Adult males are up to 1.9 m long and weigh 70-150 kg, females 1.7 m and 60-110 kg. At birth, pups are 65-100 cm and 8-12 kg (Burns 2002).

Harbour seals are mainly found in the coastal waters of the continental shelf and slope, and are also commonly found in bays, rivers, estuaries and intertidal areas. On land, harbor seals are usually extremely wary and shy unless habituated to human activities and noise sources in their vicinity. It is almost impossible to approach them when they are ashore without stampeding them into the water. Most harbour seal haul-out sites are used daily, based on tidal cycles and other environmental variables, although foraging trips can last for several days. Although generally considered a non-migratory species with a high degree of site fidelity to a haul out, juvenile dispersal, emigration and establishment of new haul out sites are all possible reasons for long range movements of harbour seals (Burns 2002).

Harbour seals are gregarious at haul-out sites. However, they usually do not lie in contact with each other. They will haul out on rocks, sand and shingle beaches, sand bars, mud flats, vegetation, a variety of man-made structures, glacial ice, and to a very limited extent sea ice in some areas. They usually lie close to the water to permit a rapid escape from threats. Sex and age segregation is common in most populations (Kovacs et al. 1990). At sea, they are most often seen alone, but occasionally occur in small groups. Localized aggregations can form in response to feeding opportunities and concentration of prey.

Male harbour seals become sexually mature when four to five years old. Female harbour seals usually become sexually mature when three to five years old. Gestation lasts 10.5-11 months, including a 2+-month delayed implantation. Throughout the range, the time of birthing varies widely and may follow a latitudinal cline (Temte 1994). Peak pupping date varies from mid March to early September. The mating system is promiscuous, or weakly polygynous, with males defending underwater calling sites (e.g. Van Parijs and Kovacs 2002). Mating usually takes place in the water, with females coming into oestrus around a month after giving birth. Moult follows the pupping and mating season. The timing of onset of moult depends on the age and sex of the animal with yearlings moulting first and adult males last (e.g. Reder et al. 2003). Most harbour seal monitoring programs are based on counts obtained during the moult and are therefore subject to possible biases due to changes in age and sex structure of the population.

Most pups shed their silvery gray lanugo coat in the uterus before birth. Exceptions to this include pups born prematurely, and some that are born early in the breeding season. Pups usually enter the water rapidly after birth, and because of tidal inundation at many sites used for birthing, this often occurs in hours (Burns 2002).

Harbour seals are generalist feeders that take a wide variety of fish, cephalopods, and crustaceans obtained from surface, mid-water, and benthic habitats (e.g. Olesiuk et al 1990, Payne and Selzer 1989). Their diet is highly varied, and animals from different populations and areas show differences and there is also variation associated with seasonal changes in the abundance of prey (e.g. Harkonen 1987, Andersen et al. 2004). Generally, a few species dominate the diet at any one location and time of year. Although primarily coastal, dives to over 500 m have been recorded (Burns 2002). Harbour seals also take many commercially important fish species such as Atlantic cod, many kinds of salmon, herring, and flatfishes to name a few, and this aspect of their foraging puts them into conflict with coastal fisheries.

Longevity is typically 35 years for females and 25 years for males, but is lower in P. v. stejnegeri and P. v. richardii which are reported to live to approximately 20 years for males and 30 years for females. Predators include killer whales, great white and Greenland sharks and possibly other shark species, Steller sea lions and walrus, gulls and ravens and in southerly locales feral dogs and eagles. There is no information on polar bear, brown bear, and wolf predation on harbor seals, but all are possible predators.