Seals, marine mammals belonging to the group pinnipeds (true seals, fur seals, and sea lions), include squid in their diet. Cephalopods are a consistent and significant prey source across many seal species worldwide. The consumption of squid, octopuses, and cuttlefish is a fundamental part of the seal’s ecological role as a marine predator. Understanding this dietary component reveals details about the foraging strategies and nutritional requirements of these aquatic hunters.
The Importance of Cephalopods in Seal Nutrition
Cephalopods provide a valuable nutritional package that supports the high metabolic rate of seals. They are composed of high-quality protein, which is essential for building and maintaining muscle mass in these active swimmers. The lipid content in cephalopods, though sometimes lower than in certain fatty fish, is rich in long-chain polyunsaturated fatty acids (PUFAs). These fatty acids, such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), are beneficial for neurological function and overall health.
A diet including high-lipid squid is often more energetically efficient for seals than one dominated by lean, high-protein fish. The digestive cost of breaking down excess protein is high, requiring seals to consume greater quantities of low-fat prey for the same net energy gain. Cephalopods are common in stomach contents and scat samples across various seal populations. For instance, certain fur seal species have diets consisting of over 50% cephalopods by mass in some regions.
How Diet Varies Across Seal Species
The extent to which a seal relies on squid is highly dependent on its species, size, and geographical location. Deep-diving oceanic seals tend to have the highest reliance on cephalopods, which inhabit the deeper water columns. Northern Elephant Seals, known for their incredible diving capabilities, regularly forage on squid and octopuses found at depths often exceeding 1,000 meters. Weddell Seals, another Antarctic deep diver, also feature cephalopods prominently in their diet, alongside fish.
Coastal-dwelling seals have a more generalized diet that includes locally available fish and crustaceans. Harbor Seals are opportunistic feeders, including squid and octopus among various fish species. Similarly, the New Zealand Fur Seal’s diet is a mix of fish and cephalopods, with some populations relying heavily on squid. For the Leopard Seal in the Antarctic, squid constitutes a portion of its diet, though this large predator is known for consuming warm-blooded prey like penguins and other seals.
Hunting Techniques and Preferred Prey
Seals employ specialized sensory and physical adaptations to successfully hunt fast-moving cephalopods. To locate prey in the ocean’s dark depths or murky coastal waters, seals rely heavily on their highly sensitive facial whiskers, known as vibrissae. These whiskers can detect subtle movements and pressure changes in the water, allowing the seal to track the jet-propelled movements of squid even with limited visibility.
Deep-diving species utilize physiological adaptations for prolonged underwater excursions to access the mesopelagic zone where many squid reside. Seals frequently target members of the oceanic Ommastrephidae and Onychoteuthidae families, which include various deep-sea squid. Smaller, benthic octopuses are also consumed by seals that forage near the seafloor. Specific examples found in seal diets include the Antarctic squid Psychroteuthis glacialis and Kondakovia longimana. Coastal species like the Guadalupe Fur Seal often feed on pelagic squid at night when the cephalopods migrate closer to the surface.