A Review of Methods for Diet Analysis in Piscivorous Marine Mammals

Graham John Pierce, Peter Robin Boyle

Research output: Contribution to journalLiterature reviewpeer-review

325 Citations (Scopus)


The methodology for analysis of marine mammal diets may be thought of as a series of choices; about the sampling regime, the type of sample, the processing of samples, the identification of prey remains, the quantification of prey importances, and the estimation of population food consumption. In addition to directed sampling, samples may be obtained indirectly or opportunistically and the most frequently used types of sample are gut contents of dead animals, and faeces. Direct observation of feeding may be possible for species living close inshore, and humane approaches to sampling (e.g. lavaging) have been successfully applied to seals, but all methods are subject to bias. Identification of fish prey usually relies on recognition of otoliths, but additional information may be obtained from other hard remains or serological analysis of proteins. Remains of invertebrate prey may also be identified from hard parts. A variety of methods exist for quantifying the importance of prey in marine mammal diets. The most useful currency is probably prey biomass but, for fish, natural variation in otolith size-fish weight relationships makes it difficult to estimate fish weights from otolith size. Also, otoliths are reduced in size or lost during digestion. Notwithstanding these problems, otoliths are widely used. If the diet of a population has been adequately characterised, the amounts of each prey type consumed may be derived from direct or indirect estimates of energy requirements. Ideally the measurements should be taken in the field. Possible improvements in methodology include improved calibrations for otolith measurements, the development of keys and guides to fish bones, and of serological methods for identifying fish proteins. Feeding is normally inferred rather than directly monitored and a means for remote detection of prey ingestion and defaecation would greatly enhance the interpretation of foraging activity. To predict diet choice it is necessary to understand the underlying behavioural mechanisms. Measurement of the energetic costs of prey capture would be an important step in this direction.

Original languageEnglish
Pages (from-to)409-486
Number of pages78
JournalOceanography and Marine Biology: An Annual Review
Issue number29
Publication statusPublished - 1991


  • seals phoca-vitulina
  • otter lutra-lutra
  • Northern fur seals
  • Moray-Firth area
  • Donana-National-Park
  • Southern New-England
  • San-Miguel-Island
  • feeding habits
  • killer whales
  • orcinus-orca


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