NOTE Sekiguchi and Best: In vitro digestibility of some prey species of dolphin 
391 
into which gelatine capsules containing red dye were 
inserted. They found that only 40 to 155 minutes 
elapsed before dye appeared in feces, but it is not 
clear how this relates to the full digestion times of 
the fish. In vivo experiments with pinnipeds (another 
marine mammal feeding largely on cephalopods and 
fish) suggest somewhat faster digestion rates than 
those in our study. Murie and Lavigne (1985) found 
no fish hard parts remaining in seal stomachs 18 
hours after feeding. However, stomachs could have 
been voided by regurgitation and gastric evacuation, 
whereas “hard parts” in our experiments could es- 
cape from the digestion bags only if they were re- 
duced to less than mesh size. Thus, their results are 
not necessarily inconsistent with those of the present 
study, although mechanical break-down actions of 
stomachs are likely to produce faster digestion in vivo. 
The in vitro digestion speeds recorded in the 
present study differed between species ( Table 3), but 
there was no consistent correlation with the taxo- 
nomic position of the prey. Three fish species in the 
order Clupeiformes (round herring, pilchard, and 
anchovy) had digestion-rate ratios in the range 2.03- 
2.76, although large and small size groups of pilchard 
had significantly different T., 0 values. However, 
maasbanker and goby, both in the order Perciformes, 
showed very different digestion-rate ratios (3.76 and 
1.74). While both squid species were digested faster 
than most fish species, chokka squid was digested 
more slowly than large round herring, hake, goby, 
and lanternfish. Bigg and Fawcett (1985) found that 
the squid Loligo opalescens was digested much faster 
than herring (Clupea harengus pallasi), both in vitro 
and in vivo (i.e. in a seal stomach). On the other hand, 
Jackson et al. (1987) found no difference in the di- 
gestion rate between fish (hake and anchovy) and 
squid ( Loligo ) in vitro. LeBrasseur and Stephens 
( 1965) reported that fish (salmonids, myctophids, and 
hexagrammids) were digested faster than squid 
(gonatids) in their pepsin-hydrochloric acid solution 
(0.2 g pepsin/1 L, 1.5% HC1, pH 1.8). These in vitro 
differences quite possibly are the result of variations 
in the acidity of the solutions used and differences 
in experimental procedures. 
It is possible that digestion rates are related to 
muscle structure. Because pepsin is an enzyme that 
dissolves protein, the protein composition of a body 
will have an effect on digestion rate. Greer-Walker 
and Pull (1975) found that active pelagic fish had 
higher proportions of red muscle than coastal or deep- 
sea fish species. They reported that the mean red 
muscle proportion was 19.8% for Clupeidae, 18.3% 
for Carangidae, 4.5% for Gobiidae, and 4.5% and 0.6% 
for the deep-sea fish families Macrouridae and 
Chimaeridae, respectively. The digestion rates of fish 
prey found in the present study (Table 3) appear to 
fit a pattern in which the prey species digested most 
slowly tend to have the highest proportions of red 
muscle. Red muscle, containing greater quantities 
of mitochondria, myoglobin, fats, and glycogen than 
white muscle, may have stronger resistance to pep- 
sin in the digestion process. 
Fish otoliths recovered in the present study were 
reduced in size, and most hake and maasbanker 
otoliths completely dissolved within 8-12 h after ex- 
posure. McMahon and Tash (1979) reported that 
otoliths in a 0.01 N HC1 solution (pH=2.0-2.5) at 25°C 
were dissolved completely in 24 h, and a herring 
otolith in a pH 1.09 to 3.09 solution disappeared in 7 
h (Jobling and Breiby, 1986). However, the erosion 
rate of otoliths of different species in acid varies 
(Jobling and Breiby, 1986), possibly depending on the 
ratio of surface area to volume (da Silva and Neilson, 
1985). On the other hand, using otoliths recovered 
from fecal samples of captive harbor seals (Phoca 
uitulina), Harvey (1989) found no significant rela- 
tion between the robustness (length/weight) of the 
otolith and the degree to which the resultant esti- 
mate of fish length was reduced. In seal stomachs, 
all otoliths were released from herring skulls within 
6 h and no otoliths were found 12 h after feeding 
(Murie and Lavigne, 1986; Murie, 1987). In the 
present experiments, only fragile, somewhat eroded 
otoliths were recovered after about 20 h of digestion 
in vitro. Consequently, it would be likely that any 
intact otoliths that are found in dolphin stomachs 
are from recently ingested fish. 
Walker et al. (1986) reported the recovery of an- 
chovy ( Engraulis mordax) otoliths from the stomach 
of a Pacific white-sided dolphin ( Lagenorhynchus 
obliquidens ) that had been held in captivity for 8 days 
without being fed anchovy; this finding suggested the 
possibility that otoliths can be retained over a pe- 
riod of one week. In the present experiments, a total 
of 16 anchovy otoliths (80%) were recovered after 20 
h; these otoliths were too eroded, however, to esti- 
mate original sizes. Because the forestomach of a 
dolphin contains no glands, gastric juice must be re- 
fluxed from the main stomach (Harrison et al., 1970), 
so that the retention of otoliths for as long as 8 days 
should be viewed as exceptional. 
The digestion sequences were similar for all ex- 
perimental species (Table 2). Because otoliths are 
located inside a fish skull, their size reduction de- 
pends on when they are initially exposed to stomach 
acids. In most cases, heads of fish had disintegrated 
when about 40-60% of the body had been digested 
(Table 2), usually some 4 to 15 h after digestion be- 
gan (Figs. 1 and 2), when most otoliths were prob- 
ably exposed to the acids and began to erode. Harvey 
