Capt. Flynn Smith, at the helm of his 30-foot sport fisherman, trolled slowly over the open sea, dragging David Gross of the National Aquarium at Baltimore behind, hanging on to a rope.

Gross, his head plowing up a briny foam, peered underwater through his face mask, searching for a big school of porkfish, a common reef denizen with black and yellow stripes against a silvery blue.

He had seen the school in the vicinity a few days earlier and wanted to catch 40 or 50 of them to make an impressive display in the aquarium's popular Reef Tank. The prime summer season at the aquarium was coming and the huge, 335,000-gallon tank was down to about 600 fish of several species. Gross manages the Reef Tank and wanted to bring the population up to between 1,200 and 1,500.

"Just saw a few," he shouted to the boat as he briefly raised his head from the water. "Just saw a few more," he called moments later.

Suddenly, Gross let go of the rope.

"The mother lode!," he yelled. "Porkers!"

Smith circled the boat back and dropped anchor. Gross climbed aboard and donned scuba gear, along with fellow aquarist John Camper, curator Jackson Andrews, who heads the aquarium's "bring 'em back alive" expedition, and Smith, a Key West charter captain who usually takes anglers out for the big ones and who catches fish for several aquariums around the country.

Every year, the Baltimore aquarium hires Smith for two weeks of daily runs to the reefs off Key West to replenish depleted captive populations. Some species live many years in captivity; others die in a year or so, either of old age or of unknown causes.

One natural cause of death is predation. In the Reef Tank there are big, silver tarpons that sometimes add variety to their artificial diets by gulping down smaller fish. Major predators, such as sharks, are kept in a separate tank.

"Sharks aren't really that much of a problem out here," Smith told a visitor as he suited up. "There was one time I had to give a lemon shark my catch. He was about 10 1/2 feet long -- big enough to get my attention."

On the ocean floor, about 15 feet down, the team set up a long barrier net with weights on its bottom edge and floats along the top to keep it stretched like a fence eight feet high. The net's length was curved to form a U, a cul-de-sac into which the fish could be driven and caught.

Then the four flippered away to find the porkfish school again. The divers, brandishing hand nets, herded the fish, most measuring between four and eight inches long, toward the barrier net.

Within minutes the school had entered the cul-de-sac. One, two or three at a time, the fish were scooped up in the hand nets and quickly transferred into what look like plastic laundry baskets with lids, sitting on the ocean floor.

A few fish slipped away, but most had entered the first stage of a six-week journey that would see them airlifted to Baltimore, acclimated to the aquarium's artificial sea water, chemically treated for external parasites, quarantined to be sure they are not diseased and eventually released into the Reef Tank, a scientifically designed simulation of their Florida coral reef habitat.

After most of the school had been caught, the divers swam off individually, small plastic fish containers trailing from their waists, looking for other fish species on the aquarium's "wish list" of about two dozen colorful and compatible crowd-pleasers. A curious barracuda hovered nearby, more interested in the porkfish waiting in the baskets than in the divers.

Andrews spotted a blue angelfish, one of the reef's most attractive inhabitants, and chased it, hoping to drive it toward the barrier net. It got away. Then a blue tang, a species that changes from yellow to blue as it ages, caught his eye just before it flashed under a low coral overhang.

To flush out his quarry, Andrews reached for a plastic squeeze bottle of quinaldine strung to his waist belt and squirted the chemical into the crevice. Quinaldine, which Andrews said is normally used as an anesthetic, irritates fish and they immediately swim away from it, usually right into an adroitly held hand net. Andrews transferred the blue tang to the plastic container and swam on to stalk a 3-inch-long foureye butterflyfish, a species with spots that look like eyes near the tail.

After several hours underwater, interrupted by returns to the boat to change air tanks and gulp down sandwiches for lunch, the hunters had netted nearly a hundred specimens.

The baskets were hauled aboard the boat and the fish were poured into a fiberglass tank of sea water. A pump continually changed the water as the boat raced back to the dock at Key West's historic turtle kraals, site of a now defunct sea turtle cannery.

At the dock the fish were transferred to a large concrete tank of sea water to await packing and shipment.

Some unlucky specimens would end up in an improvised laboratory under the coconut palms outside Smith's dockside restaurant. At a shady restaurant table, veterinary technician Sue Nevy anesthetized the fish, then checked them for parasites and evidence of their health in the wild.

"We want to compare the wild caught fish with ones that have been in the aquarium for some time," Nevy said. "From what I can tell so far, there's something funny going on."

Nevy said that some of the aquarium fish harbor three kinds of parasites: flukes, protozoans and nematodes. Newly caught fish, however, appear to carry only flukes. It may be that when they go into the Reef Tank they will pick up the additional parasites. It is not clear what harm the parasites do.

Every few days, the fish are packed for shipment.

"This is the really critical time for the fish," Andrews said one morning as the group worked quickly to pack 23 blue tangs, 18 blue-barred soldierfish, 15 squirrelfish and 54 additional fish of 18 other species. "If we lose them, it's during the flight up to Baltimore."

Each fish goes by itself into a heavy plastic bag half filled with sea water and topped with pure oxygen. The bag is sealed and put inside two more bags to guard against leakage.

The bags go inside white styrofoam boxes, like picnic coolers, and then into cardboard cartons marked "Live Animals -- RUSH" and "Keep at 70 ."

One recent morning, 30 cartons went into the aquarium's van for the four-hour drive to Miami, then a nonstop flight to Baltimore-Washington International Airport. Other aquarists would meet the flight and truck the cartons to the aquarium.

After a week of daily dives, Andrews and his team returned to Baltimore, swapping places with another group who would carry out a second week of dives and shipments.

Late one night back in Baltimore, Gross, the Reef Tank manager, and fellow aquarist Sandy Flynn met a shipment from their counterparts in Key West.

Working quickly on the aquarium's loading dock, the two cut open the cartons, opened the Styrofoam boxes, lifted out the plastic bags and tore them open, pouring the fish into big plastic bins on wheels. In many cases, the water had turned murky, fouled by ammonia-laden fish waste.

Many of the fish were swimming actively. Some were sluggish.

"Usually they perk up once we get them into some clean water and give them room to swim around in," Flynn said.

"Oh, bummer," Flynn exclaimed when she opened one carton. Inside was a beautiful, dark-blue midnight parrotfish. It was dead. The fish was large, about 14 inches long, and though it had a whole box to itself, it may have succumbed to the accumulated waste.

"Kinda lousy," Gross said.

In another carton, however, a second specimen was alive and well. Another pair of large specimens, scrawled filefish, experienced the same fate, one dead and one alive. Both fish that died had been in plastic bags that went into Styrofoam boxes, but their live counterparts had been packed directly in Styrofoam boxes without the plastic bags.

"This was an experiment to see which method was better," Gross said. "I think we found out."

The bins of fish were taken upstairs to large quarantine tanks. Because these contain artificial sea water, made from a product called Instant Ocean, the fish cannot be transferred immediately. The mineral contents differ and the fish must be acclimated by a slow change of water.

The fish rest a few days in the quarantine tanks before copper sulfate is added to kill external parasites. After three weeks, the copper sulfate is stopped, but the fish stay in quarantine for another two or three weeks. Finally, after six weeks and a 1,100-mile journey from home, the fish are released into the Reef Tank for viewing by thousands of wide-eyed visitors.

"All these pretty fish may look delicate," Flynn said, "but they're tough. Most of them come through and do just fine."