Toxicity of Chlorine to Brown Trout

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Toxicity of Chlorine to Brown Trout
By David J. Pike


This paper deals with the responses of the Brown Trout (Salmo Trutta Linn.), to free chlorine residuals in freshwater, and the potential toxicity that sometimes results when waste water containing free chlorine residuals, is discharged from sewerage and water treatment plants, into a receiving river or stream
Hatchery reared yearling brown trout were killed experimentally by exposure for 1 hour to free chlorine residuals, at levels as low as 0.04 mg/l, and to continuously maintained free chlorine residuals at levels as low as 0.01 mg/l. At 0.01 mg/l free chlorine in water, 50% of the test fish dies in 43.5 hours. The implications of the discharge of chlorinated waste water and sewerage into freshwater fisheries is discussed.


Concern at the effects of chlorine on freshwater fisheries in New Zealand, arose following a severe kill of brown trout in the Wainuiomata River in December 1970. The fish kill was observed following the discharge of waste water containing free residual chlorine, into the Wainuiomata River, from the Wellington City Water Supply Line, after disinfection treatment of a section of the line.
Earlier research had indicated that safe concentrations of residual chlorine may be extremely low, with levels of 0.08 mg/l giving a 7 day median tolerance limit (TLm) for Rainbow Trout. (Merkins, J.C., Studies of the Toxicity of Chlorine and Chloranimines to the Rainbow Trout. 1958: Water Waste Treatment Journal, Vol 1, pp. 150-151.).
Following the Wainuiomata fish kill, it was realised that the effects of free chlorine in water, on brown trout, must be determined. Experimental work is as yet incomplete, but early findings have shown far reaching implications to trout fisheries in New Zealand and to the treatment of water supplies and sewerage waste, due to the extreme toxicity of chlorine to brown trout.
It is a commonly held view among civil engineers concerned with water treatment, and with health authorities, that adequate chlorination of treated sewerage would make it acceptable, in terms of public health, for discharge into a receiving water. To be effective, a free chlorine residual of several mg/l would need to be maintained for at least 30 minutes before disinfection could be regarded as adequate. The sewerage could then be considered safe for disposal. In view of the experimental findings this chlorinated sewerage could be disastrous to trout fisheries unless measures to remove the chlorine are taken before discharge of treated wastes into a river or stream.


Bro trout used in the experiments were yearling, reared at the Wellington Acclimatisation Society’s Masterton Trout Hatchery, and the work was carried out at the hatchery. The test fish ranged in size from 12.5 cm to 18 cm in length.
The hatchery water supply is from springs on the property, and is strictly controlled. Dissolved oxygen concentrations were measured at 6.2ppm, and water temperature at 52­­o

Fahrenheit was recorded. These levels were constant throughout the experiment, and checked periodically.
Preliminary experiments carried out with brown trout, in 2 gallon plastic containers at high concentrations, indicated that exposure to 5 mg/l chlorine killed trout in less than 60 seconds. Exposure to 3 mg/l for 60 seconds killed trout within ten minutes, although some test fish subjected to 3 mg/l for up to 45 seconds, survived to 336 hours, before symptoms of intoxication appeared. Exposures to any concentration in excess of 0.04 mg/l chlorine for periods in excess of 120 seconds, caused complete mortality within 24 hours, the time taken to die being dependent upon periods of exposure, and the concentration of chlorine in the water.
Following the determination of basic levels, exposures to free chlorine were made in flow experiments, using standard hatchery troughs. Further increasing exposure times, and decreasing concentrations produced death of 50% of the test fish, at concentrations down to 0.05 mg/l within a few hours.
A second series of experiments were conducted to determine the tolerance level of brown trout to free chlorine residuals in water.
The hatchery troughs in which the experiments were conducted are arranged in pairs, with water flowing into the upper trough of the pair being controlled by a gate valve to the header trough, and thence overflowing into the lower trough through a standpipe. By drip feeding a strong solution of chlorine into the top trough, below the input valve, and adjusting the water flow through the trough, it was possible to obtain thorough mixing, and a relatively constant level of free chlorine (= 0.005 mg/l) in the second trough, receiving this chlorinated water.


