There is a strong link between the breeding pattern, body size, and feeding ecology in phocid seals. The relatively large size of phocids allows them to build body stores of fat that fuel breeding without foraging for periods of time (a phenomenon known as capital breeding). Some of the smaller phocids, however, such as the harbor seal, may have to forage some during the breeding period to rear their young successfully because they cannot store enough fat. The ability to store fat and fast during breeding means that foraging grounds can be separated by considerable distances from breeding grounds in these species. Northern elephant seals (Mirounga angustirostris), for example, are known to leave breeding and resting areas to travel about 12,000 mi (20,000 km) during two major foraging periods, after breeding and after molting.
The diets of phocid seals may be varied both within and among species, including varying seasonally as food resources change or as the location of seals change seasonally. Crab-eater seals, for example, eat krill (small shrimp-like organisms) almost exclusively, whereas leopard seals eat penguins, other seals, fish, krill, squid, octopus, and other invertebrates. Fish of one sort or another is the primary diet of most phocid species.
To obtain the energy-rich food and dense patches of fish, phocids often have to dive to considerable depths. Elephant seals will dive to depths as deep as about 5,000 ft (1,500 m), staying underwater from 20 to 60 minutes while they forage for squid. Other species, like harbor seals in some locations, regularly forage only at depths of 80-200 ft (25-60 m), whereas at other locations may forage more often at about 650-825 ft (200-250 m). The shallower dives are usually much shorter in duration, lasting more in the range of three to five minutes.
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