The hearing into the proposed Northern Gateway oil pipeline has heard that mystery spills -- or chronic discharges of oily effluent from ships -- pose a greater risk to the environment that one large oil spill.

On the opening day of the hearings in Prince Rupert, B-C, Nature Canada lawyer Chris Tollefson told an Enbridge panel that studies show the cumulative effects of constant minor oil spills have a bigger effect on marine birds than one catastrophic oil spill.

Jeff Green, who's responsible for the environmental assessment for the project for Enbridge, agreed and said that's why there's a call for increased enforcement of the law that stops discharges from ships.

But John Carruthers, president of the pipeline project, told the hearing that the project will fully comply with all regulations and laws if a tanker port is set up in Kitimat.

Nature Canada has expressed concerns that vessel accidents, oil spills and habitat disruptions could have a catastrophic effect on marine birds along the tanker route near the port.

If approved, about 200 tankers a year would travel up the B-C coast to the take on oil shipped between Alberta and B.C.'s coast.