Harbor seals are generalist predators that eat an impressively wide range of prey, with studies documenting over 120 species or genera in their diet across the Pacific coast alone. Fish make up the bulk of what they consume, but squid, octopus, shrimp, and occasionally even crabs round out the menu depending on where and when a seal is foraging. How they catch all that food is equally varied: harbor seals hunt using a combination of sharp underwater vision, remarkably sensitive whiskers, and two distinct feeding modes that let them handle everything from a darting herring to a flatfish hiding in the sand.
A Broad and Flexible Menu
If you picture a harbor seal eating the same kind of fish day after day, the reality is far messier. A long-term synthesis of diet data from Pacific harbor seals across Oregon and Washington found they consumed fish, invertebrates, and occasionally even birds and small mammals spanning 37 taxonomic orders and 62 families.1Northwestern Naturalist. Dietary Composition of Four Stocks of Pacific Harbor Seal (Phoca vitulina richardii) In the Northern California Current Large Marine Ecosystem As Synthesized From Historical Data, 1931–2013 In European waters, a 2020 study of harbor seals in the Skagerrak (the strait between Norway, Sweden, and Denmark) identified 28 fish species in their diet, with sandeel and Norway pout appearing most frequently. However, the prey that contributed the most actual body mass to the diet were larger species like haddock, pollack, saithe, and ballan wrasse.2Marine Mammal Science. Shifting Harbor Seal (Phoca vitulina) Diet May Reflect Ecosystem Changes in Skagerrak
That distinction matters. A seal might eat dozens of tiny sandeels in one foraging trip, making sandeels the most “common” item by count. But a single large pollack or wrasse delivers far more energy. Researchers track both metrics to get a complete picture. The overall pattern is one of opportunism: harbor seals eat what is locally abundant and accessible, and that varies enormously between regions. In the Pacific Northwest, salmonids showed up in roughly a quarter of diet samples, while in some European populations, flatfish and gadoids (the cod family) dominate.1Northwestern Naturalist. Dietary Composition of Four Stocks of Pacific Harbor Seal (Phoca vitulina richardii) In the Northern California Current Large Marine Ecosystem As Synthesized From Historical Data, 1931–2013
Seasonal Shifts in What They Target
Harbor seals do not eat the same things year-round. Their diet tracks seasonal changes in prey availability, sometimes dramatically. In the Wadden Sea (the shallow coastal waters along the North Sea coast of Germany, the Netherlands, and Denmark), stable isotope analysis of seal tissues revealed a clear shift from pelagic prey in spring to bottom-dwelling prey in summer. During spring, seals appeared to feed heavily on squid, while their summer diet leaned more toward benthic fish.3PubMed Central. Seasonal Variation of Harbor Seal’s Diet from the Wadden Sea in Relation to Prey Availability
Breeding and molting cycles also shape what seals eat. In the Gulf of Alaska, researchers found that diet diversity was consistently lower during the breeding season than during the molting season.4Journal of Experimental Marine Biology and Ecology. A new method to evaluate the nutritional composition of marine mammal diets from scats applied to harbor seals in the Gulf of Alaska During breeding, seals are time-constrained. Females are nursing, males are competing for mates, and neither sex can afford long exploratory foraging trips. The result is a narrower diet focused on whatever is most reliably available nearby. Once the breeding season ends and seals enter their annual molt, they have more freedom to roam and sample a wider variety of prey.
How Harbor Seals Actually Catch Their Food
Harbor seals use two fundamentally different feeding strategies depending on the prey and the situation: biting and suction feeding. These are not subtle variations of the same motion. They involve different mouth shapes, different jaw movements, and different muscular actions.
During biting, a seal opens its mouth wide, curls its lips back to expose its teeth, and lunges at the prey. This is the mode you would expect for grabbing a free-swimming fish in open water. Suction feeding looks completely different. The seal purses its lips into a small circular opening, seals the sides of its mouth, and rapidly depresses its tongue and the floor of its mouth to create a powerful inward rush of water. The prey gets pulled into the mouth by the vacuum. Researchers studying harbor seal feeding kinematics observed that suction feeding worked in two ways: sometimes the prey was drawn completely into the mouth in a single bout, but more often, the seal used multiple rounds of suction, pulling the prey partway in, holding it, then repeating the suction to finish the job.5Journal of Experimental Biology. Comparative feeding strategies and kinematics in phocid seals: suction without specialized skull morphology After the prey was inside the mouth, seals frequently expelled water out through the sides of their lips before swallowing.
