The grey seal, Halichoerus grypus, is a large marine carnivore found across the North Atlantic. As a generalist predator, its broad feeding habits allow it to thrive in diverse environments along coastlines, islands, and ice floes. Understanding what these seals eat provides important insight into their survival and distribution within marine ecosystems.
Primary Prey and Dietary Composition
Fish constitute the overwhelming majority of the grey seal’s diet, though the species consumed vary widely by location. Their generalist strategy results in a diet composed of both demersal (bottom-dwelling) fish and pelagic (open-water) schooling fish. A significant portion of their food source often consists of sand eels (Ammodytes spp.), which can make up over half the diet in certain areas.
Other commonly consumed fish include members of the cod family (gadids), such as cod, whiting, and haddock. Flatfish, including flounder and plaice, are also regular prey, reflecting the seal’s tendency to forage near the seabed. While fish are the primary focus, the seals also occasionally prey on cephalopods (squid and octopus) and various crustaceans, including lobsters.
Foraging Behavior and Hunting Techniques
Grey seals are capable of deep diving, reaching depths of up to 1,560 feet and holding their breath for as long as one hour. However, most feeding dives are shallower, typically between 100 to 230 feet. A specialized tool for hunting is the use of long, highly sensitive whiskers, or vibrissae. These vibrissae detect the subtle pressure waves generated by moving fish, allowing the seals to locate prey even in dark or cloudy conditions.
Grey seals often hunt alone, but they have also been observed hunting in small groups, which improves success when targeting schooling fish. Smaller fish are typically swallowed whole underwater. For larger prey, the seal brings the catch to the surface, using its fore-flippers to hold the fish steady. The seal then breaks the catch into smaller, manageable pieces before consuming it, sometimes discarding the head entirely.
Variables Influencing Diet Selection
The grey seal’s diet is highly dynamic, shifting in response to environmental and biological variables. The most pronounced variation is geographic, as diet composition directly reflects the local abundance of prey. For instance, the diet in the North Sea is often dominated by sand eels, while in the Gulf of St. Lawrence, cod, herring, and capelin may be more important. Seasonal changes in prey migration patterns also drive temporary shifts in feeding habits, requiring seals to switch between schooling and demersal species.
Life stage and sex introduce further variables to diet selection, as energy requirements differ between individuals. Adult males and females exhibit different foraging tactics, particularly during the breeding season, leading to sex-specific differences in targeted prey. Juvenile seals, being less experienced hunters, tend to have a broader, lower-quality diet compared to selective adults.
Consumption Rates and Ecological Impact
Grey seals have substantial daily food consumption requirements due to their considerable energy needs. On average, a grey seal consumes an amount of food equivalent to four to six percent of its total body weight each day. This rate is an average, as seals fast completely during the breeding and molting seasons, relying on stored fat reserves. An adult grey seal requires approximately 11 pounds (5 kg) of food daily to meet its energy demands.
When quantified across large populations, the total annual consumption is significant; for example, grey seals in the North Sea were estimated to consume nearly 200,000 metric tons of prey in a single year. This high consumption rate places the grey seal in a prominent position within the marine food web, leading to discussions about their ecological impact on localized fish stocks. While seals prey on commercially valuable species like cod and herring, research suggests that environmental factors and commercial fishing pressure often have a greater impact on fish population declines than seal predation alone.