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Four-day LC50s for Venezuelan Tia Juana crude oil were 0.86 mg/liter for first-stage larvae of the American lobster (Homarus americanus) and 4.9 mg/liter for third- and fourth-stage larvae. The 30-day LC50 was 0.14 mg/liter for larvae starting the test in their first stage. The threshold for retardation of larval development was about the same as the 30-day LC50. Decreased food consumption was demonstrated at 0.19 mg/liter. More "intermediate" larvae developed in oil exposures but no threshold was estimated. The ratio of "safe" to acutely lethal concentrations was about 0.03.Oil concentrations decreased during exposures; stated values could be multiplied by 0.59 to arrive at conventional average exposures. Stirring and ultrasonic vibration for 30 min dispersed averages of 7.4 and 18% of added oil. This and other techniques apparently dispersed similar components since toxicities were the same when based on measured concentrations. Aged dispersions were also equally toxic on a measured basis. Particles larger than 1.2 μm made up 84–96% of the dispersed oil and were about one-third as toxic as smaller particles and dissolved oil. For the reference toxicant DSS, the 4-day LC50 was 0.72 mg/liter for first-stage larvae indicating that lobster larvae are sensitive. Post-larval lobsters dug significantly more burrows when the substrate contained oil but did not avoid oiled substrate nor was growth or survival affected for substrates containing up to 1740 mg/liter of oil.

Three groups of Atlantic salmon (Salmo salar) parr were acclimated to water temperatures of 3, 11, and 19 C. When exposed at their acclimation temperature to lethal concentrations of zinc, cold-acclimated salmon survived longer than warm-acclimated salmon. However, the threshold lethal concentration (LC50) of zinc was highest for salmon at 19 C, i.e. there was less toxicity at the high temperature. At test temperatures lower than prior acclimation temperatures, survival times in zinc were longer but the threshold LC50 was lower, i.e. fish were less tolerant of zinc. The opposite results were seen at temperatures above prior acclimation temperatures.The threshold LC50 may be predicted by the equation LC50 = 1.9611 − 0.1873 Ta + 0.8699 Te − 0.0414 Te2 + 0.00004 Ta Te3, where Ta = temperature of acclimation and Te = temperature of exposure.In polluted rivers, zinc concentrations may be most damaging to fish populations during the winter months. Measured concentrations of zinc and copper in the Miramichi River, New Brunswick were highest in winter when estimated toxicity of metal was also greatest.

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.

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.