Diet (marine mammals)

 

The ancestors of present-day marine mammals moved into water, possibly to escape competition on land for food resources, to escape predation, or to take advantage of relatively abundant food supplies in the seas. Most likely, it was a combination of factors. Eventually, the development of echolocatory capabilities in odontocetes (toothed whales) and physiological adaptations for deep and prolonged dives allowed for the exploration ol deep waters in search for food. Some groups of cetaceans, such as beaked whales, were able to evolve the ability to dive to great depths and take advantage of food resources unavailable to other predators. From a presumed terrestrial insectivore diet, marine mammals switched largely to fish, squid, and shrimp as main prev (in addition to other crustaceans. mollusks. and zooplankton organisms).

I. Methods of Study

A variety of different methods have been used to gain insight into what marine mammals eat. Their aquatic lifestyle usually limits direct observations of feeding, except in shallow waters in geographical areas where water visibility is good. The following methods have been used to draw inferences on marine mammal diet.

A. Direct Observations of Feeding

This method is limited to what can be observed above the surface, from a vessel, from a vantage point on land, or from the air. Although much can be learned from these observations.

Especially when made systematically in a particular area, subsurface leeding behavior is generally not observed, and the picture obtained on feeding is incomplete, at best. Prey that present aerial behavior (e.g.. mullet, flving fish) tend to be overrepresented, whereas bottom-dwelling species (e.g.. toad-fish. flatfish, octopus) are underrepresented.

B. Traditional Methods

The traditional method to study marine mammal food habits has been the analysis of food remains present in vomit or scat from living animals, and the stomachs and intestines of stranded animals. This method relies on the finding and identification of structures representing a typical meal. e.g.. fish bones and the jaws of cephalopods, often referred to as “beaks” due to their resemblance to beaks of parrots. Fish ear stones (or otoliths), in particular, are diagnostic structures in the identification of prey because their size and shape vary considerably from species to species (Fig. 1). Fish otoliths are calcareous structures (their primary composition is calcium carbonate) and are more resistant to digestion than bones. In life, they are housed in capsules inside the fish’s skull, and their main function in a fish is to provide information on balance and sound reception. Each species of fish has three pairs of otoliths (the sagitta. lapil-lus. and asteriscus). With a few exceptions (e.g.. catfish), the largest pair of otoliths (the sagitta) is also the one most distinctive and recognizable, and the one used in species identification. Similarly, cephalopod beaks possess morphological features that vary and can be used in the identification of species (Fig. 2). Although the upper beaks have some taxonomic value, the lower beaks are generally the ones used in prey identification. Beaks are composed of cliitin. a similar material to our fingernails and to mammal horns and are not dissolved bv digestive processes. Otolith and cephalopod beak lengths correlate well with the length and weight of the animal from which they came: relationships with their weights are described by a power curve. These allow reconstruction of the original meal bv weight. Estimates can then be made of consumption of particular prey species bv single mammals and, sometimes, their populations. The relationships relating otolith/beak length to animal length are best described by straight lines, which indicate the target lengths of prey species.

Fish ear stones (otoliths) of different sizes and shapes retrieved from stomachs of bottlenose dolphins stranded in Florida.

Figure 1 Fish ear stones (otoliths) of different sizes and shapes retrieved from stomachs of bottlenose dolphins stranded in Florida.

Cephalopod lower (left) and upper (right) jaws or "beaks" from a giant squid (Architeuthis).

Figure 2 Cephalopod lower (left) and upper (right) jaws or “beaks” from a giant squid (Architeuthis).

The advantages of this method are (1) knowledge of prey composition and size classes allows for understanding spatial and temporal distribution of predators; (2) studies of predator-prey dynamics are possible; (3) prey species may be very poorly sampled by humans using other methods, and diets can give considerable information on the species in an area available to predators; (4) changes in diet during growth and over time can be monitored; (5) analysis requires low cost or little equipment; and (6) samples can be collected from carcasses in an advanced stage of decomposition.

