased within 4 days, although this tabulation may have been biased towards short times by a large number of static tests. The incipient LC50 (lethal concentration for 50 per cent of individuals on long exposure) is recommended as the most useful single criterion of toxicity. If this cannot be estimated, the 4-day LC50 is a useful substitute, and often its equivalent. A desirable first step in toxicity tests is to estimate median lethal time for each of a series of concentrations. A toxicity curve should be drawn by plotting median survival times against concentrations on logarithmic paper. The curve helps to reveal any unusual features of toxicity. Whenever possible, tests should be prolonged until the toxicity curve becomes parallel to the time axis, indicating a lethal threshold concentration. The incipient LC50 is then estimated by selecting an exposure time from the asymptotic part of the toxicity curve; for this exposure time, observed mortality is plotted against concentration on log-probit paper, and the LC50 is read from an eye-fitted line. Confidence limits of the LC50 may also be estimated by simplified methods. These should be given in published work along with a value for slope of the probit line. Alternative approaches use reciprocal transformations or estimate LC50's for a series of exposure times. A graph is given for estimating partial replacement times of water in tanks of continuous-flow tests. Rate of flow should give a short partial replacement time, such as a flow equal to volume in 3–5 hr, and also adequate water for respiration of fish, usually 2 or 3 l/g of fish/day, or more.

Rainbow trout ( Salmo gairdnerii Richardson) showed strong avoidance reactions to sublethal concentrations of zinc sulphate. The threshold avoidance level was 5.6 μg/l of zinc added to laboratory water. This is only 0.01 of the lethal threshold concentration. There were no significant differences in threshold avoidance levels at 9.5° and 17°C., nor when background of zinc in the water was increased during acclimation and/or testing, from 3 μg/l to 13 μg/l.

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.