Grey seals eat mostly fish, and their diet spans dozens of species depending on where they live and what time of year it is. In some regions, cod and sand lance dominate their meals; in others, perch, flatfish, or herring take center stage. They are opportunistic predators that forage primarily along the sea floor, though they occasionally hunt in open water, and their diet can include surprises like harbour porpoises. How they find and catch their prey is just as interesting as what ends up on the menu.
A Fish-Heavy Diet That Varies by Region
Grey seals are found across both sides of the North Atlantic, from eastern Canada to the coasts of the United Kingdom, Scandinavia, and the Baltic Sea. Wherever they live, fish make up the overwhelming majority of their diet. But the specific species they eat differ dramatically from one population to the next, shaped by what is locally abundant. A study of grey seal scat near the Vistula River mouth in the southern Baltic, for instance, found that perch appeared in nearly 78% of samples and pikeperch in 67%, while Baltic cod showed up in about 31% and sea trout in roughly 27%. Genetic analysis of the same samples bumped the detection rate for cod to 69% and sea trout to 63%, suggesting hard-part analysis alone underestimates some prey.
1Oceanologia. Characteristics of the grey seal (Halichoerus grypus) diet in the Vistula River mouth (Mewia Łacha Nature Reserve, southern Baltic Sea), based on the osteological and molecular studies of scat samplesIn the Northwest Atlantic, the picture looks quite different. Populations in that region tend to rely on species like sand lance, Atlantic cod, redfish, and various flatfish. Even within the same ocean basin, diets at emerging offshore colonies differ drastically from those at established coastal colonies, reflecting the different prey communities available in deeper water.
2Canadian Journal of Zoology. Dietary composition and sources of variability in emerging offshore grey seal colonies in the Northwest AtlanticThis flexibility is a hallmark of grey seal ecology. They are not specialists committed to one prey type. They are generalists that exploit whatever fish community surrounds them, and their diets shift as prey populations rise and fall.
How Season and Sex Shape What Gets Eaten
Beyond geography, two of the strongest predictors of what an individual grey seal eats are the time of year and whether it is male or female. Research using fatty acid signature analysis has shown that season and sex together explain most of the observed variation in adult grey seal diets. The differences become sharpest in spring, after the breeding season, when females are recovering from the enormous energy cost of nursing a pup. During that post-breeding period, females tend to focus on fewer, higher-quality prey species, while males forage more broadly.
3PubMed. Sex differences in grey seal diet reflect seasonal variation in foraging behaviour and reproductive expenditure: evidence from quantitative fatty acid signature analysisThese dietary differences line up with differences in where males and females go to feed. Males and females use different areas of the ocean at different times of year, effectively segregating themselves spatially even though they are not migratory in the traditional sense. Females recovering from breeding seem to prioritize energy-dense prey that helps them rebuild fat stores needed for the next pregnancy, while males, freed from that constraint, range more widely and eat a more varied assortment of fish.
4PubMed Central. Sexual segregation of seasonal foraging habitats in a non-migratory marine mammalFemale grey seals also show diel patterns in their foraging, adjusting their dive depths throughout the day to track prey that move up and down in the water column. Sand lance, for example, make daily vertical migrations to feed on zooplankton, and female grey seals appear to follow them. Females in the Northwest Atlantic consume a higher proportion of these pelagically foraging prey species compared to males.
5PubMed Central. Foraging behaviour of a continental shelf marine predator, the grey seal (Halichoerus grypus), is associated with in situ, subsurface oceanographic conditionsHunting Along the Sea Floor
Grey seals are primarily benthic foragers, meaning they hunt on or near the seabed. When a grey seal leaves its haul-out site on a foraging trip, it typically swims relatively slowly and performs distinctive flat-bottomed dives, spending roughly 60% of the total dive time at the maximum depth, cruising along the bottom. These are sometimes called “U-shape” or “square-wave” dives, and they look very different from the sharp, V-shaped dives the animals make when simply travelling between locations.
6Journal of Zoology. Movements, diving and foraging behaviour of grey seals (Halichoerus grypus)Tracking data confirm this benthic preference. In a study that logged over 324,000 dives, about 42% occurred in water shallower than 50 meters, likely near haul-out sites. Of the remaining dives in deeper water, roughly 69% were dives to the seabed and 31% were pelagic dives through open water. Benthic dives tended to cluster in runs, with one bottom dive highly likely to be followed by another. The picture that emerges is of an animal that, most of the time, works the ocean floor systematically.
7PubMed Central. Habitat-Mediated Dive Behavior in Free-Ranging Grey SealsPhysically, grey seals are built for this style of hunting. Comparative studies of skull and jaw anatomy across seal species classify grey seals as “pierce feeders,” meaning they use a combination of biting and suction to grab individual prey items. Their jaws and teeth are suited to seizing and holding slippery fish rather than filtering small organisms from the water or tearing chunks from large prey.
