The harbor seal, Phoca vitulina, is a widely distributed marine mammal found in temperate and Arctic coastal waters across the North Atlantic and North Pacific Oceans. As a non-migratory species, the seal’s life history is closely tied to its local environment, including its feeding behavior. Harbor seals are adaptable predators, and this exploration focuses on their varied diet and specialized methods for capturing prey in challenging underwater conditions.
Primary Food Sources
Harbor seals are categorized as opportunistic generalists, meaning their diet is remarkably diverse and changes based on what is locally abundant and easiest to catch. The majority of their food intake consists of various species of fish, supplemented by a range of invertebrates. They are known to consume a large volume of food daily, with a 100-kilogram adult seal eating approximately 5 to 7 kilograms per day.
The fish portion of their diet includes small, schooling species that inhabit the water column, such as herring and capelin, which are caught through active pursuit. They also target demersal, or bottom-dwelling, fish like flounder, hake, and cod. In many regions, small, elongated fish like sandeels form a major part of their diet, particularly in coastal areas of the North Sea.
Harbor seals also consume cephalopods and crustaceans. Invertebrate prey includes squid, octopus, shrimp, and crabs. They use their back molars to crush hard-shelled crustaceans. Seals typically swallow prey whole or tear it into large chunks before consuming it head-first.
Foraging Techniques and Sensory Tools
Harbor seals are highly adapted for underwater hunting, employing specific behaviors and sensory tools to locate and secure their meals. While they can swim up to 12 miles per hour in short bursts, foraging success relies more on persistence and specialized senses. Foraging occurs through diving, with the majority of dives being relatively shallow, typically less than 20 meters and lasting under four minutes. However, they possess the physiological adaptations to reach depths extending beyond 400 meters.
The most specialized tool in the harbor seal’s hunting arsenal is its mystacial vibrissae, or whiskers, which function as highly sensitive mechanoreceptors. These whiskers allow the seal to detect subtle movements in the water, a capability that is particularly useful in the turbid, murky coastal waters they often inhabit. A swimming fish leaves behind a wake, or a hydrodynamic trail, which the seal’s whiskers can sense and follow with precision, even in complete darkness or when blinded.
The undulated structure of the harbor seal’s whiskers is crucial for this function. This specialized morphology actively suppresses self-induced vibrations caused by the seal’s own swimming motion. By minimizing this “noise,” the whiskers become acutely sensitive to the swirling patterns of water movement created by distant prey. When actively searching, the seal holds its whiskers erect, allowing it to accurately determine the direction, distance, and size of the prey.
Geographic and Seasonal Diet Variation
The composition of a harbor seal’s diet changes significantly depending on its geographic location and the time of year, reflecting its reliance on local prey availability. Seals are highly flexible in their foraging and generally target the most abundant and easily accessible food sources in their immediate environment. This leads to considerable regional differences in which species dominate the diet.
For instance, studies in the Moray Firth in Scotland show that sandeels dominate the diet year-round, while populations in the Inner Hebrides rely more heavily on large gadids, such as cod and pollock. Seals in the Norwegian Skagerrak consume a large proportion of gadoids and flatfish, differing from the primary prey of seals along the North American Pacific coast. These regional differences demonstrate the seals’ adaptation to the specific fish communities of their local feeding grounds.
Seasonal shifts in diet are driven by the spawning or migration patterns of prey species. In the Wadden Sea, harbor seals shift from a diet based on pelagic (open-water) sources in the spring to one dominated by benthic (bottom-dwelling) sources in the summer. As seasons change, seals may expand their foraging range, using resources from the North Sea more frequently in the fall and winter when local estuarine prey is less available. During periods of high prey abundance, such as summer and fall, seals feed heavily to build up blubber reserves for surviving the leaner winter months.