OTTAWA -- Canadian soldiers got an up-close look at bomb-detecting dolphins during a military training exercise with the Americans off the coast of British Columbia, a newly released document shows.

The U.S. navy brought four bottlenose dolphins and a small support team to Esquimalt, B.C., last year to practise anti-mine tactics as part of Exercise Trident Fury. The large-scale training operation took place in May 2011 and involved the armed forces and coast guards of Canada and the United States.

A briefing note to the chief of maritime staff described the exercise as a rare opportunity for the Canadian military to gain valuable experience working with the animals -- something it currently does not do.

"Canada has no similar programme for the conduct of mine counter-measures (MCM) and no intent to develop one," the April 2011 briefing note says.

"However, this is a principal means by which the (U.S. navy) will conduct such operations. It is likely that any MCM operation in our Pacific approaches will involve a combined defence construct and it is thus essential that we practice such procedures."

The document suggests the chance to train with the Americans and their dolphins on mine-clearing techniques was worth any risk of bad press.

"While there is a risk of negative perception developing within the public over this activity, operating with the full spectrum of USN MCM forces, including the (Marine Mammal System), affords the Canadian navy a unique opportunity to gain exposure to some of the most advanced MCM tactics and mine hunting systems in service today."

The Canadian Press obtained the briefing note under the Access to Information Act.

The American military has long used its so-called Marine Mammal System comprised of dolphins and California sea lions to hunt for mines and stop hostile swimmers and divers.

Canada does not use aquatic animals in its military operations and apparently has no plans to do so.

What makes the animals so useful to the U.S. navy is their sensory and diving capabilities.

A dolphin's natural sonar is still superior to any device used by the military, the briefing note says. Dolphins send out sound waves by making high-frequency clicking noises. They can tell what else is in the water by reading the echoes that bounce back.

Sea lions have excellent low-light vision and can tell which direction sounds come from underwater, which humans cannot do.

The U.S. navy Marine Mammal Program has been training dolphins and sea lions since the 1960s, but it was only in the early 1990s that the program was declassified.

During the 2003 invasion of Iraq, navy dolphins helped U.S. forces clear anti-ship mines and underwater booby traps planted in the port of Umm Qasr by Saddam Hussein's forces.

Earlier this year, a retired U.S. navy admiral told National Public Radio the Americans would use dolphins to find explosives if Iran followed through on its threat to mine the Strait of Hormuz, a passage in the Persian Gulf through which one-fifth of the world's oil shipments pass.

But the use of marine mammals in military operations remains controversial.

Animal-rights groups have criticized the U.S. military for putting dolphins in harm's way. Dolphins are big enough to set off a mine by accident, although the navy insists the animals never get close enough to the explosives to detonate them.

Canadian military planners worried the public might not like the idea of dolphins being used for the training exercise.

"It is assessed that the presence of the (Marine Mammal Systems) has the potential to be negatively received and portrayed due to local attitudes concerning human interaction with dolphins," the briefing note says.

"As a result, a reactive approach will be taken with regard to the MMS."

The document also made clear the military's intent to keep the dolphins out of the spotlight.

"While the MMS will operate within the naval harbour, it may still attract attention and a notable vulnerability exists in the movement of the MMS to and from the Victoria Airport," the briefing note says.

"All possible measures will be taken to maintain the lowest possible profile during planned transportation, while ensuring the safety of the MMS and minimal disruption to road traffic."

The Defence Department did not answer questions about the dolphins.