This suction ability is particularly useful for prey that is hiding. A flatfish pressed against the seafloor, a small fish tucked into a crevice, or an invertebrate wedged in a rocky gap can all be extracted by suction even when the seal cannot reach them with its teeth. Harbor seals can also combine both methods, using suction to draw prey close enough to grab with a bite.6PLOS ONE. Feeding Kinematics, Suction, and Hydraulic Jetting Performance of Harbor Seals (Phoca vitulina) Work on Australian fur seals (a related pinniped) showed a similar pattern, with suction drawing concealed prey out of places that biting alone could not reach.7PubMed Central. Australian Fur Seals (Arctocephalus pusillus doriferus) Use Raptorial Biting and Suction Feeding When Targeting Prey in Different Foraging Scenarios Harbor seals accomplish all of this without any specialized skull morphology for suction; they generate the vacuum purely through soft-tissue action, making them surprisingly versatile feeders for their anatomy.
Whiskers as a Hunting Tool
In murky or dark water, harbor seals cannot rely on their eyes. This is where their whiskers become essential. Seal whiskers are not passive sensory hairs like the ones on your cat; they are finely tuned hydrodynamic sensors capable of detecting the tiny water disturbances left behind by a swimming fish. A fish moving through water creates a trail of small vortices in its wake, and those vortices persist for several seconds after the fish has passed. Harbor seal whiskers can pick up on those trails and follow them.
Recent research on seal whisker mechanics found that over half of harbor seal whiskers have underwater natural frequencies exceeding 80 Hz, which overlaps with the frequencies generated by fish swimming trails (above 100 Hz). This means the whiskers are physically tuned to vibrate in response to the kinds of water disturbances that fish leave behind.8PubMed Central. Wonders of Harbor and Grey Seal Whiskers: Morphology, Natural Frequencies, and 3D Modeling The detection mechanism is thought to work through a jerky, stick-slip motion as the whisker encounters each vortex in the trail. Rather than bending smoothly, the whisker snaps from one position to another in response to each pulse of disturbed water. Researchers have proposed that the rate of change of this bending, not just the bending itself, is what carries the most useful information to the seal’s nervous system, because it encodes the frequency signature of the wake even in noisy ocean conditions.9PubMed Central. Seal and Sea lion Whiskers Detect Slips of Vortices Similar as Rats Sense Textures
This whisker-based hunting system is not just a backup for poor visibility. Even in relatively clear water, detecting a fish’s wake trail can be more useful than spotting the fish visually, particularly when the prey is fast, evasive, or camouflaged. And in the turbid coastal waters where many harbor seals forage, vision degrades fast. Testing on captive harbor seals showed that visual acuity drops sharply even at moderate turbidity levels, and real-world turbidity in places like the German Wadden Sea routinely exceeds the levels where vision becomes practically useless for hunting.10ScienceDirect. Effect of water turbidity on the visual acuity of harbor seals (Phoca vitulina) In those conditions, whiskers are the primary hunting sense.
How Pups Learn to Feed Themselves
The transition from nursing to independent feeding is one of the riskiest periods in a harbor seal’s life. In most phocid seals (the “true” or earless seals), weaning has traditionally been described as abrupt: the mother nurses the pup for a few weeks, then abandons it and heads back to sea, leaving the pup to figure out foraging on its own.11PubMed. Weaning mass affects changes in body composition and food intake in harbour seal pups during the first month of independence The pup lives off its fat reserves while gradually developing the skills to catch prey.
