On a wooden boat in St. Jerome Creek one gray morning last week, Bobby McKay pulled a crab pot out of the water with a thump. His son, Mark, shook it open a crack, sending crabs skittering into a basin in the boat.

Mark scooped out empty clamshells from the bait box of the trap, put in a handful of new clams and tossed the pot back in with a splash. Then he grabbed the crabs squabbling in the basin, sorting each one into a bushel basket by size, sex and shell, as his father steered the boat to the next buoy, ready for the next round.

Over and over, they pulled, dumped, measured, hauled, culled -- working seamlessly together, not saying much, busy with their work.

Mark McKay learned to be a waterman from his father, who learned from his father, who learned from his father. Bobby McKay is 70, and he'll keep crabbing as long as he can walk onto a boat. But the family tradition might end with his son, who is 37 and wondering how much longer he can make a living on the Chesapeake Bay.

Their small hometown of Ridge, in Southern Maryland, is a place where fathers have been teaching sons to live off the water for centuries, passing on the trade: how to twist wire into a crab pot, how to sort through the catch to find the crabs about to wriggle out of their hard shells, how to be in tune with the tides.

"You watch everything as it cycles," Bobby said. The crabs come and go. The market rises and falls. Fathers teach sons, and sons become fathers. They watch the herring come in spring, then the spotfish, then the shad. "Then in the fall you get oysters, so you look forward to that . . . ," he said. "In our way of life here, there's always another thing coming that you look forward to."

But they can see the cycle ending all around them; the last generation goes to work not on the bay but on the base, the growing Patuxent River Naval Air Station just north of Ridge.

The crab catch has been shrinking for years, and once-plentiful oysters have dwindled to almost nothing. Meanwhile, the defense contractors around the base bring steady jobs and good salaries.

Of Bobby and Marian McKay's six children, only Mark became a waterman. Three of the children work with computers, two run their own businesses.

"They're all doing better than I am," Bobby said, and paused. "Except for Mark."

Bobby McKay was born in October 1933, soon after his father and grandfather lost just about everything in a hurricane.

His father, Benjamin McKay, found a piece of land by St. Jerome Creek to be the family's home. And when Bobby was 8, his father surprised him with a handmade rowboat. Bobby used to take a net to catch soft crabs around the shore early in the morning, before school. "I couldn't get home from school fast enough to get [back] out there," he said.

He went on the water with his dad, then got his own boat, working from sunrise to night, day after day. After he married, Bobby and his wife built a house just around the cove from his father's home, with a big front window looking out on the silvery water, the pier, the blue herons.

Some of his children never much liked going on the boat, he said. His youngest son, Gary, liked computers. His youngest daughter, Jackie, was more interested in doing hair. After one rough day helping her father, getting up at 4 a.m. and spending most of the time hanging over the side of the boat, sick as a dog, she never went back.

He didn't push her. "You got to like it to do it," he said.

His oldest son, Robbie, liked it but knew how uncertain the future would be; he went to work for the power company instead.

Mark always loved the water. He went out with his grandfather, who crabbed into his eighties. He fished for eel. He helped his dad. He learned how to fix boat motors at school.

Just before Benjamin McKay died in 1980, he asked Bobby for five rolls of wire: He wanted to make Mark some crab pots. He was 86 and happy that there would be another waterman in the family.

A few years later, when Bobby's mother died, Bobby and his brother and sister bought their parents' brick house on the cove, to help Mark and his wife get a home on the water. Bobby's house is back through the woods, to the right of the faded wooden McKay's Seafood sign painted with a green crab. Mark's house is to the left. Mark has been paying it off over the years.

Mark gave up diving for oysters when there weren't enough to catch anymore. Now in the winter -- which used to be oystering time -- he does some construction work. The catches are smaller now. And the days are shorter, because of state regulations.

"If it gets bad enough, I guess I'll have to do something else," Mark said. Sometimes he thinks about getting a job at the Navy base. "I wouldn't want to," he said.

Bobby and Mark don't talk much on the boat. Sometimes they listen to country music on the radio, sometimes they just work, close by but not in one another's way, bringing the crabs in with a spray of briny water. The crabs dance in the cull box of the boat, smacking each other, pinching or hiding in corners. Some land upside down, waving their legs, their front claws blue and white, as smooth and luminous as Delft china.

This spring has been better than expected for blue crabs; the McKays hope that's a good sign.

When a bushel basket got full on the boat last week, Mark slid a lid onto it, using his gray rubber boot to stop the crab that jumped out, blocking its skittering escape.

As they motored in to shore, he and his father sat on the edge of the boat, looking toward home.

"This is about as good as we've done this year," Bobby said. "It just comes in cycles, you know."

Bobby McKay, 70, pulls in a crab pot as he and son Mark, 37, work on the Chesapeake Bay. "You watch everything as it cycles," Bobby says. Mark and Bobby McKay motor from St. Jerome Creek to the Chesapeake. Mark is a fourth-generation waterman, but the family tradition might end with him.Bobby McKay and his son Mark inspect crabs they just caught on the Chesapeake. Of Bobby's six children, only Mark became a waterman. A crab is measured by the McKays, who say the crab catch has been shrinking for years.