Mortality of juvenile Atlantic salmon (Salmo salar) exposed to water withdrawn from the hypolimnion of a headpond on the Mersey River, Nova Scotia / G.J. Farmer, T.R. Goff and D. Ashfield. : Fs97-4/2016E-PDF

"A mortality of 8.6% of the 683,340 juvenile Atlantic salmon (Salmo salar) at the Mersey Fish Culture Station occurred during 12-25 August 1988. The salmon parr had been provided with water withdrawn from a depth of 9 m near the bottom of a headpond located on the Mersey River, Nova Scotia. The water was dark brown, smelled of hydrogen sulphide and had a very low concentration of dissolved oxygen. In addition, a reddish-brown material was observed to accumulate in boxes of limestone gravel placed within the hatchery ponds to aerate and alter pH of the incoming water. The same material was observed to adhere to the gill filaments of moribund parr. We suggest that ferrous and manganous bicarbonates were released from the headpond bottom sediments. Release of these bicarbonates occurs under anoxic conditions in the presence of carbon dioxide and a reducing agent such as decomposing organic matter. Subsequent aeration of the water containing ferrous and manganous bicarbonates resulted in the formation of ferric and manganic hydroxides which are insoluble precipitates. Accumulation of ferric and possibly manganic hydroxide on the gill filaments of the salmon parr appeared to cause an impairment of respiratory function which resulted in mortality. Concentrations of hydrogen sulphide in the water supply may also have contributed to the mortality of parr which occurred"--Abstract, page v.

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