BOB EVANS hefts bushel after bushel of crabs out of his boat and up to his lone crewman, Calvin ''Peewee'' Matthews, who loads the day's catch into the back of a pickup. They've got 19 bushels in all, plus about five-dozen ''peelers'' that will shed and be sold as soft-shell crabs for $20 a dozen. All told, today's catch will bring Mr. Evans about $900 before expenses. ''A pretty good haul,'' says ''Captain Bob,'' beads of sweat dotting his tanned face. ''Better than lately.'' The crabs have been late this year, Evans says, because of hot, dry weather. But he is confident Mother Nature will come through. What worries him more are the conservationist Chesapeake Bay Foundation and the regulating power of the state to restrict his livelihood. Foundation scientists have warned that the Chesapeake Bay's population of Atlantic blue crabs - the region's leading commercial fishery, worth more than $186 million a year - is in danger of collapse from overfishing. The foundation proposed a ban on deep-water harvesting starting, Sept. 1, to protect the egg-bearing females, but Maryland and Virginia are waiting for the results of a major study due in November. Virginia, in particular, where anti-regulation sentiment is strong, is disinclined to take new steps soon. The clash between depleted stocks and human livelihoods is one being played out up and down the North American coasts, from salmon fishermen in the Pacific Northwest to cod and haddock crews off New England. Here in the cobalt waters of the Chesapeake, scientists agree that existing data show signs of trouble: wide fluctuations in recent annual harvests and an indication that crabbers need twice as many pots to catch the same number of crabs as they did 10 years ago. New restrictions are already in place; but more may be needed, scientists say, to prevent the need for a complete moratorium. Evans is skeptical. ''Don't get me wrong, I agree with studying things closer,'' says the crabber, a board member of the Maryland Watermen's Association and a governor-appointed member of the Maryland Seafood Advisory Commission. ''But there's no shortage of crabs, just more crabbers.'' More than years of formal education lie at the heart of the tension between crabbers and scientists. For the Chesapeake's watermen, working the bay is an art, a storied profession with a language and a lore captured in William W. Warner's classic book ''Beautiful Swimmers.'' And now they're feeling picked on, an easy target in a complex matter. ''They could regulate sports crabbing,'' says Russell Dize, a waterman on Tilghman Island, one of the bay's major crabbing villages. ''The situation is, as soon as something happens, they jump on the watermen.'' Recreational crabbing is popular around the bay - by some estimates, nearly equaling the professional catch - but enforcing regulations is difficult. State efforts to require licenses for sportsmen have been shot down. Mr. Dize also complains that the bay's resurgent rockfish population, which regulators have tightly protected in recent years because of overfishing, is eating up baby crabs. William Goldsborough, a fisheries scientist at the Chesapeake Bay Foundation, says stomach-content analysis on rockfish shows their diet is only 5 percent crab. In an interview, Mr. Goldsborough's pronouncements about the bay's crab population seem less dire than his foundation's press releases. But he and other scientists following the crab situation agree there is no doubt that the current rate of harvest could lead to the population's collapse, if not this year then in the next several. Beth Gillelan, chief of the National Oceanic and Atmospheric Administration Chesapeake Bay office, calls the foundation's proposed no-crabbing zone ''a cautious first step that makes a lot of sense.'' With all the jurisdictions that rule the bay - Maryland, Virginia, the District of Columbia, and the Potomac River Fisheries Commission - in close consultation over the crab situation, she says, ''we have a huge opportunity to manage blue crabs on a bay-wide perspective.'' The Chesapeake crabs themselves, which account for one-third of the crab sold in the United States, are also in part responsible for their troubles: Their sweet, succulent meat has made them increasingly popular throughout the US and Asia. She-crab soup is a hit on restaurant menus. Increased demand has brought more watermen into the business, as prices have skyrocketed. Only a few years ago, female crabs fetched $5 a bushel; now crabbers are getting $35. Down at Washington, D.C.'s waterfront, consumers are paying $120 a bushel for the No. 1 jimmies, the fat juicy males. ''See these females here?'' says Evans. ''These'll be in California tomorrow.'' Evans says if the worst happened, and he was forced to give up crabbing, ''we'd catch minnows for a living.'' He would also go more for catfish, which are growing in popularity, as well as clams and oysters, whose populations are coming back. But his finances would take a big hit. ''I make two-thirds of my yearly income in June, July, and August from crabs,'' he says. It's hard work: Out in the boat by sunrise, harvesting 300 of his 400 pots a day. In by noon, then home to clean pots, get his peelers ready for shedding, sell his and other watermen's catch, and run his retail business. In bed by 9:00. Evans is also a single father of two girls, who help with his business. ''I'm the first in my family in seven generations not to be a college graduate,'' he says proudly. ''My brother has a doctorate in psychology, and I make as much as he does. But I work harder.''