About 400 dead fish washed ashore in Jamaica Pond over the Memorial Day weekend, a massive die-off apparently caused by warmer temperatures, according to the Boston Parks Department.

The fish were mostly perch that had just been placed in the pond in April as part of an annual state fish-stocking program, according to Parks Department spokesperson Ryan Woods.

“Our park rangers noticed. They pulled out about 400 perch,” said Woods.

The Parks Department called in experts from the state Division of Fisheries and Wildlife, who determined that the die-off was likely caused by a relatively common natural event. As water warms up, it cannot hold as much oxygen, which fish require to breathe. In a crowded pond–such as one just stocked with thousands of fish–that means there is not enough oxygen to go around, and many of the fish suffocate to death.

Such die-offs are fairly common around the state in the spring, and have been noticed this year in various locations, according to a state fact sheet. State officials assume that is what killed the Jamaica Pond fish.

“It’s not uncommon,” Woods said, though he added, “Four hundred is a little higher than we had in past years.”

If the die-off continues, state officials will return with a biologist and take a closer look, Woods added.

Residents who notice dead fish in bodies of water can notify the state’s “Fish Kill Coordinator.” His name is Richard Hartley, and residents can call him at 508-389-6330 or 508-479-4092. Be sure to have the location of the fish kill and the name and phone number of a person who directly saw the dead fish.