Acid rain and snowfall have wiped out all the fish and many plants in 50 percent of the high mountain lakes in the Adirondack Mountains, according to a new survey by the New York State Bureau of Fisheries.

The lakes, formerly clear, cold bodies of water known for their trout fishing, have reached such high levels of acidity that fish and plants have died and been replaced by acid-tolerant mats of algae.

The report also states that, while 50 percent of the high-altitude lakes have been killed by acid fallout, more than 10 percent of the lakes in the entire U.S. Adirondack region are now "dead." Some 280 of the region's 2,800 lakes have been found to be without fish and more than a thousand of the mountain lakes have not yet been tested.

The survey was described by Dr. Carl Schofield, professor of natural resources at Cornell University, during the meeting here of the American Association for the Advancement of Science, by Patrick Festa of the New York Bureau of Fisheries.

Another researcher, Thomas Hutchinson of the University of Toronto, said that there are many more vulnerable lakes in Canada than in the United States, and that the Canadian government has counted hundreds of "killed" lakes in Ontario and Quebec. He said that if lakes continue to reach acidity levels at the current rate, between 4,000 and 8,000 more lakes will lose their fish life in the next two decades in Canada.

The lakes in both the United States and Canada have been increasing in acidity. Researchers blame the oxides of nitrogen and sulfur which are carried by air currents from the industries and autos of the midwestern and northeastern states. The oxides then fall into the lakes with rain, or collect in snow and pour in heavy amounts into the lakes in the spring.

Many lakes can absorb and neutralize the acidic substances falling into them for many years, especially those that have thick layers of soil on the lake bottoms or a constant supply of flowing fresh water from springs or streams.

But many mountain lakes, "some of the most pristine and beautiful recreation lakes," Schofield said, "have only thin layers of soil on top of rock. These lakes can neutralize these compounds only to a limited degree." Two of the biggest and best known lakes that have lost their fish life are Colden and Avalanche lakes, both of which were fisheries for Brook trout in the 1930s.

Acidity is measured on the pH scale of zero to 7, with the numbers closest to zero being the most acidic, and 7 being neutral. Each lower whole number in the scale is 10 times the acidity of the number above it.

The average level of acidity of the rain and snowfall in the Adirondacks is now about 4.2. The average pH level in the lakes might be expected to be between 6 and 7. When the levels in the lakes drop to pH 5 or lower, fish life is likely to die out, Schofield said. For comparison, vinegar has a pH level of about 3.

Scientists have discovered that fish are not being killed directly by the acidity of the water in most cases, but instead are killed by the aluminum and other heavy metals in the lake soil.

Normally harmless, these metals are released into the water when the acidity of the lake increases.

Another result of the "acid rain" phenomenon which Hutchinson of Toronto noted is an ironic one. The nitrogen and sulfur which are killing fish seem to be acting as a tree fertilizer. "In the short run -- on the order of 30 years -- it appears that the acid rain may have a good effect," Hutchinson said, "though in the long run, it is more likely that the acidity will damage the trees."