130
Marine Mammal Physiology: Requisites for Ocean Living
Stable nitrogen and carbon isotope sources (e.g., δ 15 N and δ 13 C) at the base of food chains
vary spatially which is reflected in spatial variability in isotopic composition among food
webs (Bearhop et al. 2004). Spatial variability in δ 13 C can discriminate between inshore and
offshore feeding at a range of spatial scales from oceanic (marine) habitats (Hobson et al.
1994) to lake (freshwater) food webs (Vander Zanden and Rasmussen 1999). Stable carbon values can also differentiate between pelagic and benthic contribution to food intake
(Hobson et al. 1994; Cherel et al. 2011). Inshore/offshore and pelagic/benthic δ 13 C gradients
have been used as an effective way to investigate the habitats of coastal, neritic, and oceanic species of Antarctic fish, with inshore/benthic species having higher δ 13 C values than
offshore/pelagic species (Cherel et al. 2011).
Stable isotope composition turnover rates vary among tissues, with high rates in metabolically active tissues such as blood plasma and liver, somewhat lower in muscle, and
lowest in long-lived tissue such as bone (Tieszen et al. 1983). Keratinous structures and
whole blood are particularly suitable for studying temporal variation in diet. Since keratin is a highly stable structural protein, the δ 13 C and δ 15 N composition of keratin-based
tissues, such as whiskers, remains unchanged after the completion of growth. Whiskers
and other keratinous tissues, therefore, provide a temporal record of feeding dating back
several months to years (Kernaléguen et al. 2012; Walters et al. 2014). By comparing the
isotope ratios along the length of the whisker with those of putative prey items, changes
in food sources and habitat can be surmised for the temporal span represented by the
growth of the whisker. Whole blood, on the other hand, provides short- to medium-term
dietary signals and can be used to examine diet in discrete temporal windows, including
periods outside the limited sampling seasons of traditional dietary methods (Chaigne
et al. 2013).
Interpretation of isotopic data from animals that move between areas of differing isotopic compositions (e.g., Schell et al. 1989) can be complicated (Hobson and Welch 1992),
requiring knowledge of a migratory animal’s breeding, wintering and stop over sites.
Satellite and archival derived tracking information is increasingly used to provide this
important contextual information. For example, analysis of stable isotope ratios in the
whiskers of sub-yearling southern elephant seals in conjunction with satellite telemetry
and environmental data has been successfully used to examine habitat use and diet during their first foraging migration (Walters et al. 2014; Box 6.1).
6.4.2.2 Fatty acids
Fatty acid signature analysis (FASA) (Iverson 1993) has emerged over the last few decades
as another useful tool to investigate diet in marine mammals. As with other biochemical
techniques used to reconstruct diet, FASA techniques do not rely on the recovery of prey
hard parts and integrate the diet over ecologically significant periods of time (Newland
et al. 2009). FASA techniques have been used qualitatively to infer trophic levels and spatial and temporal differences in diets both within and among species (Iverson et al. 1997;
Smith et al. 1997; Beck et al. 2007). Unlike other nutrients, such as proteins that are readily
broken down during digestion, fatty acids are released from ingested lipid molecules (e.g.,
triacylglycerols) during digestion but are not degraded (Iverson et al. 2004). Dietary fatty
acids remain largely intact and those of carbon chain length >14 can be deposited in animal
tissue with little or no modification (Smith et al. 1997). Owing to various restrictions and
specifications in the biosynthesis and modification of fatty acids, only a relatively limited
number of fatty acids can be biosynthesized by animals (Ackman 1980). This makes it possible to distinguish dietary and non-dietary components present in animal tissues, particularly in lipid-rich tissues such as blubber and milk (Iverson et al. 2004). Dietary fatty acid
Marine Mammal Physiology: Requisites for Ocean Living
Stable nitrogen and carbon isotope sources (e.g., δ 15 N and δ 13 C) at the base of food chains
vary spatially which is reflected in spatial variability in isotopic composition among food
webs (Bearhop et al. 2004). Spatial variability in δ 13 C can discriminate between inshore and
offshore feeding at a range of spatial scales from oceanic (marine) habitats (Hobson et al.
1994) to lake (freshwater) food webs (Vander Zanden and Rasmussen 1999). Stable carbon values can also differentiate between pelagic and benthic contribution to food intake
(Hobson et al. 1994; Cherel et al. 2011). Inshore/offshore and pelagic/benthic δ 13 C gradients
have been used as an effective way to investigate the habitats of coastal, neritic, and oceanic species of Antarctic fish, with inshore/benthic species having higher δ 13 C values than
offshore/pelagic species (Cherel et al. 2011).
Stable isotope composition turnover rates vary among tissues, with high rates in metabolically active tissues such as blood plasma and liver, somewhat lower in muscle, and
lowest in long-lived tissue such as bone (Tieszen et al. 1983). Keratinous structures and
whole blood are particularly suitable for studying temporal variation in diet. Since keratin is a highly stable structural protein, the δ 13 C and δ 15 N composition of keratin-based
tissues, such as whiskers, remains unchanged after the completion of growth. Whiskers
and other keratinous tissues, therefore, provide a temporal record of feeding dating back
several months to years (Kernaléguen et al. 2012; Walters et al. 2014). By comparing the
isotope ratios along the length of the whisker with those of putative prey items, changes
in food sources and habitat can be surmised for the temporal span represented by the
growth of the whisker. Whole blood, on the other hand, provides short- to medium-term
dietary signals and can be used to examine diet in discrete temporal windows, including
periods outside the limited sampling seasons of traditional dietary methods (Chaigne
et al. 2013).
Interpretation of isotopic data from animals that move between areas of differing isotopic compositions (e.g., Schell et al. 1989) can be complicated (Hobson and Welch 1992),
requiring knowledge of a migratory animal’s breeding, wintering and stop over sites.
Satellite and archival derived tracking information is increasingly used to provide this
important contextual information. For example, analysis of stable isotope ratios in the
whiskers of sub-yearling southern elephant seals in conjunction with satellite telemetry
and environmental data has been successfully used to examine habitat use and diet during their first foraging migration (Walters et al. 2014; Box 6.1).
6.4.2.2 Fatty acids
Fatty acid signature analysis (FASA) (Iverson 1993) has emerged over the last few decades
as another useful tool to investigate diet in marine mammals. As with other biochemical
techniques used to reconstruct diet, FASA techniques do not rely on the recovery of prey
hard parts and integrate the diet over ecologically significant periods of time (Newland
et al. 2009). FASA techniques have been used qualitatively to infer trophic levels and spatial and temporal differences in diets both within and among species (Iverson et al. 1997;
Smith et al. 1997; Beck et al. 2007). Unlike other nutrients, such as proteins that are readily
broken down during digestion, fatty acids are released from ingested lipid molecules (e.g.,
triacylglycerols) during digestion but are not degraded (Iverson et al. 2004). Dietary fatty
acids remain largely intact and those of carbon chain length >14 can be deposited in animal
tissue with little or no modification (Smith et al. 1997). Owing to various restrictions and
specifications in the biosynthesis and modification of fatty acids, only a relatively limited
number of fatty acids can be biosynthesized by animals (Ackman 1980). This makes it possible to distinguish dietary and non-dietary components present in animal tissues, particularly in lipid-rich tissues such as blubber and milk (Iverson et al. 2004). Dietary fatty acid