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Chapter six: Diet and nutrition
17 months or more to rear, diet opportunities are limited to regions very close to breeding
sites, typically those areas mothers can reach in a few days. The limited foraging range
of central place foragers restricts the diversity of foraging opportunities, and can possibly lead to local prey depletion around larger breeding colonies, ultimately limiting the
population size. For central place foragers operating in temperate coastal waters, this can
be particularly important because shallow, inshore waters can be relatively depauperate in
prey compared to the open ocean, and this has been linked to the generally small population size and slow population growth rates of sea lions compared to fur seals (Arnould
and Costa 2006). Outside the breeding season, individuals are less constrained and range
widely allowing for a greater diversity of diets.
The pronounced sexual dimorphism (i.e., difference in body size between males and
females within a species) of many species of land breeding marine mammals can also
impose very different dietary requirements between males and females. Disparate body
size among genders is typically related to mating strategies. Polygamous species, where
males compete for access to females provide strong selection pressure for large males.
Monogamous species, where there is less direct competition for mates, tend to have equivalent sized males and females. Pronounced differences in body size can have significant
implications for diet and nutrition. The larger gender has greater energy requirements,
in terms of both maintenance and energy stores for breeding, as well as d ifferent oxygen
needs and different body forms. All of these factors can lead to size and gender-based
differences in diet and foraging locations. A good example is southern elephant seals
(Mirounga leonina) where males make more extensive use of continental shelf habitats than
females and also feed on large benthic prey. Females tend to use oceanic waters and to feed
on smaller pelagic fish and squid.
6.3 Feeding mechanisms
6.3.1 Filter feeding
Filter feeding, used by all the mysticete whales and two species of pinnipeds (crabeater
seals, Lobodon carcinophagus; and leopard seals), is a foraging strategy that allows individuals to capture and process large quantities of very small prey in a single mouthful.
All filter-feeding species need to feed on prey which occurs in dense aggregations. Two
feeding adaptations have evolved to allow the exploitation of these prey: baleen (mysticete
whales) and modified dentition (seals).
Filter feeding arose in both cetaceans and pinnipeds in response to the unique
patterns of productivity and prey availability in marine ecosystems, particularly in
the high latitudes during the spring and summer. Filter-feeding marine mammals
primarily concentrate their foraging in polar and highly productive coastal upwelling regions. Prior to their exploitation by humans, the highest densities of mysticetes
occurred in highly productive Southern Ocean waters. Crabeater seals, Antarctic fur
seals (Arctocephalus gazella), and leopard seals are also found primarily in the Southern
Ocean where seasonally dense aggregations of Antarctic krill (Euphausia superba) develop
(Berta and Sumich 1999).
Studies of the diving behavior and daily movement patterns of right whales
(Eubalaena spp.) have shown that they exploit dense aggregations of copepods that aggregate at oceanographic features such as fronts. Rorquals also follow seasonal and diel
patterns in the abundance and behavior of their prey. In general, the distribution and
movement patterns of most rorquals consist of a seasonal migration from high latitudes
Chapter six: Diet and nutrition
17 months or more to rear, diet opportunities are limited to regions very close to breeding
sites, typically those areas mothers can reach in a few days. The limited foraging range
of central place foragers restricts the diversity of foraging opportunities, and can possibly lead to local prey depletion around larger breeding colonies, ultimately limiting the
population size. For central place foragers operating in temperate coastal waters, this can
be particularly important because shallow, inshore waters can be relatively depauperate in
prey compared to the open ocean, and this has been linked to the generally small population size and slow population growth rates of sea lions compared to fur seals (Arnould
and Costa 2006). Outside the breeding season, individuals are less constrained and range
widely allowing for a greater diversity of diets.
The pronounced sexual dimorphism (i.e., difference in body size between males and
females within a species) of many species of land breeding marine mammals can also
impose very different dietary requirements between males and females. Disparate body
size among genders is typically related to mating strategies. Polygamous species, where
males compete for access to females provide strong selection pressure for large males.
Monogamous species, where there is less direct competition for mates, tend to have equivalent sized males and females. Pronounced differences in body size can have significant
implications for diet and nutrition. The larger gender has greater energy requirements,
in terms of both maintenance and energy stores for breeding, as well as d ifferent oxygen
needs and different body forms. All of these factors can lead to size and gender-based
differences in diet and foraging locations. A good example is southern elephant seals
(Mirounga leonina) where males make more extensive use of continental shelf habitats than
females and also feed on large benthic prey. Females tend to use oceanic waters and to feed
on smaller pelagic fish and squid.
6.3 Feeding mechanisms
6.3.1 Filter feeding
Filter feeding, used by all the mysticete whales and two species of pinnipeds (crabeater
seals, Lobodon carcinophagus; and leopard seals), is a foraging strategy that allows individuals to capture and process large quantities of very small prey in a single mouthful.
All filter-feeding species need to feed on prey which occurs in dense aggregations. Two
feeding adaptations have evolved to allow the exploitation of these prey: baleen (mysticete
whales) and modified dentition (seals).
Filter feeding arose in both cetaceans and pinnipeds in response to the unique
patterns of productivity and prey availability in marine ecosystems, particularly in
the high latitudes during the spring and summer. Filter-feeding marine mammals
primarily concentrate their foraging in polar and highly productive coastal upwelling regions. Prior to their exploitation by humans, the highest densities of mysticetes
occurred in highly productive Southern Ocean waters. Crabeater seals, Antarctic fur
seals (Arctocephalus gazella), and leopard seals are also found primarily in the Southern
Ocean where seasonally dense aggregations of Antarctic krill (Euphausia superba) develop
(Berta and Sumich 1999).
Studies of the diving behavior and daily movement patterns of right whales
(Eubalaena spp.) have shown that they exploit dense aggregations of copepods that aggregate at oceanographic features such as fronts. Rorquals also follow seasonal and diel
patterns in the abundance and behavior of their prey. In general, the distribution and
movement patterns of most rorquals consist of a seasonal migration from high latitudes