By Michael L Bass, Alan G Heath
Water Research, Volume 11, Issue 6, 1977, Pages 497-502, ISSN 0043-1354, DOI: 10.1016/0043-1354(77)90036-7.

Heart rate, ventilation frequency, coughing rate, arterial blood Po2, pH, lactate, methemoglobin, and hematocrit were monitored serially in trout exposed to free chlorine thrice daily of 0.4 or 0.5 mg l-1 peak residual concentration. A simultaneous bradycardia and hyperventilation occurred with each pulse of chlorine. Coughing rate rose 15-fold during each chlorination. All three parameters tended to return toward normal rates between pulses with recovery being noticeably better in 0.4 ppm chlorine pulses.
Arterial Po2 dropped to 40% of the control value during the peak of the first pulse and pH declined 0.25 units. Subsequent chlorine pulses caused more severe declines and progressively less interpulse recovery of these parameters. Blood lactate and hematocrit values were significantly elevated. Methemoglobin rose in concentration but not to a physiologically significant level. It is concluded that free chlorine kills fish by internal hypoxia induced by damage to the gills.

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