By Erin Bishop, Chaille Blount, and Ashly Norman

Foraging, by definition, is to search for provisions or food, or by searching for a supply of food. Almost all mammals forage for survival. Here we will only look at 3 marine mammals who forage, seals, sperm whales, and sea otters. We will look at how they forage, what they forage for, where in the world they do most of their foraging, and how they each differently forage for their food.

Researchers have many different ways of finding out this information. Some of the instruments or methods used include observation, the use of satellites, tagging with time/depth recorders, using directional hydrophones, examination of dead specimens, a "critter camera"(Marshall 1998), and photographs.

Seals, the family Phocidae, use two general types of foraging tactics. They either individually forage or forage in groups. Typically the animals which individually forage are capturing nonschooling fish, slow-moving or sessile invertebrates, or small-sized warm blooded prey. This style is normally done in coastal waters, bays, and rivers, although, the elephant seal is an exception because it forages individually in the open sea. Normally phocids prefer this method to foraging in groups. The animals which do forage in groups tend to go after large patchy distributed schools of fish or squid and feed in pelagic waters. Some animals actually use the group feeding of other animals to help them locate prey. Some seals locate their prey by looking for groups of feeding birds and then swim towards them. Another way in which some seals feed is by stealing. Many seals have been caught taking fish out of fisherman's nets. Devices are being used to deter the seals but as of now they have not been effective. (Riedman 1990)

The harbor seal, Phoca vitulina richardsi, lives off most coasts in the northern hemisphere (Riedman 1990). They tend to feed on Pacific herring, salmon, zoarcid fish, cephalopods, krill, and other types of invertebrates (Suryan and Harvey 1998, Riedman 1990). Seals that live near Washington state show a preference towards zoarcid fish and Pacific herring in July and August.

When harbor seals forage they exhibit a behavior know as milling behavior. This means that they stay in a localized area and don't move in any specific direction (Suryan and Harvey 1998). Habor seals often forage in areas where tide rips are present. Tide rips form in shallow areas around reefs or shoaling seafloors. These shallow areas possibly make it easier for the seal to catch bottom dwelling fish or consolidate schooling fish. Some harbor seals exhibit no pattern of diurnal or nocturnal foraging. A typical dive for foraging is 4-6.22 minutes long and about 110 meters deep. The dives that occur during milling behavior are longer and deeper than dives which occur when the seal is traveling or engaged in near-shore activities (Suryan and Harvey 1998).

There are two types of elephant seals. The Northern elephant seal, Mirounga augustirostris, lives off the west coast of the United States of America and the Southern elephant seal, Mirounga leonina, lives in the South Pole. They feed mostly on deep-water squid and fish. Some examples of their diet items are sharks, rays, ratfish, Pacific hake, and rockfish (Riedman 1990).

The elephant seal forages in the open ocean. This seal tends to dive more often, less deep and for shorter periods of time at night compared to during the day. This behavior is thought to occur because many of the elephant seals' prey items migrate towards the surface of the ocean at night (Reidman 1990). A typical dive for an elephant seal is between 350 and 650 meters deep and about 20 minutes long. The elephant seal is the deepest diving Pinniped and has been documented diving up to 1,250 meters deep and for 62 minutes long. A speculation on why elephant seals hang out deeper than most seals is that they are avoiding their predators, like the great white shark (Reidman 1990).

Weddell seals, Leptonychotes weddellii, inhabit the coastal waters around the Antarctic continent. The weddell seal feeds on Antarctic cod, nototheniid fish, subice fish, cephalopod, krill, and other types of invertebrates. The weddell seals which feed in McMurdo Sound, Antarctica forage primarily on small nototheniid fish and large Antarctica cod (Davis et al. 1999).

The weddell seal exhibits many patterns in its feeding times. It dives deeper in the summer and in the mid-winter. A reason could be that they are exploring different food sources or that their preferred prey exists at different depths. It also dives deeper and longer in the day compared to the night (Schreer and Testa 1996). The weddell seal dives significantly deeper during periods around the full moon, likely because they are using their vision to help them hunt prey. This seal also prefers to forage around dawn and dusk, which could be because of the diel vertical migration of their prey (Schreer and Testa 1996). The weddell seal is a very deep diver. These seals on average dive 212-400 meters and about 15 minutes long. The deepest dive recorded is 726 meters deep and 78 minutes long (Schreer and Testa 1996).

The weddell seal uses many different tactics to forage because it lives in such an unusual habitat. It will swim underneath a fish and use the ice above to silhouette the fish so that the seal can see it better. Attacking the fish from below also allows the seal to be less detectable and get a better angle of attack. Some weddell seals have been seen biting the cod near its anal fit but not killing it (Davis et al. 1999). Another tactic which weddell seals use is to blow out through their nostrils into cracks or crevices in the ice. This scares fish out of these areas so that the seal can eat them. Another way which the seal scares its prey out of the ice is to ram its muzzle into the ice. These seals will also bite the exposed tails of fish which are hiding in the ice to get them to come out of hiding. The weddell seal often uses its vibrissae during final stages of attack to give it detailed information on its prey (Davis et al. 1999).