The disadvantages of this method are (1) prey with no hard parts (e.g., invertebrates) will be underrepresented; (2) different digestion rates of prey can make calculations of reconstructed meal sizes complicated (fish otoliths can last for about a day in the gastrointestinal tracts of marine mammals, whereas squid beaks may accumulate for several days); (3) there can be potential bias in using feeding data gathered from stranded (possibly sick) animals, as they may not be representative of the population at large; and (5) a comprehensive reference collection of hard structures (fish otoliths and cephalopod beaks) of the most common prey in a particular area is of great advantage in species identification, but is not always available.

C. Use of Novel Tools (New Technology) to Understand Feeding Ecology

1. Stable Isotopes The principle of this method is that ratios of heavier vs lighter isotopes of particular elements (carbon, nitrogen, oxygen, sulfur) in tissues of predators can be traced to those of their prey as they are assimilated through the diet. The following are advantages of this method: (a) it is ideal to detect shifts in diet; (b) different tissues of the predator yield information reflecting the feeding history relative to the last days, months, or the entire life of the animal; these data can be used to gain insights into the distribution, movements, and migratory habits of the animal; (c) isotopic ratios of carbon reflect those of the primary producers in the area; isotopic ratios of nitrogen are indicative of the trophic level occupied by the organism, which are helpful in understanding habitat utilization and trophic relationships; and (d) historical reconstruction of values through time can yield intra- and interannual variability in feeding. The disadvantages of this method are that a reference database for the isotopic signature of prey is needed and the cost of the equipment used in the analyses is high.

2. Fatty Acids The fatty composition of a prey is species specific and, as these compounds are assimilated through the diet and accumulated in fatty tissues of predators (e.g., blubber of marine mammals), they can be used as tracers of diet. Fatty acids analysis can be useful in (a) reconstructing diets in time and space; (b) population studies of various marine mammal species using different feeding grounds; (c) studies of energetic transfers between mother and their offsprings; and (d) the application of the technique to free-ranging animals, which can be done by the relative noninvasive collection of tissue (e.g., through biopsy darts). Similarly to stable isotopes, this method requires a reference database for the chemical signature of the various prey species, and the cost of the equipment is high. In addition, there is stratification of fat in the outer and inner blubber layer of marine mammals, and incomplete sampling of the blubber layer may yield misleading results of dietary information. Additional variability may be associated with what part of the body the sample is taken, making interstudy comparisons difficult.

3. Molecular Identification of Prey This method involves the genetic identification of material from scats, stomach contents, and gut bacteria, which must be separated prior to analysis. The disadvantages of this method are that a reference database for the genetic signature of prey is needed and it has yet to be widely applied to marine mammal dietary studies.

4. Video-Taping Studies (Using “Critter Cams”) of Animals Feeding at Depth This method has the following advantages: (a) it documents the actual feeding behavior of the predator, and the identity of the prey species can be verified by the images; (b) prey behavior during detection and capture can be documented; and (c) different feeding strategies can also be observed (e.g., cooperative feeding). Among the disadvantages: (a) only captures of a few species can be observed; (b) ambient light must be relatively bright and the water must be clear; (c) there is difficulty in the attachment and recovery of the equipment (video camera); and (d) there are high costs associated with the equipment and its operation.

In summary, although identifying and measuring items in vomit, scats, and stomach contents have many disadvantages, it provides more information at considerably less cost than other methods and cannot be replaced effectively by any other method at present.

II. Diets in General

Marine mammals, all together, eat a great variety of animals from minute crustaceans, less than 1 mm long, to giant squids, over 15 m in length. These live in a wide range of habitats, from the shallow shelf seas and estuaries to over 2000 m deep in the deepest oceans, from near the water surface to the ocean bottom. The animals consumed vary in texture from soft-skinned and gelatinous octopods to hard-scaled, muscular fish and vary in mobility from sedentary clams to jet-propelled squids. The three species of manatees (Trichechus spp.) and the dugong (Dugong dugon) of the order Sirenia rely on sea grasses and river plants; they are grazers in the true sense.

All this variety in food organisms has led to many specializations in structure. Most obviously, the mouth has developed a great variety of tooth numbers, sizes, and shapes or, for those eating very small organisms, a special filter made of horny plates, frayed on one edge, called “baleen.” Various species dive to greater depths, so permitting an extension of their feeding grounds from the continental shelves, down the continental slopes, to depths exceeding 2000 in. Similarly, thickening of their fatty blubber layer has permitted further extension of their feeding grounds into the cold waters of the Antarctic and Arctic. The diet of any species reflects its adaptations; its mouth adaptations make it possible for it to catch certain types and sizes of prey. The actual species in the diet depends on its own and the prey’s depth and geographic distribution.

