The Chicago Police Department spent a record $116.1 million on overtime in 2015 — up 17.2 percent from the previous year — to mask a manpower shortage that has mushroomed under Mayor Rahm Emanuel, with police retirements outpacing hiring by 975 officers.

A Chicago Sun-Times report says the traditional summer surge in street violence triggered the highest monthly spending, with $16.8 million in overtime in August 2015.

But the second-highest monthly spending on police overtime — $10.4 million — occurred in November. That’s when a judge ordered the city to release the video of a white Chicago Police officer pumping 16 rounds into the body of black teenager Laquan McDonald.

As Police Officer Jason Van Dyke was charged with first-degree murder, protesters took to the streets to demand the resignations of Cook County State’s Attorney Anita Alvarez, Emanuel and then-Police Supt. Garry McCarthy. Chicago Police officers worked overtime, trying to keep those demonstrations peaceful.

Retirement numbers released in response to a separate open-records request filed by the Sun-Times help explain the need for excessive amounts of overtime.

The records show that 2,575 Chicago Police officers have retired since Emanuel took office in May 2011.