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Sensitivity of fish to low concentrations of dissolved oxygen (DO) differs between species, between the various life stages (eggs, larvae, and adults), and between the different life processes (reading, growth, and reproduction, which in turn may depend on swimming ability, and specialized behaviour which may also be influenced by DO). Any DO standards set for fisheries must take all these into account, bearing in mind the type of fishery, the times and places the fish occur, and the likely impact on the fishery of impairment of each part of the life cycle. Although there is a considerable volume of laboratory data on the effects of DO on fish life processes, much of it is incomplete in terms of the distribution of the responses within fish populations at given phyeiologioal and behavioural states, and difficult to interpret in terms of ecological significance. However, the general pattern which emerges is that, providing other environmental factors (including the absence of poisons) are favourable, a minimum constant value of 5 mg/l would be satisfactory for most stages and activities in the life cycle in that some processes, such as juvenile growth, fecundity, hatch of eggs, larval morphology and survival, upstream movement of migratory salmon, and schooling behaviour of some species, including shad, are not particularly susceptible to levels of DO above 5 mg/l. However, this value may be unnecessarily high merely to eneure satisfactory survival of the fish and adequate growth of the juveniles.

Difficulties arise in formulating DO criteria for fisheries because of the widely different patterns of DO fluctuations which can exist in inland waters even when unpolluted and the uncertainty of predicting their effect even when adequately described, unless the levels are so low as to be directly lethal to fish or so high as to have no effect on them. There have been few experiments made which attempt to simulate natural conditions, most being made at conetant DO levels; furthermore, field data are generally inadequate in that either the fish populations preeent or the DO regime are poorly described, and their interpretation is complicated by the presence of other poisons. In these circumstances, only tentative criteria can be put forward. Because DO levels in a river normally fluctuate, it is inappropriate to put forward criteria based on a single minimum value never to be violated or even as several minima each not to be violated at a certain time of year, but they should be expressed as a minimum percentile distribution (e.g., a minimum 5- and 95-percentile value) over a year or part of a year, or, with estuarine fisheries, part of a tidal cycle. From the limited data available it is suggested that for resident populations of moderately tolerant freshwater species, such as roach, the anneal 50-percentile and 5- percentile DO values should be greater than 5 mg/l and 2 mg/l respectively and for salmonids the percentiles should be 9 mg/l and 5 mg/l reapectively. These values are to be regarded as being for general guidance only, because there are special circumotanCes where more consideration should be given to the seasonal distribution of DO, for example in estuaries through which migrant salmonids pass. Moreover, all theee minimum values might need to be considerably increased in the presence of high temperature or poisons. To enable more satisfactory criteria to be formulated, further laboratory studies are required which would reflect natural DO regimes and more field observations are required on the status of fish populations where the DO is lowered and on the movements of adults and young of migratory species. These should be coupled with more complete analytical data on the DO, both where the fish populations are only marginal and where they are well established. The coneequences of reduced fecundity, growth and reproduction, and of changed bellwiour patterns, on population dynamics and production of fish and of fisheries should also be appraised.