Another property of the food is quality. Crustaceans, fish, and cephalopods vary in protein, fat, and mineral constituents and proportions. Marine mammal species can sometimes shift between these three groups during a year, as supply fluctuates or during their own migrations, but this may well drastically affect their physical condition and health. Even within one of these major groups the protein content, for instance, may vary greatly so that change to another species of prey as the main food item might affect the predator markedly. This is likely to be important when the mammal shifts from a diet of shellfish, which are very muscular, to deep-living oceanic fish, which are generally lower in protein content. Similarly, shelf-living cephalopods are mainly soft bodied, have weak muscles, and are low in protein so that twice as much has to be eaten. The high acidity and presence of ammonium salts in high concentration in some soft-bodied squids also probably require special adaptations of digestive processes.

The food of rarer marine mammals, and species that have not been caught for their oil, is not well known. Knowledge depends on information from occasional strandings, and stomach contents are often difficult to collect because of the size of the carcass. Debris from the stomachs only occasionally includes complete, readily identifiable, prey animals. Usually, information on diet has to be obtained from hard pieces, mainly fish otoliths and cephalopod beaks.

A. Cetaceans

1. Baleen Whales (Mysticeti) These possess baleen plates, and all but the gray whales collect swarming animals by skimming through the water or by gulping. They therefore primarily eat shoaling plankton or small nekton together with a few larger animals, such as fish and squids, caught with the shoals.

a. right whales (balaenidae and neobalaenidae)

The five species have very long baleen plates hanging from the roof of the mouth, whose finely frayed inner edges can trap very small plankton. They mainly eat small crustaceans ranging from minute copepods less than 1 mm long, favored by the bowhead whale (Balaena mysticetus) of the northern seas and the pygmy right whale (Caperea marginata), to small euphasiid crustaceans called “krill” as much as 25 mm long, eaten by the southern right whale (Eubalaena australis). The bowhead is also known to eat a small molluscan called a pteropod.

b. gray whale (ESCHRICHTWS ROBVSTVS) This species has very tough baleen plates that become worn, particularly at the right side, by rubbing on the sea floor from which it principally sucks, by piston action of its tongue, bottom amphipod crustaceans but also mollusks and bristle worms.

c. rorquals (balaenopteridae) The eight species have shorter baleen plates than right whales and generally favor larger prey than copepods. Blue whales (Balaenoptera muscu-lus) eat midwater crustaceans, mainly krill, in the Antarctic and other euphausiid species in the North Pacific and North Atlantic. The fin whale (B. physalus) eats krill in the Antarctic but in the North Pacific it broadens its diet to include fish such as clupeids, muscular squids, and a copepod. It eats the fish capelin in the North Atlantic. Humpback whales (Megaptera novaeangliae) eat mainly krill or “lobster krill” in the Southern Hemisphere, but mainly anchovies and cod in the Northern Hemisphere. Assorted squid are also eaten by humpback whales. The sei (B. borealis) whale eats 20 species of densely shoaling midwater crustaceans, including krill and copepods, in addition to anchovy, cod, and assorted oceanic squids. Minke whales (B. aaitorostrata and B. bonaerensis) eat assorted crustaceans in the Arctic and Antarctic seas, and also fish, including anchovy, in the North Pacific and herring in the North Atlantic. They also take assorted midwater squid in the south tropical seas and appear to rely more on fish than other baleen whales. Bryde’s whales (B. brydei and B. edeni) eat crustaceans, including krill, but also various fish, including mullet and anchovy in the Southern Hemisphere and anchovy in the North Pacific.

