There has been an extensive fish kill on the Shenandoah River, Virginia's most productive and most heavily fished small mouth bass stream.

The state Water Control Board and the Commission of Game and Inland Fisheries agree on that. They don't agree on much else in the case.

The WCB, empowered to investigate fish kills and determine who, if anyone, is at fault, contends 3,000 to 5,000 fish of all species have died of natural causes.

"Out investigations have in no way revealed any reason to suspect pollution as a causative agent for this kill," said David Chance, pollution control specialist for the WCB's division of ecological studies.

The game and fisheries people are not convinced. Although it is in WCB jurisdiction, fish and game officials are doing research. Jack Hoffman, chief of the commission's Fish Division, expects his organization to have its estimate of the scope of the kill within a week. "We approached this thing on a separate basis," he said. "They [WCB] have unfortunately placed less emphasis on the magnitude of the kill."

Hoffman has worked in the fish and game division since 1954. He remembers well the 1959 kill below Front Royal that wiped out approximately 200,000 fish. American Viscose Corp. was cited by the WCB, which traced the deaths of zinc wastes discharged from the company's Front Royal plant. American Viscose, manufacturer of rayon, polyesters and other fibers, eventually settled the case out of court and paid $40,000 to the state.

The American Viscose plant is still in Front Roayal under the new name Avtex Fobers, Inc., and with new waste treatment equipment.

The fish kill first came to light in February, and WCB officials thought immediately for Avtex. "We figured it was Avtex," said James McNeal, water pollution biologist for the WCB. "But we found nothing to indicate it."

The Water Control Board investigationm cleared Avtex and other Front Royal industries because of one simple finding: dead fish were discovered upstream from Front Royal on both forks of the Shenandoan, where discharges from factories in the city could not flow.

"Most of the dead fish we found were a way downstream, 10 to 20 miles from Front Royal," McNeal said. But they also found "a significant number of dead fish up the North Fork above Front Royal," he said.

That's the heart of the disagreement. John Kauffman, fisheries biologist supervisor for the fish and game commission, also surveyed the forks of the Shenandoah upstream from Front Royal. He found no dead fish there.

"We found a moderate to heavy kill with WCB personnel and was directed to a few dead fish. "It looked like an average natural die-off."

What he saw below Front Royal, Kauffman said, "did not look like a natural die-off."

Kauffman said WCB officials told him they had seen 35 dead fish in a small area above Front Royal. "If I saw that many I'd say the kill extended above Front Royal, too. but I didn't see anything like it. They stand behind their counts and I stand behind mine."

Part of the disagreement may be traceable to the WCB's reasoning when the kill was first discovered. WCB officials were quoted in news stories as saying the fish apparently died from the cold.

The Wcb said at that time that ice on the river was cutting oxygen levels in the water and that some fish could have died from thermal shock as they moved suddenly into warm spring-fed tributaries.

Bob Martin, research director for the Sport Fishing Institute and a former Virginia fish and game official, called the thermal shock theory ridiculous, and had severe reservations about the oxygen depletion thoery.

In his view, the fish were moving into smaller tributaries "to get away from something they didn't like in the main stream."

Ehe WCB itself has backed off that initial reasoning. Chance said the theory now is that the excessive cold added one more stree factor to the fish environment and it was enough to kill a substantial number of them.

"We believe it was a combination of environmental factors," he said. "In the past, Shenandoah fish kills have coincided with severe winters. If the fish move to warmer waters (spring-fed tributaries) as a food source or for comfort and crowd in these tributaries, the concentration creates stress and if a fish is sick the odds for spread of the bacteria is greater," Chance said.

And WCB biologist McNeal adds, "Samples from dying fish indicated diseases, empty stomachs, a very stressed condition. We're not sure what the final factor is that killed them, but we believe it's the hard winter."

The kill affected all species - carp, bass, sunfish, suckers, minnows, carfish and others.

There is differing opinion on the immediate effect on fishing; some say it will be an extremely bad year for anglers below Front Royal, others see little change.

There is one piece of good news. Because of the tremendous reproduction potential of fish, populations should be back to normal in two years at the most, barring another kill.

Martin of the Sport Fishing Institute explained:

Fish produced great quantities of roe, perhaps 20,000 eggs for a four-pound bass. nature intends for two fish to survive from that, one to replace each "parent."

But when a stream is underpopulated, far greater numbers survive because there are fewer fish around to eat the little ones. So the only offsgoot of this or any kill in two years may be fewer big lunker fish per capita and more 1- and 2-year-olds.