One day after a 15-year-old child went missing while swimming off the Coney Island shoreline, Department of Parks and Recreation Commissioner Sue Donoghue acknowledged the incident and provided clarity on lifeguard staffing at the city’s beaches and pools as the city copes with its first heat wave of the year.

The Parks Department on Thursday said the portion of the beach where the 15-year-old boy went missing had been closed all year and was not staffed by lifeguards. The area had also been marked with red flags that said “no swimming,” according to the department.

“All of New York City mourns when something like this happens, and we do certainly at the Parks Department and as the city,” Donoghue said during an appearance on “Mornings On 1” Friday.

The tragedy occurred as the city attempts to grapple with high temperatures. As part of the city’s heat emergency plan, New York City mandated all Olympic-sized outdoor public pools stay open an extra hour — to 8 p.m. instead of 7 p.m. — Thursday, Friday and Saturday.

The policy does not, however, extend to the city’s beaches. When asked whether that decision was due to the lifeguard shortage, Donoghue said it was not.

“We don’t typically keep the beaches open an hour later. Those beach lifeguards were concerned about everyone’s safety, including the safety of our beach lifeguards,” Donoghue said. “They are out in the hot sun. It’s a difficult job to do, and we worry about fatigue and heat exhaustion.”

On the topic of the lifeguard shortage, Donoghue said the city currently employs over 800 — and that all of New York City’s public pools and beaches have opened this year. However, she said the city has had to restrict access to portions of beaches and pools.

“What we do in times like this when we don’t have full capacity of our lifeguards is, we’ll open the pool or open the beach, but we restrict access,” Donoghue said. “So they’ll be open 50%. We won’t be able to open the full pool or we’ll have sections of beaches that are closed.”