The Biology of Diving–Seals Part 2
Posted by Richard Conniff on August 18, 2014
“That one there is a female,” Keith Lincoln tells the tourists on his 32-foot seal-watching boat Rip Ryder. “See her brindle coat? Come on now, sweetheart. Look at that cute little ice cream-cone face. Wait till you see one of the big males. They get ugly.” Males can weigh up to 800 pounds, about twice as much as females. Their faces are dog-like, with a dignified Roman nose bump. The species name Halichoerus grypus means “hooked-nosed sea pig.”
Lincoln, a Harwich police patrolman by night, runs Monomoy Island Ferry and Seal Cruises by day, and over the idling of twin 250-horsepower Evinrudes, he delivers a knowledgeable introduction to the strange biology—even the biochemistry—of seals. Right now he’s telling his passengers that a diving seal can stay underwater for up to 20 minutes. “How do they manage it?” he asks. Trick question. A passenger ventures that they must have enlarged lungs.
“When you dive down to 500 feet,’ says Lincoln, “the last thing you want is two big balloons full of air in your chest.” Instead, seals flood the pockets of their lungs with plasma to solidify them against the intense atmospheric pressure. They also drop their heart rate as low as 10 beats per minute. “Gray seal blood has three to five times more hemoglobin than ours,” says Lincoln, “so it can carry more oxygen out to the muscles.” The muscles likewise contain more myoglobin, so they can hold onto oxygen longer. To prolong their dive time, the seals also turn off liver, kidneys, and all other functions that are at least temporarily unnecessary. “You and me are just big F150 trucks. We motor through it,” Lincoln tells his passengers. But a diving seal is all about efficiency.
That matters, he explains, because it influences what a gray seal eats—not 200 pounds in a day, but more like 20 or 30, and not the sort of fish that would interest a commercial fisherman, but sand eels. They’re slender little fish that burrow into the bottom with only their heads exposed to feed on drifting copepods—until a seal comes along to snap them up. A study of seal scat on Cape Cod found that sand eels make up 48 percent of the local seals’ diet. Striped bass and bluefish did not show up at all, says Lincoln, because seals are not built for high-speed chases. “Eight hundred pounds and exercise is a bad thing,” he tells his passengers. “Easy livin’, that’s what it’s all about.”
New England fishermen have always believed otherwise, viewing seals as their worst, or at least their most visible, competition. From 1888 to 1962, Massachusetts and Maine together bounty-hunted up to 135,000 seals, enough to make them scarce throughout New England. Older men on the Cape can still recall the last few seals that occasionally showed up when they were boys in the 1950s. They generally shot them and handed in their noses to collect a $5 bounty.
“You can argue with me all day if you want,” says Lincoln, “but I can tell you that seals do not deplete the commercial fishery. There are no fisheries to deplete. They were killed off by our fishermen. What killed them off was technology—gill nets, and bottom trawling, and depth finders that can find a fish at a thousand feet. The fish don’t have a chance.”
Though he does not say so to his customers, Lincoln will also happily tell his neighbors that the coming of the seals to Cape Cod is a good thing economically, even for a traditional fishing community like Chatham. “There are five seal-watching companies in the area, and we each carry 5000 to 7000 people a year,” he says. It’s not just what customers pay to see the seals ($35 a person on Lincoln’s boat), but also what they spend before or after for a meal in town. He figures that adds another $1 million to the local economy.