(Fixes paragraph 10 to say the results follow five straight years of record load factors, as opposed to a fifth straight increase in load factor)
CALGARY, Alberta, Feb 4 (Reuters) - Air Canada ACa.TO and WestJet Airlines Ltd (WJA.TO), the country's biggest carriers, filled higher percentages of available seats in December, but WestJet made the gains while increasing capacity during the sector's downturn.
WestJet reported a load factor of 76.8 percent, up 0.6 percentage points from January 2008. That was against an increase in capacity, measured by available seat miles, of 9.4 percent to 1.485 billion.
Revenue passenger miles rose 10.3 percent to 1.141 billion, said the country's No. 2 carrier.
Calgary-based WestJet has kept increasing capacity as industry conditions have weakened over the past year.
However, Chief Executive Sean Durfy said WestJet is experiencing downward pressure on its yields, the money it makes per seat, pointing to lower first-quarter revenues.
He said the high-demand Easter period fell in the first-quarter last year, which contributed to record revenue.
"Given the continued economic downturn, we are cautious with respect to our year-over-year revenue per available seat mile predictions," Durfy said in a statement.
Air Canada said its mainline filled 79.4 percent of seats in January, up 0.7 percentage points.
The Montreal-based carrier cut capacity by 8.5 percent to 4.33 billion available seat miles, and it flew a total of 3.438 billion revenue passenger miles, down 7.7 percent.
Strong traffic results after five straight years of record load factors show Air Canada is effectively managing its capacity, Chief Executive Montie Brewer said.
The load factor for Jazz Air LP JAZ_u.TO, Air Canada's regional affiliate, fell 4.5 points to 64.9 percent as it cut capacity by 11.9 percent to 424 million available seat miles.
In overall traffic, Jazz flew 17.7 percent fewer revenue passenger miles at 275 million, the airline said. (Reporting by Jeffrey Jones; editing by Rob Wilson)