Authorities are investigating the mystery of what killed thousands of small fish in a large detention pond in Willband Creek Park.
Obvious possibilities – such as contamination – have been ruled out by the Ministry of Environment, and, as of Wednesday morning, the deaths were still a mystery.
The kill was discovered on Sunday by Abbotsford resident Gary Stewart, who walks the park – located at Highway 11 and Bateman Road – an average of three times a week.
Stewart often wanders off the trails to walk along the park's three large stormwater detention ponds and admire the ducks, geese, hawks, eagles and other wildlife that gather there.
On Sunday afternoon, he spotted an area where a few dozen dead fish had pooled. As Stewart followed the edge of the pond, he realized there were thousands.
He contacted the Ministry of Environment as soon as he arrived home.
"I was worried about what could have killed all those fish," he said.
Ministry spokesman David Karn said an environmental emergency response officer went to Willband Creek Park on Sunday afternoon.
Initial testing showed normal levels of pH and dissolved oxygen, and the water temperature was slightly higher than normal, likely due to the previous hot spell.
"There was no obvious source of contamination and no physical sign that the fish were exposed to a chemical contaminant," Karn said.
He said the ministry is working with Environment Canada and the City of Abbotsford to determine the next steps.
Volunteers with the Ravine Park Salmon Enhancement Society (RPSES) were also notified of the incident. It has been confirmed the dead fish were not related to the annual Mother's Day release of 10,000 coho salmon fry into the nearby Stoney Creek.
But concerns about the habitat sent volunteers to Willband Creek Park on Monday afternoon to see the dead fish for themselves.
Doug Gosling, a member of the Stoney Creek Salmon Stalkers, which is affiliated with RPSES, said they believe the small fish are three-spined stickleback. No other fish appeared to have been impacted.
"Of course, we do not know what killed these fish and can't really even speculate why they've ended up dying in such numbers," he said.
Two large detention ponds were constructed at the site in 1998 to help ease flooding in the area during rainstorms. Two hydraulic control structures allow water to flow between the ponds and Willband Creek.
As the water level in the creek subsides, flap gates allow the water temporarily stored in the ponds to flow back into the creek, although the ponds contain water at all times.
Tanya Bettles, environmental coordinator with the city, said the gate valves are opened in mid-May and closed in late September or early October to allow the passage of fish between the creek and ponds during the summer.
A new pond was constructed last year and was connected to the existing ponds via an open channel. That pond was designed to be dry during summer and filled with floodwater during winter storms.