Marsh, mud, muskrats and money. That's winter to the hundreds of Maryland hunters who begin trapping when the three-month season opens in mid-December. Some say they do it for the love of nature; others admit to an interest in the more than $3 million earned each year in the state through the sale of pelts.

Most trappers, however, would set a line for practically no money at all, just out of habit. For them, trapping, the practice of caging, snaring or clasping the leg of a wild animal for its fur or meat, is an old rite.

Passed on through generations, neither the ritual of trapping nor the business of trading fur has changed substantially since Europeans first sailed up the St. Lawrence River in 1535. The business has grown, of course, and, through the efforts of humane organizations, traps and tools have been developed to lessen animals' suffering

But mucking around in waist-deep mud at low tide on a bitter cold marsh and stretching hides and haggling with traders as they make their rounds go on much as they always did and may always do. Trapper:

"We're 20 minutes out of Washington," says Upper Marlboro trapper Jim Gentry. "When the weather's right meaning Canada-like cold , you'd think you were in Saskatchewan."

Gentry, 44 and graying at the temples, is still wearing the blue dress shirt but not the clip-on tie he wears as a Prince George's County school administrator. Glass of wine in hand, he draws on a cigarette and watches night settle over the Patuxent River from his front porch step. On the other side of the broad, smooth water are 440 acres of marsh he leases; that's where the muskrats are. He expects to take close to 1,000 of them this season.

"I love to do it only because it needs to be done," says Gentry, who expects to make less than $4 a pelt this season after watching the market fall from $10 two years ago.

"When you look at what muskrats do to the marsh--honeycomb it, aerate it, keep it from silting up--you want to keep the population high. The only way to do that is to harvest them."

Gentry maintains that leaving muskrats alone allows the older males in the population to dominate the rest by killing new litters. Trapping encourages production by taking nine males for every female, which are less active during the trapping season and, therefore, less likely to swim into traps.

Gentry learned to hunt, trap and fish at his boyhood home of Lonaconing, Md., close to where the Potomac River emerges as a stream out of the Appalachians. He speaks of the rhythms of the seasons and says he might retire in three years "just to go fishing and white-water canoeing in the spring, ginseng picking and backpacking in the summer, hunting in the fall and trapping through the winter." It is a sentiment he repeats often.

Trapping starts with, well, traps: leg-holds, instant killers and snares. In the trapper's mind, the most nefarious thing about these tools are the blanket statements made about them by "Bambi-ites" and "antis," the save-the-seals, airlift-the-burros and don't-hunt-deer folk to whom all wildlife killing is senselessly cruel.

Gentry contends that there is a trap designed for each animal hunted and that special care is taken in the design of those used on land to spare the captive pain. An instant killer, named Conibear in honor of its inventor, is restricted by law to use underwater.

The most widely used leg-hold, called a one-and-a-half, is made to hold a small foot firmly. These are Gentry's favorites for fox. He demonstrates by putting three fingers inside the open jaws of one and snapping it shut: no damage. He maintains that a cat or dog would be released from a trap in the field within 24 hours of being caught, that being the frequency with which the law requires trappers to check their lines.

To prepare his steel traps, Gentry blackens them by boiling them in a tannic acid solution of water and red oak bark, then eliminates any trace of odor with a boiling in beeswax.

When the season opens, the traps are set and baited according to the habits of "target animals." In Maryland, that includes beaver, gray fox, red fox, mink, muskrat, nutria, opossum, otter, raccoon, skunk and weasel. Gentry's craft is one born of winters spent following tracks across fields, from finding animals in the off-season and observing how each buries its food and marks its territory.

While a fur bearer such as a fox must be lured to a trap with urine (the same way dogs are lured to fire hydrants by the scent of other dogs), muskrats demand less care. Traps are simply set in water, and the constantly swimming animals find a steel-jawed death.

Each species is killed differently. Raccoons get a bullet in the back of the head. A knee pressed hard on the chest of a fox stops the blood flow to its brain. For others, the blunt side of a hand-held ax is quite effective. Gentry prefers the ax, as it kills more quickly and surely than a bullet.

Gentry carries his catch from the marsh in a hickory basket. His wife, Carol, "not a lady born to the country," handles skinning the muskrats.

