One small piece of the huge puzzle that marks the decline of striped bass on the Atlantic Coast is falling into place in a federal laboratory in Missouri.

Scientists there are studying the effects of chemical contaminants on juvenile stripers. They have discovered significant levels of poisons in tiny stripers spawned in the Hudson, Potomac and Nanticoke rivers.

Working with sophisticated equipment that measures contaminant levels in fish as small as a half-inch long, the scientists have found relatively high amounts of PCBs, lead and cadmium in Hudson River fish; lead, zinc, arsenic and selenium in Potomac fish, and arsenic and selenium in fish from the Nanticoke River on Maryland's Eastern Shore.

According to Dr. Paul Mehrle of the National Fisheries Research Laboratory in Columbia, Mo., the research showed juvenile fish with high levels of contaminants had less backbone strength than uncontaminated fish in a control group. Also, he said, trends indicate that their growth rate is slower.

"During early life stages of fish life, if you have chemicals causing altered bone development and slow growth, these fish will not be as likely as uncontaminated fish to survive environmental stresses. It decreases the organism's ability to survive."

While the indication that contaminated fish grow more slowly is only that so far -- an unsubstantiated trend -- bone weakness was measured at the lab. Hudson River fish were shown to have 42 percent weaker backbones than those from an uncontaminated control group, and the fry taken from the Potomac and Nanticoke had 20 percent weaker bone structures.

The huge majority of East Coast stripers use the Chesapeake Bay and its rivers as spawning grounds. The Hudson is another major spawning area. Stocks of these prized game and table fish have fallen off dramatically over the last eight years, reaching a 21-year low in 1978.

Chesapeake sport fishermen have all but given up on the striped bass (called rockfish locally) and have switched their principal) interest to bluefish.

Whether chemical contamination of fry plays a significant part in the decline is not known but Mehrle said, "If we're going to find a time when the effects of these contaminants are likely to be the greatest, it would be in these early life stages."

He said when fish are in the fry stage they have their hardest battle to compete for food, respond to environmental stresses and avoid predators.

He said the effects of chemical contamination would not be seen in anything as obvious as a massive fish kill.Instead, affected fish simply would be less likely to survive these normal competitive pressures as tiny organisms in a large environment.

PCBs, the contaminants found in heavy doses in Hudson River fish, are chemicals that have been banned by the federal government as a suspected cause of cancer. These organic contaminants are thought to be particularly debilitating.

Lead, cadmium, selenium and arsenic are so-called heavy metals, inorganic substances that occur naturally as well as from industrial pollution.

PCB levels in Potomac striper fry were very low, Mehrle said.

The contaminants get into the flesh of the young fish in three ways, Mehrle said. Some exist at birth, passed on by the parent.Others can be absorbed through the gills and still more are taken in through the food chain.

Mehrle said levels of all contaminants increased as the fish increased in size, indicating there was continued absoprtion of poisons as the fish matured. s

It has been known for some time that mature striped bass have contaminants in their systems, but Mehrle said the federal study was the first concerted effort to determine levels in juvenile fish and the effects of the contaminants.

The issue of chemical contaminants is only one of a number of problems the Interior and Commerce Departments are studying as part of a three-year effort to determine why striper stocks are down.

Other suspected causes are habitat deterioration, including the decline of bay grasses that formerly served as nursery areas for young stripers; suspected overfishing by commercial and sport interests; industrial development, and an extended sequence of natural events that worked against any single highly successful reproduction year.

Striper stocks have risen and fallen in years past, and some observers believe the current decline is simply one of these normal fluctuations.