Cordova-Zavaleta et al.: Food habits of Prionace glauca in waters off northern Peru 
319 
ki, 2010; Klarian et al., 2018). In this study we sought 
to reinforce these studies with a new approach, i.e., 
with the consideration of the cosmopolitan duck bar¬ 
nacle ( Lepas anatifera) as a bioindicator of scavenging 
behavior. 
The duck barnacle is a cosmopolitan species that as 
an adult is commonly found attached to floating objects 
(Hinojosa et al., 2006). Its cyprid larval stage denotes 
the shift from a free-swimming organism to a sessile 
organism. During this stage, cyprid larvae are forced to 
find a favorable place to settle and metamorphose into 
juvenile duck barnacles (Hpeg et al., 2012). The most 
common substrate used by cyprid larvae are animals 
and floating objects, such as boats, buoys (Sneli, 1983), 
wood (Minchin, 1996), macroalgae (Hinojosa et al., 
2006), turtles (Casale et al., 2012), and even fish (Zevi- 
na and Memmi, 1981). However, we believe that other 
substrates could be used by duck barnacle in the open 
ocean, for example, moribund spent females of many 
deep-water cephalopods that float passively to the 
ocean surface and die (Nesis, 1996). Indeed, Markaida 
and Sosa-Nishizaki (2010) stated that blue sharks may 
easily scavenge on these dead buoyant cephalopods. In 
our study, two stages of duck barnacle—cyprid larvae 
(60.1%, range: 1-200 individuals) and juveniles (9.1%, 
range: 1-3) (Suppl.Fig.)—were found in stomachs con¬ 
taining only the prey ‘unidentified cephalopods’ (n- 40). 
We believe that at least some of the ingestion of cepha¬ 
lopods was the result of scavenging behavior. The pos¬ 
sible scavenging behavior of blue sharks is also rein¬ 
forced by findings of duck barnacle cyprid larvae in 
samples containing skin, blubber, muscle tissue, and 
dorsal fin of marine mammals, and in the keeled ster¬ 
num of an ‘unidentified bird’ (Klarian et al., 2018). 
Dietary variability by ontogenetic factors 
Ontogenetic shifts have been described in the diets of 
several shark species—shifts that are mainly due to 
energetics, metabolism, or changes in foraging ability 
(Grubbs, 2010). In the case of blue sharks, some stud¬ 
ies have tried to prove these ontogenetic shifts, how¬ 
ever, with no success (Markaida and Sosa-Nishizaki, 
2010; Hernandez-Aguilar et al., 2016). 
Length at first maturity of blue sharks in the 
southeastern Pacific Ocean has been reported to be 
around 200 cm TL (Bustamante and Bennett, 2013). 
Therefore, size class II in our study was considered to 
comprise both juvenile and small adults, with a large 
predominance (82%) of juveniles. Juvenile blue sharks 
(TL<200 cm) have a narrow coastal distribution before 
they take part in larger-scale migrations (Vandeperre 
et al., 2014). Litvinov (2006) supported the idea that 
this spatial isolation of juvenile blue sharks is caused 
by limitations on prey consumption at the earliest age, 
specifically during the period of development of teeth 
cusps. In addition, Vandeperre et al. (2014) stated 
that coastal areas may provide juvenile blue sharks 
(fork length<185 cm) with optimal growth conditions 
because of the availability of food resources that are 
associated with the diversity of topographic features 
(seamounts and islands), and localized oceanographic 
processes. In our study, the large abundance of small- 
size octopods, such as Argonauta spp., in the diet of 
individuals from size class II and in the diet of those 
captured in the coastal zone (Fig. 2), reaffirms the hy¬ 
pothesis that small-size blue sharks occur in coastal 
areas and feed upon available prey items. 
Vertical and horizontal movements of blue sharks 
expand progressively as body size increases, and ac¬ 
cording to migratory patterns throughout their life his¬ 
tory (Nakano and Stevens, 2008). The smaller quanti¬ 
ties of Argonauta spp. and the appearance of the oce¬ 
anic squid A. lesueurii in the diet of individuals from 
size class III (100% adults) and from oceanic areas 
(Fig. 2) may indicate longitudinal movements of larger 
blue sharks from coastal to oceanic areas (Vogler et 
al., 2012). Moreover, the presence of cephalopods from 
deeper depths, such as J. diaphana or V. infernalis, 
in the diet of size class III may indicate progressive 
dives to greater depths by larger individuals (Roper 
and Young, 1975). 
In this article we have provided important new 
information about the food habits of blue sharks off 
northern Peru. Blue sharks feed on small-size (Argo¬ 
nauta spp.) and larger size (A. lesueurii) prey species, 
which indicate a surface and mesobathypelagic for¬ 
aging behavior. In addition, we identified prey items 
with commercial importance, such as jumbo squid, D. 
gahi, Peruvian anchoveta, and flying fish eggs in the 
diet of blue sharks, which highlight the importance 
of the results from this study for Peruvian ecosystem 
management. 
Acknowledgments 
We thank F. Galvan-Magana and J. Xavier for assist¬ 
ing with cephalopod beak identification, as well as J. 
Ramon-Ortega for his advice with statistics. The au¬ 
thors also thank all onboard observers who participat¬ 
ed in this study, as well as E. Alfaro, J. Coasaca, S. 
Pingo, A. Jimenez, and ProDelphinus staff. This study 
was funded by the Fondo para la Innovacion, la Ciencia 
y la Tecnologla (contract no. 369-PNICP-PIBA-2014), 
the Darwin Initiative, and the U.S. State Department 
through the U.S. Embassy in Lima. 
Literature cited 
Adams, G. D., D. Flores, O. Galindo Flores, K. Aarestrup, and 
J. C. Svendsen. 
2016. Spatial ecology of blue shark and shortfin mako in 
southern Peru: local abundance, habitat preferences and im¬ 
plications for conservation. Endang. Species Res. 31:19- 
32. 
Alfaro-Shigueto, J., J. C. Mangel, M. Pajuelo, P. H. Dutton, J. 
A. Seminoff, and B. J. Godley. 
2010. Where small can have a large impact: structure and 