"It took a little while for her to come around," Gentry said, "but she can do 40 in an hour, have the fur stretched and the meat wrapped for the refrigerator."

Muskrat meat is considered good eating on the eastern and western shores of the Chesapeake Bay, where dealers will pay about 75 cents a "rat" to sell it as "marsh hare" in fish markets or restaurants.

The muskrat is a water rat 10 to 14 inches long, not counting the tail, with dark brown or black fur. It builds lodges, keeps underwater entrances, burrows in banks and feeds on cattails, grasses and mussels.

The meat of the other game (raccoon being an occasional exception) goes to the renderer. Many trappers will keep the sex and scent glands of their catch as lures for the next time out.

Although trapped animals may be sold whole to a buyer, Gentry, who prefers making just one end-of-season run to his dealer, stretches, flays the fat from, and air-dries hides himself.

"One thing with muskrats," Gentry said. "You have to get them when the tide is out." Since the law requires trappers to check water traps at least once every 36 hours, having a daytime job can mean a foray after dark. "I don't mind," Gentry said. "You probably couldn't get me to go out there on a cold night for any other reason." The Dealer:

John R. Stieff, near-40, is a former rodeo rider, scuba diver and logger (to name a few vocations) turned auctioneer and purveyor of country miscellany. The phone rings in his country-style home near Annapolis: it's a trapper with fur to sell.

"This is John Stieff . . . . Are you standing up? Well, sit down, I've got some bad news for you. The market's hitting bottom. You're not going to get what you got last year from anyone."

He quotes prices on raccoon and fox and ends the conversation by saying, "I'd hold on to the pelts, if I were you."

Stieff says the Europeans, consumers of 75 percent of the fur taken in America, are enjoying a fur glut. Even with dealers asking 10 to 40 percent less than last year's rates, the Europeans refuse to speculate.

"I'll call Thomas Saltz a Baltimore-based fine-clothes seller and others just to keep on top of what's happening at the end of the line," Stieff said. "They'll tell me coats above $8,000 are moving but the middle-class pieces (ranging around $2,000) are hanging on the rack."

The rich, apparently, haven't stopped being able to afford furs, but Stieff cites the worldwide recession and weak European currencies as causes for the collapse in demand generally.

Stieff, licensed to deal furs in 15 states, said he sells directly to collectors, the few who invest in fur the way the Hunt brothers did in silver, and that a buyer working full time can handle furs worth close to $20,000 in a week.

Although he won't reveal how much time he puts in ("Less as I get older"), he said he makes the rounds through Maryland, Virginia, West Virginia and Pennsylvania to collect fur from trappers. After conducting business over the pickup's tailgate (with whole animals and hides packed to the ceiling of the cargo cabin) he feels obliged to zigzag home to avoid robbery, he said.

His problems are not new to the trade. "The business hasn't changed much since the first white man poled a boat onto a bank to trade with the Indians," he said. "You can just about bet a few canoes got shanghaied back then, too."

One small change in the trade, however, may be significant: "You just don't find the younger generation getting into it. It's not a prestige field, it takes a long time to learn fur and--especially this--you have to have a love for these animals."

Stieff, a trapper from the time a family friend brought him out on the marsh at age 5, took the initiative to travel through New York and Canada and train with the best buyers in the business. Few if any newcomers follow so deliberate a path, he said.

Most who enter the business are trappers dealing over the heads of established local buyers--cutting out the middleman. They expand by taking on the accounts of less ambitious friends.

Stieff says it's a perilous adventure. The buyer puts out cash for fur and takes a loss every time he misjudges it or the market. Also, he says, greed between new dealers for new accounts leads to some mutually destructive price warring. Most of the new boys, 98 percent, go under, he said.

The profit margin is thin, too. According to another dealer, "If you buy a good fox worth $45 to $60 and sell him for $3 more, consider yourself lucky." Top markup on a muskrat might be 25 cents, he said.

Add to all this the fact that, like diamonds, fur is a secretive business and, as Stieff said, "You get people following you around to see how you operate. It's just not an easy dollar." The Deal:

Early in the afternoon, a pickup truck rolls down the gravel driveway to the front of Stieff's home. Two men lay out three raccoons and a red fox on the tailgate. Two of the raccoons are frozen stiff because they were caught earlier in the week and stored in an outdoor refrigerator. The third is wet, scruffy, perhaps thin-haired. A short line of blood marks one of its paws; two closely spaced toothpick-sized holes adorn its right flank. "This fellow met a few dogs," Stieff said.

