Harbor Seal

The seal that lives almost everywhere

If you have ever seen a wild seal, chances are very good it was this one. The harbor seal is the most widely distributed of all pinnipeds, ringing the entire Northern Hemisphere. It hauls out on rocks in Maine, sandbars in the Netherlands, glacial ice in Alaska, and quiet coves in Japan. Somewhere between 350,000 and 500,000 of them are alive right now.

They are mid-sized as seals go, roughly five to six feet long, with the build of a well-fed torpedo. Coat colors run from pale silver-grey to dark brown, always scattered with rings and speckles. No two are patterned quite alike, which is exactly how researchers tell individuals apart without ever tagging them.

Whiskers that see in the dark

Coastal water is murky. Silt, plankton, and shadow can drop visibility to almost nothing, and yet harbor seals hunt successfully in exactly those conditions, often at night. They are not finding fish by sight.

The answer is in their whiskers. A seal's whiskers, properly called vibrissae, are not stiff hairs like a cat's. Each one is a wavy, flattened ribbon anchored in a bundle of nerves, and each is sensitive enough to detect the faint swirl of water a fish leaves behind as it swims away. That trail lingers for many seconds. A harbor seal can pick it up, follow it through pitch darkness, and catch the fish at the end of it. In experiments, blindfolded seals wearing headphones have tracked a moving target using nothing but the wake it left behind.

Pups that swim the day they are born

Most baby mammals need weeks before they can do much of anything. A harbor seal pup does not get weeks. It is born on a tidal sandbar or rocky ledge that may be underwater in a matter of hours, so it arrives ready.

Pups weigh up to about 35 pounds at birth, and can swim and dive almost immediately. At two or three days old, a pup can already hold its breath underwater for around two minutes. It also arrives having already shed its white baby fur, which happened in the womb, so it can enter cold water right away without waiting to grow a waterproof coat.

The mother nurses for only four to six weeks on milk so rich it is closer to butter than to anything you would drink. The pup can roughly double its weight in that time. Then she leaves, and it is entirely on its own.

Built to hold a breath

Harbor seals usually dive for three to seven minutes at a stretch while feeding, but they can stay under far longer, up to roughly half an hour, and reach depths of several hundred feet when they need to.

The trick is not big lungs. In fact a diving seal exhales first, deliberately emptying its lungs before going down, which keeps compressed air from causing trouble at depth. The oxygen is stored elsewhere: in the blood, which a seal has far more of than a land mammal its size, and in the muscles themselves, loaded with a protein called myoglobin that hoards oxygen like a battery.

On the way down the body triggers what is called the dive response. The heart rate falls dramatically, sometimes to just a few beats per minute, and blood is shunted away from the skin and gut toward the brain and heart. Non-essential organs simply wait their turn.

The freshwater seals almost nobody knows about

The harbor seal as a species is listed as Least Concern, which is genuinely good news. But the species contains several subspecies, and they are not all doing equally well.

In the lakes of Quebec's Ungava Peninsula lives a population of harbor seals that never goes to sea at all. These freshwater seals, cut off since the last ice age, number only somewhere between roughly 120 and 600 animals. That is a small enough population that genetic diversity itself becomes a threat, on top of the usual pressures. The Kuril seal of the western Pacific and the western Pacific harbor seal are also far scarcer than the global headline number suggests.

It is a useful reminder about conservation math. A species can be abundant overall while particular branches of it sit right at the edge.