No one knows where they come from. Suddenly, over night, they're there . . . swimming and eating and making little fish as if they belonged.

But they don't belong. They are intruders. Unwanted water guests. They will, given the smallest opportunity, take over a river or lake. In a very short time they can ruin good fishing.They are illegally planted fish. Or, fish put into streams or lakes that for sound biological reasons shouldn't be there.

So-called "midnight stockers" put them there. They'll bring them, alive, in buckets or boats, and release them with no concern for the ultimate outcome.

Starvation Reservoir was one of the state's best rainbow trout fisheries at one time. Within two years it fell to one of the state's worst. Utah chubs moved in and pushed trout out.

Strawberry, once regarded as one of the most productive waters in the country for trophy-size trout, is being treated next week to kill what few trout remain. Utah chubs and Utah suckers took over.

Scofield was once Utah's consistent "hot spot" for good trout fishing. Water quality became a concern, but even more frightening to fish biologist was the discovery of spawning walleye.

Newcastle, once a trout fishing showcase, is today a shadow of itself. Crayfish have eliminated plantlife from the lake and and much of the trout's food source.

Fish Lake once produced some of the state's largest trout. Chubs pushed out the rainbow, then lake trout were put in to control the chubs, which they did. Not to be left alone, yellow perch were put in the lake and are now pushing out the chubs. Good? Biologists expect the lake trout to switch to the rainbow soon.

At Deer Creek the walleye were illegally planted, at Gunlock the green sun fish, at Bullock the white sucker, at Panguitch the Utah chub, at Hyrum the gold fish, at Recapture the black bullhead, and on and on and on.

It's not that some of these fish don't have a value to fishermen, it's just that in most cases they don't belong where people put them.

Take walleye for example. Many consider it the most dangerous fish ever brought into Utah. Trout and walleye cannot, under any circumstances, co-exist. Walleye are a voracious feeder and can wipe out a large trout population in a few years.

In the midwest the walleye is the No. 1 game fish. But back there, explained Glenn Davis, fisheries program director for the Utah Division of Wildlife Resources, there's a large enough food chain to support the walleye.

"We don't have that in Utah. Instead of going after a forage fish, walleye go after trout. They'll hammer trout. Another thing is that fishermen don't know how to fish for them here, so they're hard to catch. They just don't satisfy a lot of fishing pressure," he said.

Wyoming, too, is having serious problems with the walleye. To try and solve the problem there, fish managers are planting larger trout, between 10 and 12 inches, to see if their size will protect them from the walleye. Catchables in Utah are generally between six and seven inches.

The DWR has never planted walleye. Nor, have they planted chubs, suckers, perch, crayfish, or sunfish in abundant numbers and yet they show up with unsettling regularity.

Why?

Self-proclaimed biologists, mainly. A person will decide a certain fish should be in a lake or river and put it there. Or, people will fish a water with illegal live bait and when they're through dump the live fish in the water.

Or they'll catch fish in another state, put them in a bucket or the "live well" of a fishing boat, transport them back to Utah and release them. Two fish is all it takes.

Once the fish have a foothold, then nothing short of total treatment, such as is happening with Strawberry Reservoir at a cost of $2.8 million, can correct the problem.

One of the best examples of this is Pineview Reservoir. The lake has trout, bass, crappie, perch, carp and bullhead catfish. The only fish legally planted there is the trout.

In the 1970s, Pineview was one of the best trout lakes in the state. Then came the illegal plants. The DWR tried stocking seven-inch fish to try and get some survival, but got instead a four percent return and have since discontinued planting trout there.

Strawberry is another example. In 1981, 140,000 fishermen took out nearly half a million trout. Then the chubs moved in. The theory is that fishermen brought Utah chubs and suckers there as illegal live bait. Today there are 95 chubs for every five trout. Chubs have an unappealing appearance and taste and are better feeders than rainbow and better survivors.

Newcastle Reservoir was once a popular family fishing spot. Somehow crayfish were introduced. Within a few years the DWR had to cut back its trout planting program by two-third because of the high number of crayfish.

One thing that concerns Utah fish biologists is that once crayfish are in a waterway, they can never be eliminated.

Some waters can handle the new species. At Flaming Gorge, for example, chubs are a good food source for the large Mackinaw. But, the chubs have knocked out the rainbow.

At Lake Powell, crayfish are important in the food chain for smallmouth and largemouth bass.

And, for a time, chubs proved beneficial in Fish Lake.

In most cases, however, illegal plants prove disastrous, both in fishing success and economically.

This year fishermen will put in about 50,000 fishing days at Strawberry. Once treated and pressure returns to what it was during peak year, it is estimated fishermen will spend 300,000 fishing days there.

When a favorite fishing spot turns bad, some fishermen simply quit fishing while others shifted to other waters. In some cases, those waters have been unable to handle the added pressure.

But the loses go deeper than just fish. The economic impact is tremendous, particularly on the smaller communities where fishermens' dollars are important to the economy.

A 1985 study by the U.S. Department of Interior Fish and Wildlife Service, showed fishermen spend on average $29 a day on fishing-related services - food, transportation, gas, equipment.

"The problem," explained Davis, "is that these illegal plants take management choices away from us. Often we end up doing things simply to try and keep a balance. Sometimes it works, sometimes it doesn't.

"Sometimes we simply bend. At Hyrum and Newton (reservoirs), for example, we're stocking bluegill, bass and perch. As soon as we treated them these fish showed up. So now were managing it for warm water fish. There wasn't much else we could do."

The solution to the problem is education.

"We've got to convince these people that it's a bad thing," said Davis.

"I think most of it (illegal planting) is intentional myself, but it's hard to prove. But these people are just misinformed," he added.

These so-called "midnight stalkers" are costing the state millions of dollars in treatment projects and lost revenue. They, too, are costing fishermen some of their best fishing spots.

When you lose one fishery, the impact is tremendous. When you lose them at the rate Utah is, and for no good reason other than to satisfy one fisherman's selfish and uneducated decision, then it is crippling.