Species and size differences in the digestion of otoliths and beaks: Implications for estimates of pinniped diet composition

D J Tollit, M J Steward, P M Thompson, Graham John Pierce, M B Santos, S Hughes

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173 Citations (Scopus)


We examined the digestion of hard remains of between one and four different size ranges of nine key North Sea prey taxa fed to seven captive harbour seals (Phoca vitulina). Percentage length reduction (mean 27.5%) and recovery rates (mean 42%) of experimental otoliths varied between species and were positively correlated to fish size and otolith robustness (mass/length). Mean length reduction of egested otoliths increased systematically with increasing size of ingested whiting and sandeel otoliths (p < 0.001), indicating that the size of larger fish may be underestimated. Intraspecific variation in otolith digestion was high (CV = 0.48-1.30), and to control for the artificial conditions of a captive study, external morphological features of otoliths were used to grade the degree of digestion and provide grade-specific correction factors. Bootstrap simulations were used to estimate 95% confidence intervals around correction factors and when partitioned indicated that calculation errors were in general less important than resampling errors. The application of species-, size-, and grade-specific correction factors progressively improved reconstructed estimates of prey biomass fed. As a consequence, estimates of prey size and diet composition require otoliths from faeces to be graded and more complex correction factors applied.

Original languageEnglish
Pages (from-to)105-119
Number of pages15
JournalCanadian Journal of Fisheries and Aquatic Sciences
Issue number1
Publication statusPublished - Jan 1997

Bibliographical note

We thank all the staff at the Sea Life Centre and Bruce Greig for their assistance. We thank Don Bowen, Heather Corpe, Simon Greenstreet, Kate Grellier, Ailsa Hall, Phil Hammond, John Hislop, Paul Racey, David Rosen, Phil Smith, Ben Wilson, and an anonymous reviewer for constructive criticism to 116 Can. J. Fish. Aquat. Sci. Vol. 54, 1997 © 1997 NRC Canada
earlier manuscripts. Thanks also to everyone at the Lighthouse Field Station for advice and encouragement. The majority of the work was carried out under a series of contracts from the
Scottish Office Agriculture and Fisheries Department to Professor P.A. Racey and P.M.T. G.J.P. was supported by a Commission of the European Community (CEC) contract (AIR 1 CT92 0573) and M.B.S. was supported by a CEC fellowship (ERB 4001 GT93 3630).


  • seals phoca-vitulina
  • feeding-habits
  • harbor seals
  • gastric evacuation
  • Moray-Firth
  • grey seals
  • gray seals
  • North Sea
  • Scotland
  • prey


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