Local populations have been reduced or eliminated through disease (especially the phocine distemper virus) and conflict with humans, both unintentionally and intentionally.

They prefer to remain relatively close to shore in subtidal and intertidal zones, and have not been seen beyond the Channel Islands as a pelagic form; moreover, they often venture into bays and estuaries and even swim up coastal rivers.

[7][8][9][10][11][12] Considerable scientific inquiry has been carried out by the Marine Mammal Center and other research organizations beginning in the 1980s regarding the incidence and transmission of diseases in harbor seals in the wild, including analysis of phocine herpesvirus.

[13] In San Francisco Bay, some harbor seals are fully or partially reddish in color, possibly caused by an accumulation of trace elements such as iron or selenium in the ocean, or a change in the hair follicles.

Large populations move with the season south along the west coast of Canada and may winter on the islands in Washington and Oregon.

People are advised to stay at least 50m away from harbor seals that have hauled out on land, especially the pups, as mothers will abandon them when there is excessive human activity nearby.

[15] Historically, the range of the harbor seal extended from the mouth of the St. Lawrence River and Greenland to the sandy beaches of North Carolina, a distance of well over a thousand miles (greater than 1600 km) Evidence of their presence in these areas is consistent with both the fossil record as well as a few landmarks named for them during colonization: Robbin's Reef, off of Bayonne, New Jersey, gets its name from the Dutch word robben, meaning "seals".

The evidence for this is found in documents all along the coast of New England which put a bounty on the head of every seal shot, as well as the accounts of harbormasters.

Raw sewage had been dumped in the harbor since the late 1800s and the stench of fecal matter in the Charles River was overpowering, as evidenced by the song "Dirty Water" by the Standells, written in 1966.

They never were extirpated from Canada and certain pockets of the Maine coast, and thus an important mother population was created from whence the species could reclaim the home of their ancestors.

Currently, they are sighted as far south as the barrier islands of North Carolina on a regular basis,[18] with Massachusetts being the southernmost point of known pupping areas along the Atlantic Coast.

Others will head south from these areas to "vacation" in warmer waters, particularly young seals unable to compete with adults for food and territory; they do not return north until spring.

"[16] This refers to the habit of young seals leaving Cape Cod and even some Arctic waters to inhabit the harbor in winter.

[25] Both the northern and southern shores of Long Island have a reliable population of harbor seals as well as greys, where they will take sand lance as well as some species of crab as part of their diet.

[citation needed] Resting sites may be both rugged, rocky coasts, such as those of the Hebrides or the shorelines of New England, or sandy beaches, like the ones that flank Normandy in Northern France or the Outer Banks of North Carolina.

[1] Harbor seals frequently congregate in harbors, bays, sandy intertidal zones,[1] and estuaries in pursuit of prey fish such as salmon,[28] menhaden, anchovy, sea bass, herring, mackerel, cod, whiting and flatfish, and occasionally shrimp, crabs, mollusks, and squid.

[32][31] Researchers have found males gather under water, turn on their backs, put their heads together, and vocalize to attract females ready for breeding.

The timing of the pupping season varies with location,[34] occurring in February for populations in lower latitudes, and as late as July in the subarctic zone.

However, they do utilize non-harmonic vocalizations to maintain breeding territories and to attract mates during specified times of year,[38] and also during mother and pup interactions.