"We usually do that in the morning, when we can pick out the best ones to ship to Boston," I reminded her.

"Yes, but I don't want to ship any pregnant ones, and there are a few that have catfish wounds. Let's release twenty now," she said. I selected a large female, one that almost topped two feet in length, and when I lifted her up, she weighed easily five pounds with her bulging belly. Virtually every ray above 113/4 inches in length was pregnant this time of year, in July. As gently as I could, conscious of her condition, I lowered her down to the water and let her drop.

Tagging rays was important. If Narcine brasiliensis really turned out to be a good medical model for studying Alzheimer's disease, there would one day be an increased demand on their stocks. While collecting fish, we were also gathering fisheries data to try to estimate a sustainable yield, how many could be collected without depleting the population. On the basis of statistics applied to the two-thousand-odd rays we had accumulated, ranging from newborns to two-footers, it appeared that they grew to maturity in two to three years.

The more we tagged, and the more data we accumulated, the more questions about these remarkable fish arose. Where did they go when they were not on the ray bars? And how did they find those special spots on the seafloor? What drew them? Did they orient to magnetic fields-for the ray bars certainly seemed to coincide with magnetic anomalies-and did the crystals in their acoustolateralis systems play a part in their orientation?

We watched the first ray start to waddle its way down to the depths, almost outside our field of vision. Then suddenly, a six-foot shark materialized beside it like a menacing gray apparition, scrutinized it with the rapt attention of the hungry, and faded back into the shadowy murk.

We both laughed. "He knows better; he doesn't want any part of it," Anne chuckled, leaning over the side to get a better look. "Now that's what I call an example of learned behavior." "Could be," I countered, "or it could be that the ray discharged its electric organs when it felt the pressure wave. Remember the porpoises."

Several trips before, we released a tagged ray and suddenly two porpoises came rocketing up from beneath the boat, their mouths wide open, displaying rows of sharp pointed teeth, ready to swallow it. They got within two feet of the ray, then broke off the attack in unison, veered sharply, and fled. The response seemed too quick, too simultaneous to be purely visual.

Only once during the entire tagging project did we ever find conclusive evidence that a shark will hit an electric ray. The ray had a fresh set of jagged tooth marks arched over its entire dorsal surface and part of its belly. The serrated wound looked as if it had been inflicted by a shark, but it only penetrated about half an inch down into the tissue. The ray was alive and in good shape, and we took it back to the lab and kept it for months in our aquarium.

That ray provided an astonishing piece of information about sharks. A shark is a creature that bites with the force of three thousand pounds per square inch in a fraction of a second, yet has the ability to change its mind in midbite. When it felt the electricity flowing through its body, probably causing excruciating pain to all its ultrasensitive sensory organs used to detect prey emitting weak electric fields, its jaws sprang apart.

As I was thinking, Frenchie started dumping the picked-over trash fish overboard. We had a heaping basket of white shrimp picked out and watched the discarded pinfish, croakers, skates, and rays sinking down into the depths and being scattered by the waves. The sea gulls that had been hovering behind the boat, letting out impatient and scornful cries, berating the crew for their slowness, went wild.

Any fish small enough to get into their beaks was fair game. It didn't matter whether it were an anchovy, or a small catfish, or even a baby scorpionfish: they crammed it all down. But when we released baby electric rays, the gulls hovered above them, watching as the little ones moved deeper to safety. I often wondered whether the gulls could read the brown markings that looked like a warning inscription on their backs. Amazingly, when a dead one was thrown back, they didn't hesitate to swoop down and grab it.

What a sight our shrimp boat was, with its flocks of gulls overhead, screaming and screeching like an avian cloud, porpoises and sharks following along, becoming more excited with each shovelful of trash pushed through the scupper holes, spreading out fish juice, slime, and scale into the water with even more tantalizing smells. Every time a stingray or fish hits the water, they rose up from the depths to grab it. They were eating machines, and with each new shovelful of trash, the sea boiled with sharks. They were going in circles behind the boat, splashing, grabbing, and gobbling up all the little fish that drifted by. A discarded Texas skate, bearing the characteristic two handsome yellow and black symmetrical spots that looked like large fierce eyes, swam by on the surface, stunned but alive. Without hesitation another six-foot shark shot forward, engulfed it in one smooth wide-mouthed bite, and then was gone.

Perhaps the "eyes" looking up from the muddy seafloor might scare off a hungry predator, but not out there at the surface. The highly reflective eye structure of a shark identifies shapes and detects motion in the feeblest light. For more than 140 million years they have relied on a wide range of very specialized senses to survive.

With each new blinding show of lights from the skies, more sharks were revealed splashing and striking behind the boat. We wondered whether the electric discharges were stirring them up because they were far more active than usual, more sharks than we had seen on any previous trip. We watched the next ray attentively as it swam off, bearing its festive orange tag. It faded out of sight unobstructed, but suddenly we saw a shark explode out of the water tail first, stiffen, and shoot back down. "There went that ray," I said with surprise. "I don't think we should release any more." The electric organs could protect rays from sharks most of the time, but not when they were crazed by blood, fish juice, and the competition of their own growing numbers.

Edward stepped out of the wheelhouse and made his way back to the stern, when he saw our rapt attention focused on the churning wake. "See what I told you about West Pass," he said triumphantly. "You'd be out of your mind to try diving here."