But research using stomach temperature loggers has challenged this picture for harbor seals specifically. By tracking the temperature drops caused by swallowing cold prey items, researchers detected solid food ingestions in some harbor seal pups as young as 12 days old, well before weaning was complete. These pups were still nursing from their mothers while also beginning to eat fish on their own. This suggests that weaning in harbor seals occurs more gradually than in many other phocid species, with a period of overlap where the pup gets both milk and solid food.12PLOS ONE. Stomach Temperature Records Reveal Nursing Behaviour and Transition to Solid Food Consumption in an Unweaned Mammal, the Harbour Seal Pup (Phoca vitulina)
Even so, the early weeks of independence are tough. Newly weaned pups that were studied during their first month on their own did show increasing food intake over time, reflecting growing foraging ability. But for most individuals, what they managed to catch still was not enough to cover their daily energy needs. Heavier pups at weaning had a larger fat buffer to survive this learning period, suggesting that the mother’s investment during nursing has direct consequences for whether the pup makes it through the transition.11PubMed. Weaning mass affects changes in body composition and food intake in harbour seal pups during the first month of independence
Not Every Seal Eats the Same Thing
Even within the same population, individual harbor seals can develop distinct dietary preferences. This is not just random variation from meal to meal; some seals appear to specialize. In Puget Sound, research identified what could be described as “forage fish specialists” and “salmon specialists” among harbor seals living in the same area. Differences in foraging behavior by site and sex pointed to prey specialization or consistent exploitation of particular habitats.13PubMed Central. Fine-Scale Variability in Harbor Seal Foraging Behavior
Similar patterns show up in European populations. Studies tracking individual harbor seals found that local geographical differences in diet were linked to differences in foraging habitat, but even within the same area, individual seals differed in which water depths and bottom types they used.14Journal of Zoology. Variations in harbour seal Phoca vitulina diet and dive-depths in relation to foraging habitat Some seals consistently foraged in deeper water over sandy bottoms while others stuck to shallower, rockier areas. These habitat preferences translate directly into different prey: a seal that forages over sand is eating different species than one that hunts among kelp beds, even if they haul out on the same beach.
This kind of individual specialization has practical consequences for how we understand harbor seal populations. Averages across a colony can mask the fact that different seals are playing different ecological roles and putting pressure on different prey communities.
Foraging Under Fear
Harbor seals are not apex predators. They are prey for killer whales and large sharks, and this predation risk shapes where and how they hunt. In Alaska, harbor seals face a double threat: killer whales hunting them in shallow waters and Pacific sleeper sharks lurking in the deep.15Evolutionary Ecology Research. Inferring prey perception of relative danger in large-scale marine systems Seals have to balance the need to find food against the risk of becoming food themselves, which constrains the depths and locations they are willing to forage in.
Some seals have found creative solutions. In one documented case off the coast of Scotland, a harbor seal used shellfish aquaculture infrastructure (the frames and lines of a mussel farm) as a refuge to evade foraging killer whales. The seal spent over 94% of its time at the farm displaying anti-predator behavior or resting from exhaustion rather than feeding, suggesting it was hiding, not foraging among the mussels.16Aquatic Mammals. Predation in the Anthropocene: Harbour Seal (Phoca vitulina) Utilising Aquaculture Infrastructure as Refuge to Evade Foraging Killer Whales (Orcinus orca)
Human-generated noise adds another layer of complexity. Experiments with captive harbor seals tested how underwater sounds mimicking tidal turbines and pile driving affected their willingness to forage. When the noise source was near a high-quality prey patch (one with lots of food), the seals tolerated the disturbance and foraged normally. But when the noise was at a low-quality prey patch, foraging success dropped by roughly 16–28%. The seals were essentially running a cost-benefit analysis: is this patch worth the risk? When the reward was high enough, they pushed through the noise. When it was not, they stayed away.17Journal of Applied Ecology. Acoustic risk balancing by marine mammals: anthropogenic noise can influence the foraging decisions by seals This risk-reward balancing is worth keeping in mind as offshore energy development expands into harbor seal habitat.
