The Department of Fisheries, Aquaculture and Environment released results Thursday of pesticide analysis of water samples taken from the Souris River where several hundred fish died last month.
A stream enhancement crew discovered the dead fish in the east branch of the Souris River in the Harmony Junction area the morning of July 20. Investigators collected approximately 300 fish in excess of four inches, including rainbow trout, speckled trout and salmon. A number of smaller fish were also lost but were not counted.
Bruce Raymond, head of rivers and estuaries in the Department's Water Resources Division, released the results of the pesticide analysis.
"Two agricultural pesticides were detected in water samples collected during the investigation," Raymond stated in a news release.
Lab results from the Atlantic Veterinary College toxicology lab showed a concentration of dithiocarbamates and metribuzin in the water samples.
Raymond explained dithiocarbamates are a group of pesticides which include mancozeb, the active ingredient in a fungicide which was reported to have been sprayed in the area the morning before the fish mortalities were discovered.
"The maximum concentration of dithiocarbamates in the water samples, which were collected 30 hours after a rainfall, was 131 parts per billion (ppb). Concentrations may have been higher at the time of death," he said. "Compared to reported toxicity values, this concentration is of concern for fish health."
With regard to metribuzin, a herbicide used for weed control, Raymond said the lab results showed a maximum concentration of 9 ppb in the water samples. He added this concentration is below that considered to be of concern for fish health.
Raymond noted there was heavy rain in the Souris area the morning of July 19, the day before the fish mortalities were discovered.
"Between 9 and 10 a.m., there was 7 mm of rainfall, with 6.2 mm of rain falling in the short span of nine minutes," he said.
Raymond said a farmer in the area reported he had applied mancozeb immediately prior to the rainfall. The farm weather forecast for Souris that morning was a 29 per cent probability of rain between 5 and 8 a.m. with zero per cent probability after that.
In other results of the investigation, Raymond indicated water temperature and dissolved oxygen levels in the water were normal.