Few people likely know the Gorge is home to a native species of oyster, but this little creature has played a part in delaying the construction of the new Craigflower Bridge.

"The oysters are probably the key issue," said Jim Hemstock, Saanich's special-projects manager in the engineering department.

The law requires that the bridge-replacement project results in no net loss of habitat, he explained. "If we're disrupting the oysters, then we need to compensate."

While Olympia oysters are listed as a species of special concern, they are more abundant in the Gorge than anywhere else on Vancouver Island.

The World Fisheries Trust has been monitoring their population every July since 2009.

The counts show the numbers increase as you travel up the Gorge, said Nicole Barrette, summer student in charge of this' year's count. Last week, she and other researchers collected the oysters at three index sites and transported them to headquarters in Vic West. The oysters lived in tanks while she counted and measured them before returning them to their beds.

For this first time, the was included in the survey. Investigation revealed that the Olympia oyster population is significantly more dense between the wooden pilings than any other oyster bed along the waterway.

Under the bridge, there are 400 oysters per square metre, compared to 250 oysters per square metre elsewhere.

The findings have big implications for Saanich and View Royal, which are jointly replacing the bridge.

While the environmental mitigation plan has yet to be approved by the federal government, it likely involves transplanting each oyster, Hemstock said. "It's going to be very expensive."

World Fisheries Trust has been hired to help design the compensation plan.

"The new pilings will be concrete, which oysters like," said Trust executive director Joachim Carolsfeld.

To help oyster larvae settle on the pilings, he recommends texturing the concrete with horizontal ridges.

To help predict other factors in the oysters' survival, he is overseeing related studies in the Gorge. One is looking into the presence of invasive species and their interaction with Olympia oysters. Pending the receipt of a grant, he also plans to look at the movement of oyster larvae.

Keeping watch of the oyster population is important because they provide a good sentinel of water quality and health of the ecosystem, Carolsfeld said. They're also important in their own right, he said, as they filter water and provide habitat for other sea life.

rholmen@vicnews.com