A dip in the return of red salmon to the Kenai River prompted biologists on Monday to close Upper Cook Inlet to commercial drift and setnet fishing -- though anglers have not yet been affected.

The popular personal use dipnet fishery at the mouth of the Kenai River is due to close Friday night.

"The sockeye salmon return to the Kenai River appears to be much smaller than forecast," the Department of Fish and Game said in its press release.

Through Saturday, some 444,000 red salmon had passed the sonar 19 miles upstream from the Kenai River's mouth. And while that number is more through July 25 than any of the past three years, biologists believe the run is falling off rapidly.

The minimum escapement goal to maintain the health of future runs is 650,000 fish, and the Fish and Game's In-Season Alaska Salmon Summary of July 24 said, "We should be in good shape ... to achieve the escapement goal."

More than 56,000 reds passed the sonar on July 16, but fewer than 8,000 were counted Saturday. Some years, the run sees a second peak in late July or early August -- but not every year.

Jeff Fox, a commercial fish biologist for Fish and Game, said biologists now predict the full run will be 20 percent below the preseason prediction of 2.5 million fish.

"We're always guessing exactly what's coming," Fox said. "It's not an exact science."

And the timing of the run differs year to year. Three years ago, for instance, only 270,000 reds were past the sonar on July 25 -- or nearly 40 percent fewer than by the same date this year. But by the time the 2006 run ended in late August, some 1.5 million reds were upstream.

That year's run was exceptionally late, about nine days, Fox said. By contrast, this year's appears to be running a couple of days early.

Thus far, personal use and sport fishing regulations for Kenai reds have not changed. Some dipnetters working the mouth of the Kenai over the weekend reported poor fishing.

"At this stage," said Fish and Game biologist Tom Vania, "we expect we'll proceed until the end of July. In river, we'll probably be OK so long as our estimates hold true."