454 
Fishery Bulletin 11 5(4) 
Table 1 
Composition of prey identified in stomachs of smooth hammerhead (Sphyrna zygaena ) col¬ 
lected from December 2012 through June 2015 in northern Peru: %N=percentage by number; 
%W=percentage by weight; %0=percentage by frequency of occurrence, %IRI=index of relative 
importance, and SD=standard deviation. 
Prey category 
%W 
%N 
%0 
%IRI (SD) 
Chordata 
Teleostei 
Fish remains 
2.7 
12.9 
Scomber japonicus 
0.3 
0.2 
0.4 
0.3 (3.72) 
Odontesthes regia 
0.0 
0.0 
0.2 
0.0 (0.13) 
Peprilus sp. 
3.2 
3.3 
4.0 
3.3 (16.24) 
Opisthonema libertate 
0.4 
0.5 
0.9 
0.5 (6.16) 
Sardinops sagax 
0.3 
0.1 
0.2 
0.2 (3.87) 
Selene brevoortii 
0.1 
0.2 
0.2 
0.0 (0.86) 
Scomberesox saurus scombroides 
0.1 
0.1 
0.2 
0.1 (2.55) 
Trachurus murphyi 
0.1 
0.1 
0.2 
0.1 (1.64) 
Hemanthias peruanus 
0.2 
0.1 
0.2 
0.1 (2.74) 
Merluccius gayi peruanus 
1.7 
1.4 
2.2 
1.9 (11.80) 
Engraulidae 
0.6 
0.4 
0.9 
0.5 (6.10) 
Engraulis ringens 
0.0 
0.4 
0.7 
0.2 (2.34) 
Anchoa nasus 
0.3 
0.3 
0.2 
0.3 (5.42) 
Fistulariidae 
0.1 
0.1 
0.2 
0.1 (2.15) 
Mollusca 
Cephalopoda 
Teuthoidea 
Octopoteuthis sicula 
1.2 
1.3 
5.1 
1.3 (7.33) 
Gonatus antarcticus 
7.5 
6.6 
17.3 
7.1 (18.44) 
Stigmatoteuthis hoylei 
2.2 
1.3 
4.7 
1.8 (9.05) 
Mastigoteuthis dentata 
9.1 
12.2 
20.9 
10.9 (25.18) 
Dosidicus gigas 
30.5 
23.8 
30.9 
26.6 (36.28) 
Ommastrephes bartramii 
1.0 
1.2 
2.4 
1.1 (9.08) 
Doryteuthis (Amerigo) gahi 
34.5 
38.4 
44.0 
37.5 (41.56) 
Ancistrocheirus lesueurii 
1.6 
3.5 
12.0 
2.3 (7.96) 
Architeuthis dux 
0.1 
0.6 
2.2 
0.4 (5.55) 
Octopodea 1 
2.1 
4.1 
9.8 
3.4 (14.28) 
1 Argonauta spp., Tremoctopus violaceus 
(4.3), indicating that the smooth hammerhead is a top 
predator. 
The trophic niche width was narrow with a low 
Levin index value (<0.6) for the 2 groups of prey 
taxa: cephalopods and fishes. Also the value of d for 
the Berger-Parker index was closer to 1 than 1/N for 
both groups. This value means that the diet of smooth 
hammerhead is dominated by few prey species (Table 
2). Moreover, the average frequency of occurrence of 
cephalopods (89%) in gut contents was higher than the 
average frequency of occurrence of fishes (11%). The 
most common number of prey species per stomach was 
1, followed by 2 prey species (41% of stomachs con¬ 
tained 1 prey species, and 26% of the stomachs had 2 
prey species) (Fig. 2). For stomachs that contained 1 
prey species, the single prey species was the Patago¬ 
nian squid in 60% of stomachs and the jumbo squid in 
20% of the cases. 
The average number of prey items per stomach was 
5.3 with a mode of 1 and a maximum value of 74. The 
greatest number of prey items was found in a shark 
that measured 230 cm TL which contained 74 pairs of 
squid beaks (equivalent to 74 cephalopods). 
Comparisons of dietary composition 
There were statistically significant differences in diet 
based upon body size. For this analysis, we divided the 
sharks into 4 size classes: I (53-70 cm TL, n=40), II 
(71-100 cm TL, n=175), III (101-190 cm TL, *=111) 
and IV (191-294 cm TL, *=10). The stress value (0.11) 
of the nMDS plot indicated that this plot provides an 
accurate representation of the data, and an overall 
ANOSIM showed significant differences among the size 
classes (J?-statistic=0.4, PcO.OQl). The nMDS plot and 
the overall i?-statistic indicate that the diet of the size 
classes overlap but are clearly distinct. 
By analyzing each of the pairwise comparisons, we 
found that all size classes had significant differences 
(P<0.05), except size classes III and IV. Among the size 