Sperm whales (Physeter catadon or Physeter macrocephalus) are a unique species considering their foraging habits. Because they dive for such long periods of time and to such great depths, many of their foraging techniques are merely theories. According to the Whale Watching Web, the use of dead beached whales provides much information concerning anatomy and physiology of the whale (1999). By whale watching and observing photographs, it can be determined where the whales are located, how they dive, and the social classes that are held within the species. Listening to the vocalizations of the animal also helps to pinpoint where it is located and how far they descend when diving. This is done with a directional hydrophone that is attached the animal or simply is placed in the water. This directional hydrophone is an acoustical listening device that aids in locating the sperm whale (Whitehead, 1989).

Giant squid are the sperm whales meal of preference (Herman, 1980). They will, however, eat schooling fish of cod and ocean perch ( Ridgeway, 1972). The American Cetacean Society has also determined that skates and octopi are eaten as alternate food sources (1997). They will eat about 907 Kg of this diet to survive daily (1997).

Males and females are very different in their grouping strategies. Males can be found forming social clusters of about 20 individuals at the surface coordinating their movements (Whitehead, 1989). They will form smaller groups of about four individuals when foraging (1989). They will "fluke up" when descending straight down. It is not necessary for them to forage together though. The males are perfectly capable of foraging solitarily (1989). Females and calves form a group that is a "nursery school" of sorts. While one female dives, the others will watch her calf until she returns (American Cetacean Society, 1997). This group tends to run from 20-50 individuals, and is very consistent with the individuals that stay within the group (1997). The males, when reaching sexual maturity, will leave this group and form "bachelor group" until they are ready to begin mating (Whithead, 1989).

Again, the distribution of the males and females is very different. Males are found at the latitudes of 70N - 70S (Ridgeway, 1972). They will migrate to the lower latitudes in the summer to breed. The females are found at the latitudes of 40N - 40S all year ( 1972). This is presumably due to the calves and the food sources at the warmer latitudes. In the colder waters their primary food source is squid, while in warmer waters their principle food source is schooling fish (Whitehead, 1989). An average dive for the sperm whale would be 20 - 50 minutes long at a depth of 300 - 600 meters ( American Cetacean Society, 1997). They have been found to dive over an hour long and exceeding depths of 1 kilometer.

Sperm whales have three methods of feeding. Although these, again, are theorized due to the mystery of their dives. Because theses whales have a narrow, pointy, underslung jaw with no erupting upper teeth, it is thought that it is difficult for them to grasp food (Pennisi, 1999). It has been found that the muscular attachments of the tongue suggest that backward and forward movement is all that they are capable of doing (1999). This movement will allow for folding and twisting capabilities as well. This movement will cause a suction in the back of their throat, thereby "slurping" the prey inside (1999). The squid have been known to attach their suckers alongside the mouth of the sperm whale in the attempt of getting away. Perhaps they had the chance while being sucked into the whale’s mouth. Bottom scraping is another method of feeding. Massive amounts of sand, rocks, and bazaar debris have been found in the stomach contents of sperm whales (Ridgeway, 1972). They presumably scrape their jaw along the bottom floor of the ocean to gather food. The American Cetacean Society has also suggested a scaring tactic (1999). They will approach their prey and emit a large sound burst to stun the unsuspecting creature. After the sound, the sperm whale is left with an easy target meal.

Sea Otters, Enhydra lutis, have been listed as threatened under the Endangered Species Act (ESA) since 1977. They are easy to research and observe because they rest, feed, breed, and socialize on the surface. They are underwater only for a minute or so during feeding dives.

There are two types of sea otters: the Northern and the Southern. The Northern sea otter live off the coast of the Alaskan and Soviet shores, while the Southern sea otter live off the central coast of California. (http://www.aquarium.org/otter/seaotterfact.htm) They can dive up to a maximum of 328 ft (100 m), but only dive 6 – 128 ft while foraging for food, near shore and in shallow waters (Ridgeway, 1972).

Their diet includes octopus, clams, and oysters, which are high in calorie and protein. Where otter populations are high and mollusks and echinoderms are depleted, fish also become an important food source. They also eat the gonads of sea urchins, and the holdfast of kelp (which are not digested thoroughly, just passes through). They also at times eat birds when food depletion is advanced, this also is not thoroughly digested (Ridgeway, 1972). Other organisms they feed on include worms, crabs (Doroff, 19994), snails, sea stars, and squid.

Since sea otters live in waters that are 35 – 60 degree Fahrenheit and they have little blubber, they require a high metabolic rate and a high food intake to maintain their body temperature. Therefore, they must ingest 1/4 (23-33%) of their body weight per day.(VanBlancom, 1988.)

