The bright red bodies of kokanee slipped and darted through the riffles and runs, sometimes pulling toward the bank of Hayden Creek for a brief rest.

The fish grouped together in pools, searching for the easiest way around woody debris and other natural obstacles in the stream.

Kokanee, a land-locked sockeye salmon, were easy to spot Tuesday as they swam upstream in Hayden Creek. Some were in the throes of death, lying on their sides and twitching. Plenty of others already were dead and decomposing.

It's all a new sight on Hayden Creek, as the fish is a newcomer to Hayden Lake and its primary feeder stream.

"This is the first year we've had (Hayden Lake kokanee) that have attempted to spawn," said Phil Cooper, a spokesman for the Idaho Department of Fish and Game. He said the department counted a couple hundred in the creek.

The agency stocked 100,000 kokanee fry in Hayden Lake in both 2011 and 2012, marking the introduction of the fish to the lake. Another 150,000 were added earlier this year.

Many of the fish have matured and are spawning, though they should be done soon, he said. They are a strain of early, or September, spawners.

"The expectation was that most of those fish would have been caught" by fishermen trolling on the lake, he said.

Fishermen gave it try this year. Some anglers this spring reported catching kokanee in the 14-16 inch range.

Cooper said Fish and Game doesn't anticipate the natural reproduction will be successful because of water temperatures, he said.

But that's OK, he said. It will allow Fish and Game to better manage the population in Hayden Lake through stocking.

Dennis Scarnecchia, a fisheries professor at the University of Idaho, said kokanee mature and are ready to spawn after two to four years in a lake.

Population density and how much food the lake can produce "have an effect on how old and large the fish are when they mature," Scarnecchia said.

Some of the kokanee's more obvious physical characteristics are tied directly to spawning.

Scarnecchia said the hooked jaw, or kype, that kokanee males develop is a "secondary sexual characteristic."

He said it's a signal of dominance.

"It makes them look more formidable, just as large antlers might on a buck deer or bull elk," he said. "The males are then more likely to be able to mate and pass on genes to the next generation."

He said it's less clear if it has an actual advantage in a combat situation against other males.

As for the bright red color, he said, it works as a signal to other kokanee that the fish is mature and ready to spawn after spending the rest of its life with a silver color.

"At spawning, there is a strong selection for red as a sign of a good breeder for both sexes," he said.