How Scientists Figure Out What Seals Eat
Studying the diet of an animal that eats underwater and digests its meals before returning to shore is not straightforward. Historically, researchers relied on analyzing fecal samples (scat) collected from haul-out sites, looking for hard parts of prey that survive digestion: fish ear bones (otoliths), squid beaks, and crustacean shells. Otoliths are particularly useful because their shape and size can identify fish species and estimate how big the fish was. The Skagerrak study mentioned earlier used otolith counts to determine that sandeel was the most frequently eaten prey.2Marine Mammal Science. Shifting Harbor Seal (Phoca vitulina) Diet May Reflect Ecosystem Changes in Skagerrak
The problem with hard parts analysis is that it is biased. Fish with large, sturdy otoliths are easier to identify and count, while species with small or fragile otoliths get underrepresented. Soft-bodied prey like squid and jellyfish leave almost no trace. DNA metabarcoding has emerged as a powerful complement to the older method. By extracting and sequencing DNA from scat samples, researchers can identify prey species that would be invisible to hard parts analysis. A large-scale effort in the Salish Sea used both approaches side by side, running DNA metabarcoding alongside traditional hard parts analysis on the same scat samples to get the most complete picture possible.18Scientific Data. Data on the diets of Salish Sea harbour seals from DNA metabarcoding The DNA approach also allows researchers to determine the sex of the seal that deposited each scat, opening up questions about whether males and females eat different things.19PubMed Central. Large‐scale molecular diet analysis in a generalist marine mammal reveals male preference for prey of conservation concern
Stable isotope analysis offers yet another angle. Rather than identifying individual prey items from a single meal, isotopes in seal tissues (blood, skin, whisker keratin) reflect the average diet over weeks or months, revealing broader patterns like the spring-to-summer pelagic-to-benthic shift observed in the Wadden Sea.3PubMed Central. Seasonal Variation of Harbor Seal’s Diet from the Wadden Sea in Relation to Prey Availability Each method has blind spots, and the best modern diet studies combine two or three of them.
Harbor Seals and Commercial Fisheries
Because harbor seals eat commercially valuable fish, their relationship with fisheries has been contentious for as long as commercial fishing has existed in seal habitat. Seals eat cod, herring, salmon, and other species that fishers also want. But the actual magnitude and direction of the interaction is more complicated than “seals take fish from fishers.”
Modeling work in Swedish coastal waters estimated that a local harbor seal population removed about 32 tonnes of fish from bottom-set gillnet fishing areas over a 73-day period, along with about 20 tonnes from Danish seine areas. Seal predation on the fish targeted by these fisheries likely reduced catches. But in the shrimp trawl areas, harbor seal predation on bottom-feeding fish that compete with or prey on shrimp may have actually benefited shrimp yields.20ICES Journal of Marine Science. Interactions between harbour seals, Phoca vitulina, and fisheries in complex coastal waters explored by combined Geographic Information System (GIS) and energetics modelling The relationship is not simply competitive; seals can indirectly help some fisheries while hurting others, depending on which prey they remove and what the downstream effects are in the food web.
On a larger scale, ecosystem modeling for the North Sea suggests that maintaining recovered seal populations and sustaining commercial fisheries are not inherently incompatible, though both face threats from climate-driven changes in fish stocks. The highest fisheries catches in the models were associated with seal biomass well below its unfished level, but viable catches still emerged at higher seal abundances, especially under harvest control rules that accounted for environmental variability.21ICES Journal of Marine Science. Seal population recovery and conservation can be reconciled with viable commercial fisheries in the North Sea, although both may be threatened by climate change The framing of seals as straightforward competitors for fish quotas is an oversimplification that ignores the complex trophic dynamics at play, including the role seals play in controlling populations of fish that themselves prey on commercially valuable juvenile fish.
Climate Change and Shifting Prey
Harbor seals eat what is available, which makes them vulnerable to changes in what is available. As ocean temperatures rise and fish distributions shift, harbor seal diets are already changing. The Skagerrak study documented a shift in diet composition compared to earlier decades, with the authors suggesting it reflected broader ecosystem changes in the region.2Marine Mammal Science. Shifting Harbor Seal (Phoca vitulina) Diet May Reflect Ecosystem Changes in Skagerrak When a key prey species declines or moves to different waters, seals can switch to alternatives, but this flexibility has limits. If the replacement prey is less energy-dense or harder to catch, the seals end up working harder for fewer calories.
Habitat suitability modeling for the Baltic Sea projects that harbor seal habitat will decline under future climate scenarios, driven by changes in sea surface conditions and the loss of haul-out sites from sea-level rise.22PubMed Central. Forecasting shifts in habitat suitability of three marine predators suggests a rapid decline in inter-specific overlap under future climate change If seals lose access to foraging grounds they have used for generations, they will need to find new ones, and those new areas may already be occupied by other predators or may lack the prey densities that made the old ones productive. Harbor seals’ dietary flexibility is one of their greatest strengths as a species, but it is not an infinite buffer against an ocean that is reorganizing its food web from the bottom up.