Departing seats at ONT compared to the same month in 2011 will continue to decline through October, according to the Official Airline Guide, an index on the travel industry.
There will be 25,000 less seats departing from ONT in April compared to April 2011, a 9.8 percent decline.
Also, a 8.9 percent decline is expected in May while June and July will experience dips in the 7 percent range, according to the forecast.
The Official Airline Guide can be used as a gauge of how much air service is available at ONT.
As the number of seats decrease, there is a natural fall off in passenger traffic because there are fewer seats to book.
Los Angeles World Airports last week released passenger figures that showed overall traffic at ONT is down 5.8 percent from last year.
Domestic passenger traffic fell 4.21 percent in February compared to the same month in 2011.
During the same time frame, overall traffic at Los Angeles International Airport - which like ONT is operated by Los Angeles World Airports - increased by 6.71 percent.
Several reasons have had a direct impact on those figures, said Jack Keady, a Playa Del Rey-based transportation analyst. The merger of several major airlines is among the top reasons.
"That's a whole lot of excess capacity that gets squeezed out," Keady said.
Increasingly, airlines are shifting their focus from medium hub airports such as ONT and opting for major markets such as LAX.
Low-cost carrier Southwest Airlines accounts for more than half of traffic at ONT.
In recent months, Southwest has cancelled flights into ONT and instead has opted to expand into larger markets in Texas and New York, Keady said.
"If Ontario wants traffic to grow, they have to convince Southwest that it's in their best interest," Keady said.
An airport executive who had been tasked with marketing and ensuring airlines such as Southwest increase flights at ONT departed in December.
LAWA's Executive Director Gina Marie Lindsey has assured the Board of Airport Commissioners that the director would be replaced.
On top of that, LAWA Commissioner Val Velasco requested a written report be given in public session by February about how ONT would be marketed by the person filling the position.
"In May, we will return to the Board of Airport Commissioners with a comprehensive set of additional management actions, which will include a proposed approach to air service marketing," ONT manager Jess Romo said.
LAWA officials do acknowledge there is a decline.
In 2007, there were 9 million seats - departing and arriving - at ONT. That figure has currently dropped to between 4 million and 5 million seats.
Having an air service marketing director for an airport is crucial, Keady said.
"Air service serves as link to airline schedules," he said. "Those people only put planes where the passengers and money is."
The director must convince an airline that it is an economic advantage if they bring flights to the airport.
The director also emphasizes, "if you put a flight in our airport it will work and let me show you the numbers," he said.
But air service marketing has always been an inherent conflict for LAWA, which has to sell LAX as well as ONT to airlines, Keady said.
"They have to talk about both airports," he said. "That's always been a problem with LAWA, like a split personality. This isn't to say they haven't tried to promote Ontario."
One of the issues is LAWA has not had an "especially close relationship" with Southwest, ONT's largest airline, Keady said.
"It may or may not have led to Southwest cancelling flights at ONT."
Coming out of the downward trend will depend the economy as well as the price of oil, he said.
An improvement in the economy would lead to more consumer spending and more leisure traveling.
On the other hand, if the price of oil goes up, airlines may start increasing costs or cutting flights.
ONT is not a strong market and there will be cuts at the airport, Keady said.
"During these times, airlines are looking at contracting in order to reduce costs," he said.
"The program for Ontario isn't really good."