2. Toothed Whales (Odontoceti) Within this group, comprising seven families and 52 species, teeth are developed and the main prey items are fish and cephalopods. The cetacean species that live on the continental shelf eat muscular fish, such as herring, pilchards, whiting, and soles; muscular cephalopods, such as inshore squid, cuttlefish, and octopods, are occasionally taken as well. Odontocetes living in the deep ocean eat mainly lantern fishes and soft-bodied, often gelatinous, squids. Around oceanic islands, both lantern fishes and more muscular species, such as horse mackerel and trumpeter fish, are often taken.

a. dolphins (delphinidae) In 45% of the species in this family, cephalopods comprise over 75% and fish less than 25% of the diet. In 24% of the species, cephalopods comprise 50-75% and fish 25-50%. Depending on the species of dolphin, the diet can be muscular species living on the continental shelf, as for many populations of the short-beaked common dolphin (Delphinus delphis) and the common bottlenose dolphin (Tursiops truncatus); or as in spinner (Stenella longirostris) and spotted dolphins (S. attenuata and S. frontalis) the diet may include many soft-bodied oceanic species. However, on the whole, dolphins favor muscular squid rather than soft-bodied ones, even in oceanic waters; this also applies to pilot whales (Globicephala spp.). A few species include other prey groups in their diet, e.g., Commerson’s dolphins (Cephalorhynchus commersonii) eat some krill and killer whales (Orcinus orca) also prey on seals and other cetaceans.

b. porpoises (phocoenidae) In half of the species in tliis family, cephalopods comprise over 75% and fish less than 25% of the diet. In the other half of the species, cephalopods comprise 50-75% and fish 25-50%. Being inshore cetaceans, food consists of common (often economically important to humans) species: muscular inshore squid, cuttlefish, and octopus, as well as fish such as herrings, whitings, and bottom-living soles.

c. beaked whales (ziphiidae) In over half the species of beaked whales, more than half of the food is cephalopod and the rest is fish. These whales are deep divers and at least one species favors soft-bodied squids. The number of teeth is very reduced in this family and, in one species, the two teeth in the lower jaw grow over the upper jaw and limit it to a narrow gape. It is remarkable that this does not seem to inhibit capture of oceanic squids.

d. narwhal and beluga (monodontidae) The diet of the narwhal (Monodon monoceros) includes fish such as Greenland halibut and polar cod, muscular squid, and shrimp. The suggestion that the long tooth of male narwhals is regularly used to stir prey from the mud is unlikely to be true. Belugas (Delphinapterus leucas) feed on fish such as capelin and sand lance, as well as larger species such as cod and flounder. Sand-and bottom-living worms show that they probably feed on the bottom as well as in midwater.

e. sperm whales (physeteridae and kogiidae) All three species feed mainly on squids, although a few fish are taken, including large sharks. Sperm whales (Physeter macro-cephalus) eat mainly deep-living oceanic squids and most of these are soft-bodied or gelatinous, luminous, and weak swimmers. Contrary to common belief, the average weight of their prey is not great, varying from 0.5 kg off South Africa to 7 kg in the Antarctic, although some large sperm whales can eat squids over 15 m in length. Pygmy and dwarf sperm whales (Kogia breviceps and K. sima) eat some of the same species as their larger relative but, because they spend some time on continental shelves, they also include muscular, shelf-living squids and octopods.

f. river dolphins (iniidae, pontoporiidae, lipoti-dae, platanistidae) This group includes four species of dolphins, three of which inhabit the freshwater systems of major rivers in South America, China, and the Indian subcontinent. The fourth species has a marine distribution and is found in coastal waters of the Atlantic coast of South America. The riverine species feed on a variety of freshwater fish (including sharks) and prawns, and occasionally also prey on other groups, such as freshwater turtles. As cephalopods are strictly marine in distribution, they are not part of the diet of riverine dolphins. The marine species in this group (the “franciscana,” Pontoporia blainvillei) eats mainly bottom-dwelling fish, coastal species of cephalopods, and several species of shrimp.

B. Pinnipeds (Seals, Sea Lions, Walruses)

All but 2 of the 36 species of pinnipeds (seals and sea lions) probably include both fish and cephalopods in dieir diet. The exceptions inhabit freshwater systems where cephalopods do not occur. Most pinniped species inhabit coastal regions or seas close to oceanic islands, which partly influences their choice of diet.