Chlorine concentrations were adjusted in five pairs of troughs, to give readings of 0.01, 0.02, 0.03, 0.04, and 0.05 mg/l, continuously in the second trough of the pairs. A field method of chlorine determination was employed, using, acid ortho-tolidine as reagent, and a Lovibond nessleriser with suitable colour disc to make the chlorine estimation. Detection limit with any certainty, was about 0.005 mg/l chlorine.
Twenty brown trout were introduced into each of the lower troughs, and also into a sixth trough of unchlorinated water as a control. The time of introduction was recorded, and the fish were kept under observation for symptoms of intoxication, Fish were recorded as dead upon cessation of respiratory movements, and a failure to respond to stimulus with a probe.
As expected 50% of the fish at the two higher levels died inside 6.5 hours. Unexpectedly 50% of the test fish died at 0.03 mg/l chlorine within 7.5 hours, at 0.02 mg/l within 10.5 hours, and at 0.01 mg/l chlorine, at 50% mortality occurred in 43.5 hours.
First indication of response to chlorine were seen when the trout failed to take fright and dart to shelter when disturbed. They became increasingly more docile, could be handled readily, and appeared to lose vision. When not swimming they appeared to have difficulty remaining upright, and movement of the opercula was observed as rapid and arythmic. Ultimately the fish swam violently about the trough for 10-20 seconds, usually arched the body laterally then turned over. Death generally followed with ten minutes of turnup, even when the fish were placed in freshwater. The response to limited periods of exposure to chlorine ultimately occurring to all test fish. No mortality occurred in the control trough over the duration of the experiment.


Continuous, very low concentrations of chlorinated water are toxic to trout. Higher concentrations, at levels of 1-5 mg/l, are rapidly fatal to hatchery reared trout, if exposure periods exceed 45 seconds. Exposure periods ranging from 120 seconds to 60 minutes, to concentrations of chlorine from 0.04 to 1.0 mg/l caused total mortality to test fish within 24 hours, even though the fish were kept in clean flowing water after the period of exposure. If water supply and sewerage treatment undertakings are required to chlorinate treated effluent or disinfect water supply lines, before discharge of waste water into a receiving river is acceptable, then these undertakings will need to recognise their legal obligations towards freshwater trout fisheries. Experimental evidence indicates that trout are intolerant of chlorine at levels near the limit of detection by ordinary means. This means that it is not safe to discharge chlorinated water into a river at any concentration, particularly if the discharge is continuous.
The design of sewerage and waste water treatment, where it involves chlorination of the finished effluent, or chlorine residuals could be contained in waters discharged into a river or stream, will need to take into account some form of de-chlorination before discharge into a trout stream or river. This could be achieved by chemical means with sodium thiosulphate; or by retention in lagoons or ponds until “chlorine demand” of the water achieves de-chlorination.
There is a need for diagnosis of intoxication by chlorine, since the cause of  fish mortality due to chlorine. May not be detectable at the time a fish kill occurs. It is possible for fish kills to appear up to four days after exposure to low concentrations of chlorine for a limited time, and at the time of the observed fish kill the presence of chlorine is undetectable.
Histological changes in the gill-lamellae are not characteristic of chlorine intoxication alone, but resemble changes caused by other irritants and corrosives. It appears that the chlorine destroys the mucous and attacks the tissues, of the gills resulting in ultimate suffocation of the fish. Chlorine used in the experiments, was a commercially preparation of sodium hypo-chlorite (HTH) commonly used by local authorities in the treatment of water supplies, and by the public in the chlorination of domestic swimming pools, as well as for the chlorination of treated sewerage effluent.


I am most grateful to the Wellington City Council, for placing the facilities of their laboratory at my disposal; and particularly for the co-operation and assistance of the chemists, Mr Ian Vidal and Mr. Albert Collins, who set and monitored the chlorine levels throughout the investigation at the Masterton Hatchery.
(Copyright, David J. Pike 1971) - courtesy NZ Wildlife Magazine.