8Journal of Anatomy. The better to eat you with: the comparative feeding morphology of phocid seals (Pinnipedia, Phocidae)When They Hunt in Open Water
Although bottom foraging dominates, grey seals do switch to pelagic hunting under certain conditions. That same large dive dataset showed that pelagic dives became more common at night: around 44% of dives were pelagic after dark compared to about 28% during the day. The type of seabed also mattered. Over fine, soft sediment, pelagic dives were more frequent than over rocky or coarse ground. Over fine sediment at night, more than half of dives were pelagic.
7PubMed Central. Habitat-Mediated Dive Behavior in Free-Ranging Grey SealsThe likely explanation is prey behavior. Many small schooling fish that grey seals eat, like sand lance and herring, rise off the bottom at night to feed on plankton. If the prey is no longer sitting on the seabed, a bottom-hugging foraging strategy makes less sense. Over complex rocky substrate, on the other hand, prey like wrasse and other reef-associated fish tend to stay put, so bottom foraging remains productive around the clock. Grey seals appear to read these conditions and adjust, which fits their overall profile as flexible, opportunistic hunters.
Tracking Prey With Whiskers
One of the most remarkable tools in a grey seal’s hunting kit is its whiskers. The stiff, sensitive vibrissae on a seal’s muzzle are not just for touch. In phocid seals, which include grey seals, whiskers function as a highly sensitive hydrodynamic receptor system, capable of detecting and analyzing water movements created by swimming fish.
9PubMed Central. Flow sensing by pinniped whiskersExperiments have shown that seals can follow the hydrodynamic trails left by fish that passed by seconds earlier, effectively tracking prey by the wake it leaves behind. They are highly sensitive to the tiny pressure disturbances caused by a fish’s tail beats, and can use that information to determine which direction the fish went.
10PubMed. Hydrodynamic perception in true seals (Phocidae) and eared seals (Otariidae)This matters enormously for an animal that hunts along a dark seabed. Visibility in coastal and shelf waters is often poor, and at the depths and times of day when grey seals forage, light can be minimal. Whisker-based trail-following gives them a way to locate prey that does not depend on sight at all. It helps explain how grey seals manage to be successful hunters in murky Baltic waters and on pitch-dark winter nights alike.
How Much Fish a Grey Seal Goes Through
Grey seals are large animals. Males can weigh over 300 kilograms and females around 150 to 200 kilograms, and fueling a body that size in cold ocean water requires a lot of fish. Estimates from a colony in France’s Iroise Sea found that adult grey seals consume roughly 380 to 850 kilograms of fish per quarter, depending on the season. Sub-adults eat an estimated 250 to 290 kilograms per quarter, and pups take in 140 to 185 kilograms. Scaled up to a whole colony, researchers estimated that about 115 tons of fish were consumed over a year by that single population.
11ICES Journal of Marine Science. Foraging behaviour and prey consumption by grey seals (Halichoerus grypus)—spatial and trophic overlaps with fisheries in a marine protected areaThose numbers vary with body size, water temperature, reproductive state, and prey quality. A female rebuilding blubber after nursing a pup may eat more aggressively than a male in the same period. And a seal eating energy-dense herring or mackerel needs fewer individual fish than one subsisting on leaner species like sand lance. But the order of magnitude is clear: each adult grey seal is removing roughly a ton or more of fish from the ecosystem every year.
Not Just Fish
While fish account for the vast majority of the diet, grey seals occasionally eat other things. Squid, octopus, and crustaceans appear in scat samples from time to time, though rarely in large quantities. The most startling non-fish prey item documented in recent years is the harbour porpoise. A forensic study in the Netherlands concluded that grey seals are a major predator of harbour porpoises there, and that grey seal attacks are one of the main causes of death in the Dutch harbour porpoise population.
12PubMed Central. Exposing the grey seal as a major predator of harbour porpoisesThe attacks leave distinctive parallel claw marks and large, clean-edged bites on porpoise carcasses. This behavior was only definitively documented in the 2010s, and it remains unclear how widespread it is across the grey seal’s range. It may be more common in areas where grey seal populations have grown rapidly while porpoise populations overlap with them in shallow coastal waters. Whether individual seals specialize in this behavior or whether many seals take porpoises opportunistically is still an open question, but the evidence that it happens regularly in at least some populations is solid.
Raiding Fishing Nets
Grey seals have learned that fishing nets are an easy source of food, and this habit causes serious conflict with commercial fisheries, particularly in the Baltic Sea. Research tracking what happens to cod caught in gillnets found that daily disappearance rates averaged around 45% for cod. On occasions when nets soaked in the same spot for three days or more, the entirety of the cod catch vanished. From the third day onward, more than 60% of daily cod catches were being taken. The pattern suggests that seals discover a productive net and then return repeatedly in the following days to harvest freshly trapped fish.
13Fisheries Research. Disappearing fish: Grey seal depredation in a Baltic net fisheryFlatfish, interestingly, are much less affected. Disappearance rates for flounder, plaice, and turbot stayed below 20% even in heavily raided nets. Seals seem to strongly prefer cod and do not bother returning to the same net for flatfish alone. A separate study in the central Baltic found that in gillnet fleets raided by seals, roughly 44% of marked cod was lost, with another 13.5% left visibly damaged in the net.