Sea otters do not acquire any of their food on land, it is all obtained in the water column. They use their front paws and vibrissae to find food, both during the day and at night. (Ralls, 1995) Much of the time they just dive for their food, but on some occasions they do threaten California sea lions by quickly stealing their fish and even steal food from other otters.

Sea otters have a variety of foraging tactics. They use at least 14 different types of tools, such as rocks, cement slabs, glass bottles, crab claws, shells, kelp, wood, and bricks to help them while foraging and/or eating. Here are a few foraging tactics that are used by sea otters: (http://www.calacademy.org/pacdis/issues/fall96/otter.htm)

1) An otter places a flat rock on its’ chest as an anvil and pounds mollusks against it until the tough outer shell breaks open. Underwater, it is used to bash off abalone stuck tightly to rocks. On the surface, it is used as a "table" when rested on its chest.

2) Their teeth are used for ripping open mussels, and to crack open urchins.

3) When two or more large crabs are captured, to keep the second crab from escaping or pinching while the first one is being eaten, the otter wraps kelp or surf grass tightly around the crab until it looks like a mummy. This is called "straightjacketing".

4) The "hind-flipper crab block" is used by imprisoning its’ catch by folding its hind flippers tightly over the crab.

5) The sea otter will rapidly spin an urchin with both paws before eating it – called "urchin rolling". It will also turn the urchin over and tap it on its bottom causing the urchin to flatten its spines. Both these methods make it easier for the otter to eat the urchin with little problem of getting poked by spines.

6) During a "sea star stick" the otter will fold the arms of a sea star down and inward so the sticky tube feet adhere to each other, then eat it without it sticking to it’s fur.

7) After finding a mollusk to eat, the otter will rotate the shell and at the same time push it with its powerful forelegs. This allows the clam to open up easily. "mollusk squishing"

8) The otter will "siphon strip" a clam to open a tightly clamped clam shell. Otters grab the siphon tube of the clam with its teeth and remove it with a quick jerk. This siphon tube is used by clams to suck in tiny food particles and push out waste.

9) "Spoons" are used from pieces of shells to scoop out the meat.

10) During the "snail shake" the otter shakes the kelp canopy allowing snails to fall off the kelp plant and into its paws. As they are doing this they will store snails in their pouches under their forelegs until there are ready to eat. The pouch can hold up to 25 small sea urchins or snails at one time (Ridgeway, 1972).

11) Sea otter have also learned to catch seabirds such as grebes, gulls, surf scoters, common loons, and cormorants by attacking them. They would sneak up on the bird from underwater, grab the bird by its feet, and fatally bite its neck or head.

All marine mammals have different techniques in which they forage. Some may do it within groups, while other prefer to forage individually. The similarities and differences among marine mammal foraging exist because of differences in habitat, diet, anatomy, and size.

 References

A Smorgasbord for Sea Otters >http://www.calcademy.org/pacdis/issues/fall96/otter.htm by Marianne Reidman

http://www.aquarium.org/otter/seaotterfact.htm

The Whale-Watching Web <http://www.physics.helsinki.fi/whale/> 1999. Acc: October 9,1999.

The American Cetacean Society <http://www.acsonline.org/> 1997. Acc: October 3, 1999.

Davis, R.W., Fuiman, L.A., Williams, T.M., Collier, S.O., Hagey, W.P., Kanatous, S.B., Kohin, S., Horning, M. 1999. Hunting behavior of a marine mammal beneath the Antarctic Fast Ice. Science 283: 993-995.

Doroff, A.M.; DeGange, A.R. 1994. Sea otter prey composition. Fishery Bull. 92: 704 – 710.

Marshall, G.J. 1998. Crittercam: An animal-borne imaging and data logging system. Marine Technology Society Journal 32: 11-17.

Pennisi, E. 1999. Rocky mountain rendezvous. Science. 283: 475-477.

Ralls,K.; Hatfield, B.B.; Siniff, D.B. 1995. Foraging patterns of california sea otters as indicated by telemetry. Canadian Journal of Zoology 73: 523 – 531.

Ridgeway, S. H. 1972. Mammals of the sea : biology and medicine. Thomas Books, Springfield. pp. 34-38 & 208-210.

Riedman, M. 1990. The Pinnipeds: Seals, Sea Lions, and Walruses. University of California Press, Berkley/ Los Angeles. pp.75-80, 140-144, 166-175.

Schreer, J.F. and Testa, J.W. 1996. Classification of weddell seal diving behavior. Marine Mammal Science 12: 227-250.

Suryan, R.M. and Harvey, J.T. 1998. Tracking Harbor Seals to determine dive behavior, foraging activity, and haul-out site use. Marine Mammal Science 14: 361-372.

VanBlancom, G.R.; Estes, J.A. 1988. The Community Ecology of Sea Otters: 157.

Whitehead, H. J. 1989. Diving behavior of the sperm whale. Canadian Journal of Zoology. 67: 2131 - 2139.

Return to Presentation Schedules page