About 500,000 young salmon died over the weekend when a water pump failed at the Issaquah Salmon Hatchery, state Department of Fish and Wildlife officials said yesterday.

Even though "the loss is substantial," it probably won't affect the year's fishing season, said Doug Hatfield, complex manager of the hatchery. A backup reserve of salmon eggs and fish — usually supplied to schools and cooperative rearing projects — will offset the impact because many groups chose not to pick them up this year, he said.

"If I had to guess today, I'd say (the loss) won't be noticeable later in the year," Hatfield said.

A pump funneling water into the hatchery's egg-incubation room malfunctioned Saturday night after its motor burned out, the state fisheries officials said. The incubators housed more than 1 million coho salmon in different stages of development. Nearly half of the coho died, the statement said.

There was no backup pump. "It's a relatively new pump," Hatfield said. "It was only about four years old."

Hatchery staff tried to restart the pump, but that didn't work, so they moved by hand about 420,000 young coho to a nearby rearing facility with a different water source.

It's likely that some of those fish will die over the next several days because they were too young to withstand the trauma of moving, said John Kerwin, the state hatcheries division manager.

The state is replacing the water pump on the incubator and will install a backup pump.

Sonia Krishnan: 206-515-5546 or skrishnan@seattletimes.com