The Weddell seal (Leptonychotes weddellii) is the world’s southernmost-ranging mammal, thriving in the extreme cold of the Antarctic. They primarily inhabit the stable, land-fast ice that rings the continent and its coastal areas. Living in this remote and challenging environment requires a highly specialized diet to sustain their large body size. Their feeding ecology, which involves navigating frigid waters beneath thick ice to access food, is a remarkable example of polar adaptation.
Composition of the Weddell Seal Diet
The Weddell seal’s diet consists mostly of fish, supplemented by other marine organisms. The most consumed prey item is the Antarctic silverfish (Pleuragramma antarcticum), a small, pelagic species that dominates the fish biomass in their foraging areas. This fish is a staple across all seasons for many Weddell seal populations.
Seals also regularly consume members of the Nototheniidae family, commonly known as Antarctic cod. Larger prey, such as the Antarctic toothfish (Dissostichus mawsoni), are pursued by adult seals. The rich, highly unsaturated fatty acid content of these cold-water fish provides the energy and blubber necessary for survival in freezing conditions.
Cephalopods, particularly various species of squid, make up a smaller portion of their intake. They are opportunistic feeders; crustaceans and other bottom-dwelling invertebrates are occasionally consumed when primary prey is scarce. Their diet is generally less varied geographically than that of other seal species, reflecting the simplicity of the Antarctic marine food web they exploit.
Foraging Strategies and Dive Behavior
Weddell seals are exceptional divers. They routinely dive to depths between 200 and 400 meters, though maximum recorded dives can surpass 900 meters. These deep dives can last over an hour, with recorded durations extending as long as 96 minutes, making them among the most capable deep-diving pinnipeds.
The seals hunt in both the pelagic zone (mid-water) and the benthic zone (sea floor), adjusting their strategy based on prey distribution. They rely heavily on non-visual senses when hunting in the dark, light-limited environment beneath the ice. Highly sensitive vibrissae, or whiskers, are used to detect the wake and movement of swimming fish in the water column.
Foraging behavior is often timed to coincide with the seasonal movements of their prey, which vary due to the polar light cycles. To maintain access to hunting grounds, the seals use their teeth to rasp and maintain breathing holes, known as saimas, in the thick ice. They may also employ a technique of blowing bubbles into ice cracks to disturb and flush out fish from hiding spots.
Metabolic Requirements and Daily Intake
The Weddell seal’s diet must meet the energy demands of maintaining body temperature in sub-zero water and performing deep dives. The blubber layer is built and maintained by the high caloric intake from their oily, fatty fish prey, providing the fuel needed for thermoregulation.
The daily food requirement fluctuates depending on the seal’s activity level. A sedentary adult seal hauled out on the ice may consume approximately 10 kilograms of food per day. An active, foraging adult, however, may require over 50 kilograms of prey in a single day to support its energy expenditure.
Seals typically fast or significantly reduce their intake during periods spent on the ice, such as the breeding and molting seasons. Stored energy in their blubber reserves is used to survive these periods of reduced foraging. This ability to switch between high-intensity foraging and fasting is a fundamental adaptation to the highly seasonal Antarctic ecosystem.