Fur seals are marine carnivores belonging to the family Otariidae, commonly known as eared seals. These predators are distributed globally, inhabiting a vast range from subpolar to equatorial waters. Understanding their dietary habits is important because they act as indicators of marine ecosystem health. Fur seals often compete with commercial fisheries and are susceptible to environmental changes, making the study of their food intake important for conservation and management.
Main Components of the Fur Seal Diet
The fur seal diet is broad, consisting primarily of small, schooling fish, cephalopods, and crustaceans, though the exact composition varies significantly by species and location. Small, bony fish are a common food source across most populations, including species like Pacific sand lance, sardines, anchovies, and hake. Northern Fur Seals in the North Pacific, for example, consume a variety of oceanic fish and squid, including Atka mackerel and Pacific salmon.
Cephalopods, particularly squid and octopus, form a major part of the diet for many fur seals, often representing the largest proportion of prey biomass. New Zealand Fur Seals frequently consume arrow squid and lanternfish, with squid sometimes accounting for over 90% of the prey biomass in certain areas. In contrast, Antarctic Fur Seals have a diet heavily dominated by small crustaceans, relying on the immense abundance of Antarctic krill.
This dietary flexibility allows fur seals to adapt to shifts in prey availability. The consumption of lanternfish (myctophids) is a common thread, as these small, oily, deep-sea fish are widely distributed and often migrate vertically in the water column. The diet is generally centered on energy-rich, readily available prey, whether it is a small pelagic fish or a dense patch of krill.
Specialized Foraging and Hunting Techniques
Fur seals employ specialized behaviors and physical adaptations to successfully acquire their prey in the marine environment. Their bodies are built for efficient underwater pursuit, allowing them to perform impressive dives that can reach depths of 150 meters, with some species capable of holding their breath for up to 15 minutes. This allows them to exploit food resources below the surface layer.
Many seals utilize diel vertical migration, foraging at night when prey species such as squid and lanternfish move closer to the surface. Foraging strategies are broadly categorized as pelagic or benthic; pelagic foragers pursue schooling prey throughout the water column, while benthic foragers target more cryptic prey on the seafloor. Australian Fur Seals, for example, often engage in benthic foraging, using movements along the bottom to surprise and capture prey.
The vibrissae, or whiskers, on a fur seal’s muzzle are a sensory tool for hunting in the dark, deep ocean. These whiskers are sensitive, allowing the seal to detect and follow the hydrodynamic trails left behind by swimming fish or squid. This ability to sense prey movement through water vibration allows the seal to locate food even in complete darkness or murky water where visual cues are absent.
How Geography and Season Affect Food Intake
The food intake of a fur seal is influenced by its geographical location, the time of year, and large-scale oceanographic events. In nutrient-rich cold-water environments, such as the Southern Ocean, the seals’ diet is often concentrated on a single, abundant resource like krill. Populations in more temperate zones show a broader diet of fish and cephalopods. The distribution of walleye pollock in the eastern Bering Sea, for example, is affected by temperature, which then dictates the foraging success of Northern Fur Seals.
Seasonal changes also drive shifts in diet composition as prey species migrate or become more or less abundant. New Zealand Fur Seals often switch from a cephalopod-dominated diet during the summer and autumn to one that includes more fish in the winter months. For lactating females, reproductive demands impose a constraint known as central-place foraging, requiring them to alternate between nursing their pup on land and foraging at sea.
Large-scale environmental events, such as El Niño, impact the availability of prey and force fur seals to alter their foraging behavior. During El Niño events, warmer waters often cause prey species to become less abundant or move to deeper waters. This forces seals, like the Peruvian Fur Seal, to undertake longer foraging trips and dive to greater depths to find food, which can ultimately lead to lower pup growth rates and survival.