Environmentalist group Greenpeace criticised authorities for towing the burning fishing vessel out to sea after it caught fire in Las Palmas port early on Sunday. The Oleg Naydenov, carrying 1,409 tonnes of fuel oil, sank late on Tuesday 15 nautical miles south of Gran Canaria.
Television images on Thursday showed oil floating on the water. Spain has sent three tugboats and two light aircraft to control the leak, Public Works Minister Ana Pastor told reporters.
Experts were trying to locate a robot which could dive 2.4 kilometres deep to inspect the wreck.
The state prosecutor for the Canary Islands has started an investigation into the circumstances surrounding the sinking, a government spokesman said.
The waters to the south of the islands are populated by turtles and dolphins.