14Journal of Northwest Atlantic Fishery Science. Grey Seal Predation in Cod Gillnet Fisheries in the Central Baltic SeaFor fishers, this is more than a nuisance. Losing nearly half of a cod catch to seals, plus gear damage from animals tearing through nets, can make certain fisheries economically unviable. Various deterrent technologies have been tried, including acoustic devices and modified trap designs, with mixed results. The problem has intensified as grey seal populations have recovered from historically low levels across much of their range.
The Cod Recovery Question
Because grey seals eat so much cod and because several Atlantic cod stocks are depleted, there has been heated debate about whether seal predation is preventing cod from bouncing back. The evidence is genuinely mixed and depends heavily on which stock you look at.
In waters west of Scotland, modeling work found that seal predation on cod follows a pattern where seals eat proportionally more cod when the stock is small, creating a kind of predation trap. Projections suggested that cod recovery is possible under current conditions, but the recovery is fragile and sensitive to even small increases in either fishing pressure or seal population size.
15Canadian Journal of Fisheries and Aquatic Sciences. The effects of grey seal predation and commercial fishing on the recovery of a depleted cod stockA study of cod in the southern Gulf of St. Lawrence went further, concluding that the decline in fishing mortality over the previous decade had effectively been replaced by rising seal predation on a shrinking stock, and that the stock was unlikely to recover even at reduced fishing levels. That analysis warned that ignoring predation by top predators when setting fishing quotas could lead fishery managers to overestimate the benefits of cutting back on fishing.
16Journal of Applied Ecology. Grey seal predation impairs recovery of an over‐exploited fish stockOther stocks tell a different story. A model of cod on the Western Scotian Shelf and Bay of Fundy found that most natural mortality in that population could not be explained by seal predation across a wide range of scenarios, suggesting that other factors like environmental conditions, disease, or changes in the food web were playing larger roles.
17ICES Journal of Marine Science. Evaluating the potential impacts of grey seal predation and fishery bycatch/discards on cod productivity on the Western Scotian Shelf and in the Bay of FundyThe upshot is that grey seal predation can be a significant brake on cod recovery in certain populations, particularly ones that are already small and concentrated. But it is not a universal explanation for why cod have failed to rebound. The interaction between seal predation and overfishing is the real problem: when humans drive a fish stock down to low levels, the remaining fish become more vulnerable to predation, and seals can hold the stock in a depleted state even after fishing pressure eases.
How Scientists Figure Out What Seals Eat
Studying the diet of an animal that feeds underwater and digests its meals before returning to shore is not straightforward. Historically, the most common method has been examining scat samples collected at haul-out sites and identifying fish from the hard parts, like ear bones and vertebrae, that survive digestion. This works but has known biases: soft-bodied prey leaves few traces, and some fish bones dissolve more readily in stomach acid than others, so certain species get undercounted.
Molecular techniques have helped fill the gaps. DNA extracted from scat can identify prey species that left no recognizable bones. In the Baltic study mentioned earlier, genetic analysis detected cod and sea trout at much higher rates than hard-part identification alone, showing how DNA methods can substantially change the picture of what seals are eating.
1Oceanologia. Characteristics of the grey seal (Halichoerus grypus) diet in the Vistula River mouth (Mewia Łacha Nature Reserve, southern Baltic Sea), based on the osteological and molecular studies of scat samplesA third approach uses fatty acid signatures from seal blubber. Because fatty acids from prey are deposited into a predator’s fat stores in characteristic patterns, researchers can work backward from a blubber sample to estimate what the animal has been eating over a period of weeks to months. The technique was developed and validated on captive grey seals and has proven effective at capturing diet information that scat studies miss, particularly for soft-bodied or quickly digested prey. It also provides a longer time window than a single scat sample, which represents only one or two recent meals.
18Ecological Monographs. Quantitative Fatty Acid Signature Analysis: A New Method of Estimating Predator DietsParasites That Ride the Food Chain
Grey seals sit at the top of a marine food web, and that position comes with a parasitological cost. One well-studied example is the sealworm, a nematode that cycles between seals and fish. Fish become infected by eating invertebrates carrying sealworm larvae, and seals become infected by eating those fish. The adult worms live in the seal’s stomach, release eggs into the water through the seal’s feces, and the cycle continues.
19Developments in Marine Biology. Aspects of the sealworm Pseudoterranova decipiens life-cycle and seal-fisheries interactions along the Norwegian coastThis matters to people because sealworm larvae end up in the flesh of commercially important fish like cod. In areas with large seal populations, infection rates in fish can be high enough to cause economic losses for the fishing industry, as consumers reject wormy fillets. The problem tends to be worst in localized areas near major haul-out sites, where seal density is highest and the parasite life cycle runs most efficiently. It is one more way that what grey seals eat ripples outward to affect human economies and marine ecosystems in ways that go well beyond the direct removal of fish from the water.