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How do we determine what marine mammals eat? 

Some marine mammals, like humpback whales, feed at certain locations during the summer, so it is possible to observe them feeding.  It is usually difficult to observe most marine mammals feeding, because they feed underwater.  However, there are other ways to find out what marine mammals eat.   Some ways to find out what marine mammals eat are:

(1)  Observe marine mammals while they are feeding at locations that they return to each year.  In some cases, researchers have been able to attach small cameras to marine mammals to observe underwater behavior.  

(2)  Collect the animal's "scat" (droppings).  The scat usually contains bones and hard parts from the food it has been eating (fish, squid, seals), and can be washed so that these remains can be analyzed.  Obviously, it is easier to collect scats from pinnipeds on land than from cetaceans in water. 

(3)  Collect the animal's stomach contents when it vomits.  This can occur naturally (some pinnipeds throw up bones and squid beaks).

(4)  Dissect dead marine mammals and look at the contents of their stomachs.  This is usually done if a marine mammal drowns in a net or is found dead on a beach. 

m-pd-13 dallsporp dissectionmd.jpg (11061 bytes)

A scientist is dissecting a stranded, already dead Dall's porpoise.

(5)  Examine the diving behavior of the marine mammal; the depth to which the marine mammal dives can give clues to the prey that it is feeding on.

(6)  Observe marine mammals while they are hunting and eating. 

m-pp-5 harborporpsurveymd.jpg (9702 bytes)

Harbor porpoise observer.

Why study food habits?

By determining the kinds of food that marine mammals eat, we can gain insight into the role that these animals play in the marine ecosystem.  We can determine whether marine mammal species directly compete with commercial or sports fisheries, and this information helps in managing fisheries.  Food habit information can also help us to understand population trends of marine mammals--for example, if survival of young animals is low, and young animals eat different food than adults, it may indicate that the food for young animals is not as abundant as that of adults. 

What do marine mammals eat? 

Marine mammals feed on a variety of other species:  some eat fish, some eat squid, some eat shellfish, some even eat other marine mammals.  Many marine mammal species are specialized for the food that they eat.  For example, baleen whales use large baleen plates to sieve small food out of the water.  Crabeater seals use complex teeth to sift small krill out of the water.  Sea otters have teeth with a flat, broad surface area for crushing sea urchins and shellfish.   Manatees have teeth that continually grow and move forward, because they feed entirely on aquatic vegetation and need to continually replace worn teeth.  To read more about what different marine mammal species eat, check out our section  about marine mammals

What are the advantages of knowing what marine mammals eat?

By determining what food marine mammals consume, we can gain insight into the role that these animals play in marine ecosystems. We can also find out if different age groups feed on different types of food.  A current study focus has been concerned with how marine mammals affect commercial fisheries and whether there is direct or indirect competition between fisheries and marine mammals.  Most recently the consumption rates by marine mammals are being considered as examples of sustainable consumption rates.  (Understanding mammal consumption rates allows humans to understand how much they can consume while leaving the ecosystem healthy). 

What are the difficulties in determining marine mammal food consumption?

There are many difficulties in determining marine mammal food consumption as direct observation of underwater feeding pinnipeds and cetaceans is difficult at best.   For pinnipeds, the scat can be collected although bones in scats deposited by pinnipeds on land may not reflect what they are eating while at sea. Some bones are represented more in scats, because they pass through the gut faster than other kinds of bones. Proportions of bones in the scat does not necessarily indicate the proportions of different kinds of prey that are eaten. Any ideas on how to collect the scat of a whale?!?!

Would you like to learn more?

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Information within the education website should not be cited in scientific journals or publications.

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