Raccoon season opens a little more than a week before muskrat time. This gives trappers a chance to clean them off the leased land before they maraud the traps by, as reportedly occurs, eating all of one muskrat and part of another and claw cutting three more for no other reason than instinct.

Stieff tells the trappers about the market. They don't seem to care. There's a light drizzle and it's cold standing by the truck. They're just showing the dealer what they trapped. Stieff arranges to take the fur, dry it and pay them afterward. They trust him. If they didn't, there are at least 80 more licensed fur dealers in the state they could bargain with. The Animal Protectionists:

Next to fashion models and politicians, the most image-conscious group in the country may be hunters and trappers. In the fight to protect their way of life, hunters and trappers often are locked in combat over legislation with groups such as the Fund for Animals.

"Cleveland Amory is against trapping altogether," said Glenn Chase, Washington coordinator for the Fund for Animals, speaking about the organization's president. This is the group, more than 200,000 strong, trappers say they would most like to see defused.

The fund has worked successfully to have the leg-hold trap banned in Florida, Hawaii, Massachusetts and South Carolina; it has had similar legislation before Congress since 1971, so far without success. It also has been blamed by hunters for animal deaths, the most recent being the mass starvation of an Everglades deer herd because the organization delayed a state-approved "mercy kill" for excess population.

"As a practical matter," Chase said, "we're mostly concerned with the humaneness of trapping. We want to control the way it's done."

The fund provides literature on what it considers alternatives to the leg-hold trap: cages and a few new types of snare. It describes the Conibear instant killer as a substitute that prevents struggle and starvation by snapping an animal's back for a quick kill. (Since it's not the kind of tool you'd want the family cat to stick its nose into, its use in Maryland is restricted to water, where it can be used to trap muskrat).

What the fund abhors most is leaving an animal in the trap to die of disease, starvation or predators, to experience long hours of terror, or to maim itself in getting free. State laws requiring a trap check every 24 hours (on land) and every 36 (in tidewater) attempt to address this concern.

"The humane organizations have made us more professional," Stieff said. "The traps of the '20s and '30s are gone. We're looking for better ones all the time, some with rubber jaws, others with jaws offset. What I don't understand is why people who never read an issue of Outdoor Life a well-circulated sportsman's journal would put down $15 for a group whose sole appeal is emotional."

State statistics show that Maryland's fur harvest in the winter of 1980-81 was: 538 beaver; 1,605 gray fox; 2,898 red fox; 387 mink; 120,304 muskrat; 104 nutria; 5,460 opposum; 298 otters; 2,645 raccoon; 98 skunk; 161 weasel.

"The concerns of the 'antis' are justified," said Bob McKee, director of the state natural resource department's fur-bearer program. "But they're dealing with emotion, not the real world."

In McKee's world, the prognosis for rabies, which became a serious problem this year in Montgomery County, is for its continued spread in urban areas.

"It's a density-related disease," he said, "and we have a high population of raccoons in those areas."

According to some who trap, the high urban raccoon population stems from bans in Montgomery and Prince George's counties on the leg-hold in all but farmlands and wildlife management areas.

However, "most will tell you that's a crock," said Bob Beyer, southern regional wildlife manager for the natural resources department. "They know you can't wipe out the disease without wiping out all the animals in a population and that's impossible. Second, the raccoon is here not because it isn't being trapped but because it's finding food and shelter. Montgomery is trapping them now for some immediate reduction and to get some figures on rabies, but the long-term answer is to keep the garbage cans shut, attics and sheds sealed."

For Donald Peed, 25, a trapper from Brandywine with a land lease six miles south of Gentry's on the Patuxent, the marsh means extra money.

"It's not expensive to get into, but you buy a boat and a motor and figure in your time . . . . I wouldn't do it if there weren't any profit in it."

That's what he says, this tobacco farmer and third-generation trapper. But ask about his 8-month-old son, Austin. What's he going to be?

"I hope he'll be a trapper, a hunter and a fisher, too," Peed said.