Gray Seals vs. Harbor Seals: How to Tell Them Apart in Chatham
Short answer: Cape Cod has two seal species. Gray seals are large — bulls reach 800 pounds — with a long, straight, horse-like muzzle. Harbor seals are much smaller, rarely over 250 pounds, with a short concave face that looks like a Labrador. If it’s summer in Chatham and the animal is huge, it’s a gray seal.
Most people arrive thinking “seal” is one animal. Then they’re on the water looking at four hundred of them and the differences start jumping out. Here’s the field guide we’d give you on the boat.
The one-second test: look at the head
Everything else is detail. The head tells you immediately.
A gray seal has a long, flat, straight profile — the muzzle runs out from the forehead in nearly a single line, with wide-set nostrils that make a broad V. The old whalers’ name for them was “horsehead,” and once you’ve heard it you can’t unsee it. Big males develop a heavy, wrinkled neck and a distinctly Roman nose.
A harbor seal has a short face with an obvious dip where the muzzle meets the forehead, and nostrils that meet at the bottom in a narrow V. The result reads as puppyish. If your first thought is “that looks like a dog,” you’re looking at a harbor seal.
Size, if you have something to compare against
| Gray seal | Harbor seal | |
|---|---|---|
| Adult male | up to ~800 lbs, 8–10 ft | ~250 lbs, ~6 ft |
| Adult female | ~500 lbs | ~180 lbs |
| Face | long, straight, “horsehead” | short, concave, dog-like |
| Nostrils | wide apart, broad V | close together, narrow V |
| Coat | dark with light blotches (males darker) | light with fine dark spots |
| In Chatham | year-round, the colony | mostly fall through spring |
Size is only useful with a reference. Alone on a sandbar, everything looks mid-sized. Next to each other, a bull gray seal beside a harbor seal is not a close call — it’s roughly a pony beside a spaniel.
Coat and coloring
Gray seals are, unhelpfully, often not gray. Males run dark — near-black when wet — with scattered lighter patches. Females tend to be lighter silver-gray with darker blotching. The pattern is in large irregular patches rather than fine spots.
Harbor seals go the other way: a pale to tan base covered in small, dense, dark spots and rings, like freckling.
Wet animals of both species look darker and glossier, and a seal that’s been hauled out drying in the sun can look like a completely different color than the same animal an hour earlier. Go by the head shape.
Which ones will I see on a Chatham seal cruise?
Gray seals, almost certainly, and probably several hundred of them.
The Monomoy and Chatham haul-outs are a gray seal aggregation — one of the largest on the U.S. East Coast — and they’re residents. They’re here in July and they’re here in January.
Harbor seals are the seasonal ones. They’re common around the Cape from roughly September through May and largely move north to breed in summer, so a peak-season trip is a gray seal trip. In the shoulder months you can see both on the same bar, which is when the size difference really lands.
We run Chatham seal cruises May through October, so May and October are your best chance of catching both species on the same bar — the tail end of the harbor seals’ season at one end, the start of their return at the other. The size difference is far easier to appreciate side by side than from any photograph. Through the height of summer, it’s gray seals.
Why are the gray seals the ones in the news?
Because there are so many of them, and because of what follows them.
Gray seals were hunted almost out of Massachusetts — there was a state bounty into the 1960s — and the Marine Mammal Protection Act of 1972 stopped it. The population rebuilt over the following decades into the colony you see today. That recovery is also why white sharks returned to these waters in numbers, which we cover in are there sharks where the seals are.
Harbor seals never crashed the same way and were never as concentrated, so they generate fewer headlines.
When do they have pups?
This surprises people: winter. Gray seals pup on Cape Cod between roughly December and February, on the outer bars and the remote stretches of Monomoy. Pups are born with a white coat and are weaned in about three weeks, which is one of the fastest weaning periods of any mammal.
Harbor seals pup in late spring and early summer, but generally north of here — Maine and Canada — which is another reason the summer Cape is gray seal territory.
How close can you get?
Both species are protected under the Marine Mammal Protection Act. NOAA asks people to stay at least 150 feet away, on the water and on the beach. Approaching, feeding, or crowding them is a federal offense, and it’s also just bad for the animals — a disturbed haul-out flushes into the water and burns energy it needed.
If you come across a seal alone on a beach, leave it alone. Resting on shore is normal, not distress, and most “abandoned” pups are simply pups whose mother is offshore feeding. If something genuinely looks wrong — visible injury, entanglement, obvious distress — call IFAW’s marine mammal rescue line or the Center for Coastal Studies and let trained people decide.
Quick recap
- Long straight horse face, enormous → gray seal
- Short dog face, spotted, much smaller → harbor seal
- Summer in Chatham → gray seals, in the hundreds
- Fall through spring → your best shot at seeing both
Want to put it into practice? Book a Chatham seal cruise — an hour on the water from the closest harbor to the colony. And for timing your trip around the tide, see the best time to see seals in Chatham.