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Fishery Bulletin 95(2), 1 997 
Table 3 
The list of calculated mean times and standard deviations for each species in the artificial digestion experiments when remaining 
weights reach 20% of the original weight (T 20 ). The digestion rate ratio shows the T 20 value for each species in relation to that of 
lantern fish. (L=large size and S=small size groups — see Table 1). 
Sample species 
n 
Time (h) to reach 
20% of original wt. (T 20 ) 
Digestion 
rate ratio 
Mean 
SD 
Maasbanker 
Trachurus t. capensis 
5 
21.35 
3.09 
3.76 
Cape anchovy (L) 1 
Engraulis capensis 
5 
15.67 
2.81 
2.76 
Pilchard (L) 
Sardinops sagax 
5 
14.82 
0.80 
2.61 
Round herring (SP 
Etrumeus whiteheadi 
5 
13.35 
3.33 
2.35 
Cape anchovy (S) J 
Engraulis capensis 
5 
12.85 
2.60 
2.26 
Pilchard (S) 
Sardinops sagax 
5 
12.57 
0.62 
2.21 
Chokka squid 
Loligo v. reynaudii 
5 
11.82 
1.04 
2.08 
Round herring (L) 2 
Etrumeus whiteheadi 
5 
11.54 
1.96 
2.03 
Hake 
Merluccius sp. 
5 
11.36 
0.98 
2.00 
Goby 
Sufflogobius bibarbatus 
5 
9.88 
0.39 
1.74 
Red squid 
Todaropsis eblanae 
5 
8.44 
0.70 
1.49 
Lanternfish 
Myctophidae 
8 
5.68 
0.66 
1.00 
1 T 20 for large and small anchovy = 14.26 ± 2.95 h. 
2 T 20 for large and small round herring = 12.45 ± 2.75 h. 
(f=5.02, df=8, P=0.001). Because there was no sig- 
nificant difference between the two size groups of 
anchovy and round herring, data were combined for 
one-way ANOVA on all sample species. 
The T., 0 values for 10 sample groups (maasbanker, 
large and small pilchard, anchovy, round herring, 
hake, goby, lanternfish, chokka squid, and red squid) 
showed a significant difference (one-way ANOVA, 
F= 27.3, total df=62, P<0.0001). The Newman-Keuls 
test indicated maasbanker, goby, red squid, and 
lanternfish had different T., 0 values from other spe- 
cies (P<0.05). 
Discussion 
Compared with previous digestion experiments, com- 
plete digestion of samples took longer than expected 
(Figs. 1 and 2). Bigg and Fawcett (1985) reported 
that whole herring and squid were digested within 10 
h in an artificial solution of 1% HC1 and 1% pepsin. 
Jackson et al. (1987) found that about 10-15 h were 
required to digest whole anchovies in vitro (pH=1.25- 
1.35). These time differences are probably the result of 
differences in acidity of the digestion solutions. In the 
present experiments, the solutions had a pH of ~2.3. 
The pH of the solution used by Bigg and Fawcett ( 1985) 
can be calculated as about 1.1. Therefore, their solu- 
tion was far more acidic than ours, resulting in more 
rapid digestion of fish and squid tissues. 
As noted, there was a general tendency for the di- 
gestion rate to decline when the remaining weight 
was less than 20% of the original weight. This was 
more pronounced for cephalopods than fish (Figs. 1 
and 2). Bigg and Fawcett (1985, Fig. 16.1) reported 
similar trends: declines in rates of digestion can be 
caused by the accumulation of less digestible mate- 
rial, i.e. squid beaks and pens (Table 2; also Table 
16.3 in Bigg and Fawcett, 1985). 
Although their procedure was different from that 
used in the present study, the digestion experiment 
ofNordpy et al. (1993) for herring (Clupea harengus ) 
also showed a rapid decline in digestion rate after 
about 70% of “dry matter disappearance” (DMD), and 
stated that the maximum DMD of herring is about 
80%. The digestion rate decline at 80% in the present 
study may also be related to the digestibility of prey 
species of dolphins, or cetaceans in general. Undi- 
gested prey remains may be voided via gastric evacu- 
ation or, possibly, by regurgitation, as proposed for 
squid beaks (Clarke, 1980; Pitcher, 1980). 
The validity of in vitro experiments in represent- 
ing in vivo situations remains a matter of debate, 
but technical and other considerations make in vivo 
digestion experiments with dolphins impractical at 
this stage. Although not engaging strictly in a diges- 
tion experiment, Kastelein et al. (1993) fed captive 
Commerson’s dolphins (Cephalorhynchus commer- 
sonii) on North Atlantic herring (Clupea harengus) 
and Columbia river smelt ( Thalechthys paci ficus), 