1. Fur Seals and Sea Lions (Otariidae) Of die 16 species, 10 take benthic cephalopods and 11 eat midwater squids. At least 14 eat muscular cephalopods and at least 3 of these eat oily squids while only 1 eats soft-bodied squids. Three species consume all these on the continental shelf while 8 eat them in oceanic waters.

a. northern fur seal (callorhinvs ursinvs) On the continental shelf this species eats primarily small shoaling fish and muscular squids. In offshore waters, they eat mainly muscular oceanic squids.

b. guadalupe fur seal (arctocephalus townsendi)

They eat oceanic cephalopods and lantern fish.

c. juan Fernandez fur seal (a. philippii) They apparently eat muscular, oceanic squids.

d. galapagos fur seal (a. galapagoen si s) This species mainly eats muscular, oceanic squids.

e. cape and australian fur seals (a. pusillus)

They eat both shelf and oceanic fish and cephalopods, as well as both midwater and bottom species. They favor muscular rather than soft-bodied cephalopods.

f. new Zealand fur seal (a. forsteri) This species eats midwater fish and muscular cephalopods, but also takes penguins.

g. antarctic fur seal (a. gazella) They eat mainly krill in the Antarctic, but further north take oceanic fish, as well as muscular and soft-bodied squids.

h. sub-antarctic fur seal (a. tropicalis) They eat oceanic squids, both muscular and soft-bodied.

i. steller sea lion (eumetopias jubatus) They eat muscular, bottom octopods as well as oily oceanic squids and polar cod.

j. california, galapagos and japanese sea lions (zalophus californianus, z. wollebaeki, and z. japonic us) These species eat shelf-living, muscular squids and octopods, as well as muscular, oceanic squids.

k. south american sea lion (otaria flavescens) They eat mainly on bottom and midwater shelf fish but also take some shelf cephalopods.

l. australian sea lion (neophoca cinerea) This species eats shelf octopods and cuttlefish.

m. new zealand sea lion (phocarctos hookerl) This species eats shelf fish, cephalopods, and crustaceans.

2. Earless (true) Seals (Phocidae) This group eats a variety of fish, cephalopods, and crustaceans of both inshore and oceanic species, depending on their locality. Of the 19 species, at least 15 eat muscular cephalopods, 5 eat oily species and 4 eat soft-bodied squids. Some slight deviations from this pattern are given.

a. harp seal (pagophilus groen landicu s) This species eats mainly fish and crustaceans, especially amphipods and euphausiids, although both bottom and midwater oceanic cephalopods are also taken.

b. bearded seal (erignathus barbatus) They eat mainly bottom shelf invertebrates, such as clams, and also fish, in addition to a few species of octopods.

c. gray seal (halichoerus grypus) Gray seals eat schooling fish, squids, octopods, and occasionally sea birds.

d. crabeater seal (lobodon carcinophaga) They eat krill almost exclusively.

e. ross seal (ommatophoca Rossii) Ross seals eat oceanic species of fish and squids of both muscular and soft-bodied species.

f. leopard seal (hydrurga leptonyx) This species eats krill, fish, soft-bodied squids, and occasionally mammals.

g. weddell seal (leptonychotes weddellll) They eat mainly cephalopods, including muscular and soft-bodied species, and bottom octopods.

h. elephant seals (mirounga angustirostris and m. leonina) These species eat oceanic species, including muscular, soft-bodied, and oily species and, seasonally, shelf squids.

3. Walrus (Odobenus rosmarus) These are benthic feeders in shallow Arctic seas at depths less than 100 m. Their main food is clams but they also eat a small quantity of bottom octopods and have been known to attack other seals.

C. Sea Otter (Enhydra lutris)

These eat bottom invertebrates on the continental shelf, usually very close to shore, including clams, sea urchins, and other invertebrates.

D. Polar Bear (Ursus maritimus)

This species eats mainly harp seals but also feeds on other seals, young beluga and narwhal, young walrus or sick animals, and fish such as Arctic char. Polar bears also feed on terrestrial species of mammals and birds, on carcasses of bowhead and gray whales, and occasionally on humans. Polar bear males are known to kill and eat cubs of their own kind, possibly in order to incite the female to come into estrus again rapidly.

E. Sirenians

The manatees and dugong feed on tropical grasses and roots and rhizomes in nearshore areas in saline environments and on water hyacinths, water lilies, and other vegetation in rivers and lakes. The extinct Stellers sea cow (Hydrodamalis gigas) was a cold-adapted species, last found off the Kamchatcka Peninsula in far east Russia; it fed on cold water kelp.

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