Cordova-Zavaleta et al.: Food habits of Prionace glauca in waters off northern Peru 
317 
Figure 4 
Cluster analysis of 3 size classes with total length intervals of 40 cm for blue sharks (Prionace 
glauca) sampled off northern Peru between February and December 2015. Hierarchical clustering 
was performed by using the Bray-Curtis index of dissimilarity. 
The nonmetric MDS ordination for fishing ground 
longitude showed that the diets of both groups over¬ 
lapped (Fig. 5B). This overlap explains the small dif¬ 
ference calculated with ANOSIM (Table 2). SIMPER 
analysis revealed that, in order of importance, Argo- 
nautidae and Chiroteuthidae contributed more to the 
diet of blue sharks in the coastal zone, whereas An- 
cistrocheiridae and Argonautidae contributed more to 
the diet of individuals in the oceanic zone. Trophic 
positions in both longitudinal zones were similar and 
Levin’s standardized index revealed that blue sharks 
had a higher degree of diet specialization in both zones, 
although higher values were estimated for the oceanic 
zone (Table 2). Values of %PSIRI for longitudinal zones 
(Fig. 2) showed that the coastal zone diet (n= 36) was 
predominated by Argonauta spp. (33.2%), flying fish 
eggs (6.3%), Gonatus antarcticus (5.2%), and Peruvian 
anchoveta (4.2%). In contrast, the oceanic zone (n= 79 
stomachs) had a much lower percentage of Argonauta 
spp. (8.3%), in addition to A. lesueurii (8.2%) and dia¬ 
mond squid (Thysanoteuthis rhombus, 7.6%). Further¬ 
more, ‘unidentified cephalopods’ varied greatly between 
coastal (15.3%) and oceanic (38.6%) zones. 
Finally, the PERMANOVA test (pseudo-F=0.7961, 
P>0.05) indicated no interaction between the 2 size 
classes and coastal and oceanic fishing grounds. 
Discussion 
The present study provides important new information 
on the diet of blue sharks in Peruvian waters. In this 
study, cephalopods represented 87.4% PSIRI of overall 
diet, and 66.7% of total prey species identified. These 
results validate, for Peruvian waters, the preference 
of blue sharks to prey on squid in the eastern Pacific 
Ocean (Markaida and Sosa-Nishizaki, 2010; Loor-An- 
drade et al., 2017). Furthermore, our results showed 
that small quantities of fish were consumed (8.5% 
PSIRI). It is important, however, to note that fish con¬ 
sumption is likely underestimated because both fish 
flesh and otoliths degrade faster than cephalopod flesh 
and beaks (Tricas, 1979; Hernandez-Garcfa, 1995). 
Blue sharks have been described as meso- and 
bathypelagic predators (Clarke et al., 1996). They are 
known to swim at depths near the thermocline at night, 
whereas during daylight hours, they prefer to dive and 
complete long incursions to great depths (Carey and 
Scharold, 1990). Campana et al. (2011) stated that 
this behavior was highly related to the diel vertical 
migrations of their preferred prey (i.e., cephalopods), 
which feed in the epipelagic zone at night and move to 
greater depths during daylight hours. Our identifica¬ 
tion of mesopelagic (e.g., Ancistrocheiridae, Histioteu- 
thidae, Gonatidae cephalopods) and bathypelagic (i.e., 
Vampyroteuthis infernalis ) prey in significant quanti¬ 
ties, suggests that, at least for the area studied, they 
also completed vertical migrations to feed on preferred 
or available prey, or both (Roper and Young, 1975). 
Argonauta spp. off the coast of northern Peru have 
been studied little and have been described only as 
fauna associated with prospections associated with the 
Peruvian anchoveta fishery in the northern Humboldt 
ecosystem (between 4-16°S and 0-148 km from shore) 
