Edward Bruce Mason fired the gun that killed Germain Arthur Gaulin, Yukon Supreme Court heard this morning.

Charged with second-degree murder for the shooting death of Gaulin on April 20, 2004 in Watson Lake, Mason has pleaded not guilty to all charges.

Gaulin, 43, died from two gunshot wounds to the chest.

Mason, 64, is making his plea under section 16 of the Criminal Code, which is the defence of mental disorder.

This means the court must determine Mason's state of mind at the time of the shooting, as well as shortly before and afterward, said Crown prosecutor David McWhinnie.

'Reduced to its simplest, (the court) must discern whether (Mason) is entitled to section 16 or whether there is appropriate doubt to charge him with second-degree murder or manslaughter,' he said.

The issue for visiting Justice Rene Foisy is to decide whether Mason's state of mind at the time of the shooting makes him criminally accountable for Gaulin's death.

Mason's level of intoxication at the time is also at issue, McWhinnie told the court.

After he died, Gaulin's body was mutilated by several stab and cut wounds, the court heard.

'It appears fairly shortly after the shooting ... the body was disfigured by stabbings, several stabbings,' McWhinnie said.

One of the post-mortem injuries included part of the ear being cut off and placed on Gaulin's back.

Mason has no memory of inflicting these injuries, according to McWhinnie.

He said key testimony will come from an eyewitness account by a man who was present the night of the shooting.

The trial began this morning and is set to continue for two weeks. It's being heard by Justice Foisy alone, meaning there is no jury.

While Mason originally chose a jury trial, he re-elected to be heard by a judge of the Yukon Supreme Court.

The trial was adjourned this morning for Justice Foisy to read reports by psychologists who interviewed Mason.

The trial was scheduled to resume at 2:00 this afternoon.