392 
Fishery Bulletin 95(2), 1 997 
( 1989) found that lengths of prey estimated from the 
sizes of otoliths in seal feces were underestimated 
by an average of 27.5%. Although the erosion rate of 
otoliths may be different for each species (Jobling 
and Breiby, 1986), it should be possible to apply cor- 
rection factors to avoid underestimating fish size. The 
stage of digestion of fish prey in a stomach, for in- 
stance, could be used as an index to suggest how 
much time has passed since feeding. 
A significant difference in T 20 values for different 
size groups of a particular prey species was only 
found in pilchard. Smaller anchovy were digested 
about 1.2 times faster than larger ones. On the other 
hand, larger round herring were digested about 1.2 
times faster than smaller ones (Table 3). These dif- 
ferences were not significant, however, although 
there was more variation among samples for anchovy 
and round herring than for pilchard (Fig. 2). Larger 
sample sizes may be required to test for differences 
in digestion rates between different-size individuals 
of a prey species. 
Although it has not been possible to calibrate these 
in vitro experiments with in vivo information, this 
paper indicates interspecific differences in relative 
digestion rates for several prey items taken by dol- 
phins. It should, therefore, be possible to apply “cor- 
rection factors” to estimate the original amount of par- 
ticular prey consumed when prey of different digest- 
ibility occur together in a stomach. However, the wider 
application of such a method would require the exami- 
nation of digestion rates for additional prey species. 
Acknowledgments 
We acknowledge W. R. Siegfried for allowing us to 
use the laboratory at the Percy FitzPatrick Institute, 
University of Cape Town as well as S. Jackson and 
N. J. Adams who provided us with assistance with 
laboratory equipment. We also thank K. Findlay for 
his assistance during experiments. M. Hiroki, Tokyo 
University of Agricultural Technology, assisted KS 
with the statistical analysis. Useful comments on this 
manuscript were received from T. Jefferson, anony- 
mous reviewers, and the scientific editor. This study 
was supported by the Benguela Ecology Programme. 
PBB was supported by the Foundation for Research 
Development and the South African Marine Corpo- 
ration through WWF-South Africa. 
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