Harbor seal records run backward from most of the mammals on this list: sightings peak in the cold months and drop to almost nothing by summer. Cape Cod National Seashore alone accounts for a large share of the state’s named sightings, its sandbars and haul-out beaches drawing seals that spend summer farther north and move into Massachusetts waters as temperatures cool.
That winter pattern, heaviest from November through March, tracks a real seasonal migration rather than just easier viewing. Seals hauled out on a sandbar in January are hard to miss, while the same beaches see far fewer animals in July and August, when much of the population has shifted toward Maine and Canadian waters to pup and molt.
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