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Chapter six: Diet and nutrition
Similarly, fish otoliths (ear stones) primarily composed of calcium carbonate are more
resistant to digestion than bones.
These techniques provide detailed taxonomic data (Slip 1995) but are biased toward
the most recent prey intake at the end of foraging trips, and hard part structures (or remnants of) prey that can be visually identified in stomach contents and feces (Staniland
2002). Soft-bodied species that have no hard parts are largely undetectable after digestion
leading to complete under-representation of these groups in the diet. In the case of fecal
studies, prey parts that do not survive digestion (or digest or erode at different rates) or
whose hard parts are not consumed, are often missed. Controlled feeding trials of captive
seals have revealed up to a 10-fold disparity with what has been recovered through fecal
analysis (Tollit et al. 2003), leading to inappropriate conclusions about niche breadth utilization. Moreover, for a large proportion of cetacean species involved in single-stranding
events (e.g., Gales et al. 1992), the cause of death is unknown but may be associated with
old age, disease, or injury (Evans and Hindell 2004). In such instances, hard part identification from these animals may not be indicative of that of the larger population. However,
dietary assessments of mass-stranded individuals are less biased by old, sick, or injured
individuals and thus, more likely to be more representative of the larger, healthy population (Evans and Hindell 2004).
Due to limitations of conventional hard part diet analyses, researchers have
increasingly turned to using biochemical markers contained in the tissues of predators to make inferences about what they have been eating. There are three types of
markers, stable isotopes, fatty acids, and DNA, each of which has their own strengths
and weaknesses. Reliable insights are most likely to be achieved by using a combination of techniques.
6.4.2 Novel, new techniques to investigate the
feeding ecology of marine mammals
6.4.2.1 Stable isotope ratios
Over the past 20 years, stable isotope ratios of carbon ( 13 C/ 12 C; δ 13 C) and nitrogen ( 15 N/ 14 N;
δ 15 N) have been increasingly used to study variation in resource and habitat use of elusive or highly migratory animals, such as marine top predators (Newsome et al. 2012).
Their use in dietary studies is based on the fact that stable isotope ratios in the proteins
of consumers reflect those of the proteins in their diet in a predictable manner (Hobson
and Clark 1992). Although coarse in taxonomic resolution, the use of naturally occurring
ratios of stable isotopes in animal tissues can be a powerful alternative method of dietary
analysis. Stable isotope ratios can yield a data time-series from assimilated and not just
ingested food (Tieszen et al. 1983).
Stable isotope ratios of carbon and nitrogen are the main elements used in dietary
analyses (see reviews in Peterson and Fry 1987; Gannes et al. 1998; Kelly 2000; Dalerum
and Angerbjörn 2005). This is because δ 15 N exhibits a stepwise and predictable increase
with trophic transfers. Consequently, the δ 15 N values in the tissues of consumers tend
to be relatively high compared to those of their diets and can, therefore, be used to
estimate the diet and trophic position of consumers in a food web (McCutchan et al.
2003; Vanderklift and Ponsard 2003). Stable carbon isotope ratios also increase per trophic transfer but to a much lesser degree than δ 15 N. In the marine environment, carbon isotopes are mainly used to indicate the foraging habitats of predators (Kelly 2000;
McCutchan et al. 2003).
Chapter six: Diet and nutrition
Similarly, fish otoliths (ear stones) primarily composed of calcium carbonate are more
resistant to digestion than bones.
These techniques provide detailed taxonomic data (Slip 1995) but are biased toward
the most recent prey intake at the end of foraging trips, and hard part structures (or remnants of) prey that can be visually identified in stomach contents and feces (Staniland
2002). Soft-bodied species that have no hard parts are largely undetectable after digestion
leading to complete under-representation of these groups in the diet. In the case of fecal
studies, prey parts that do not survive digestion (or digest or erode at different rates) or
whose hard parts are not consumed, are often missed. Controlled feeding trials of captive
seals have revealed up to a 10-fold disparity with what has been recovered through fecal
analysis (Tollit et al. 2003), leading to inappropriate conclusions about niche breadth utilization. Moreover, for a large proportion of cetacean species involved in single-stranding
events (e.g., Gales et al. 1992), the cause of death is unknown but may be associated with
old age, disease, or injury (Evans and Hindell 2004). In such instances, hard part identification from these animals may not be indicative of that of the larger population. However,
dietary assessments of mass-stranded individuals are less biased by old, sick, or injured
individuals and thus, more likely to be more representative of the larger, healthy population (Evans and Hindell 2004).
Due to limitations of conventional hard part diet analyses, researchers have
increasingly turned to using biochemical markers contained in the tissues of predators to make inferences about what they have been eating. There are three types of
markers, stable isotopes, fatty acids, and DNA, each of which has their own strengths
and weaknesses. Reliable insights are most likely to be achieved by using a combination of techniques.
6.4.2 Novel, new techniques to investigate the
feeding ecology of marine mammals
6.4.2.1 Stable isotope ratios
Over the past 20 years, stable isotope ratios of carbon ( 13 C/ 12 C; δ 13 C) and nitrogen ( 15 N/ 14 N;
δ 15 N) have been increasingly used to study variation in resource and habitat use of elusive or highly migratory animals, such as marine top predators (Newsome et al. 2012).
Their use in dietary studies is based on the fact that stable isotope ratios in the proteins
of consumers reflect those of the proteins in their diet in a predictable manner (Hobson
and Clark 1992). Although coarse in taxonomic resolution, the use of naturally occurring
ratios of stable isotopes in animal tissues can be a powerful alternative method of dietary
analysis. Stable isotope ratios can yield a data time-series from assimilated and not just
ingested food (Tieszen et al. 1983).
Stable isotope ratios of carbon and nitrogen are the main elements used in dietary
analyses (see reviews in Peterson and Fry 1987; Gannes et al. 1998; Kelly 2000; Dalerum
and Angerbjörn 2005). This is because δ 15 N exhibits a stepwise and predictable increase
with trophic transfers. Consequently, the δ 15 N values in the tissues of consumers tend
to be relatively high compared to those of their diets and can, therefore, be used to
estimate the diet and trophic position of consumers in a food web (McCutchan et al.
2003; Vanderklift and Ponsard 2003). Stable carbon isotope ratios also increase per trophic transfer but to a much lesser degree than δ 15 N. In the marine environment, carbon isotopes are mainly used to indicate the foraging habitats of predators (Kelly 2000;
McCutchan et al. 2003).