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Aggressive interactions between juvenile swordfishes and blue sharks in the Western Mediterranean: a widespread phenomenon?


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There are numerous reports of billfishes spearing objects, marine organisms, and even humans. Whether or not this behaviour is intentional and, if so, what is its functional meaning, are open questions. In 2016, an adult blue shark (Prionace glauca) was found to be killed by a juvenile swordfish (Xiphias gladius) in the western Mediterranean. Here we report on three more recent cases involving both species in the same area. In February 2017, an adult male blue shark was found stranded in Garrucha (Spain) with a fragment of a juvenile swordfish’s rostrum (18cm long x 2cm wide at proximal end) inserted in its cranium. In March 2017, an adult pregnant female blue shark was stranded alive on the coast of Ostia (Italy) but died shortly afterwards; a fragment of a juvenile swordfish’s rostrum (25x3cm) was found allocated between the eye and the cranium. Finally, in February 2018, an adult female blue shark appeared stranded in the coast of Vera (Spain), with a putative impalement injury anterior to the right eye but without an associated bill fragment. Surprisingly, X-ray and computed tomography revealed an older injury in the right nostril, with a small piece of a juvenile swordfish’s rostrum (5.3x1.2cm). These cases suggest that juvenile swordfish would drive their rostrum into blue sharks as a defensive strategy that is likely to be far from anecdotal. We suggest that no regular cases of these interactions are reported because they occur at high sea and evidence of them, when available, can easily be overlooked.
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314 Medit. Mar. Sci., 20/2, 2019, 314-319
Mediterranean Marine Science
Indexed in WoS (Web of Science, ISI Thomson) and SCOPUS
The journal is available on line at
Research Article
Aggressive interactions between juvenile swordsh and blue sharks in the western Mediter-
ranean: a widespread phenomenon?
Jaime PENADÉS-SUAY1,2, Pablo GARCÍA-SALINAS2,3, Jesús TOMÁS1 and Francisco Javier AZNAR1
1 Marine Zoology Unit, Cavanilles Institute of Biodiversity and Evolutionary Biology, Universitat de València, València, Spain
2Associació LAMNA per a l’estudi dels elasmobranquis a la Comunitat Valenciana
3 Fundación Oceanogràfic
Corresponding author:
Handling Editor: Fabrizio SERENA
Received: 23 June 2018; Accepted: 4 April 2019; Published on line: 4 June 2019
There are numerous reports of billfishes spearing objects, marine organisms, and even humans. Whether or not this behaviour
is intentional and, if so, what is its functional meaning, are open questions. In 2016, an adult blue shark (Prionace glauca) was
found to be killed by a juvenile swordfish (Xiphias gladius) in the western Mediterranean. Here we report on four more recent
cases involving both species in the same area. In February 2017, an adult male blue shark was found stranded in Garrucha (Spain)
with a fragment of a juvenile swordfish’s rostrum (18cm long x 2cm wide at proximal end) inserted in its cranium. In March 2017,
an adult pregnant female blue shark was stranded alive on the coast of Ostia (Italy) but died shortly afterwards; a fragment of a
juvenile swordfish’s rostrum (25x3cm) was found allocated between the eye and the cranium. In February 2018, an adult female
blue shark appeared stranded in the coast of Vera (Spain), with a putative impalement injury anterior to the right eye but without
an associated bill fragment. Surprisingly, X-ray and computed tomography revealed an older injury in the right nostril, with a
small piece of a juvenile swordfish’s rostrum (5.3x1.2cm). Finally, in August 2018, an adult male blue shark appeared stranded
alive in the coast of Manacor (Spain) with a fragment of swordfish bill (6.8x1.5cm) embedded in the snout. These cases suggest
that juvenile swordfish would drive their rostrum into blue sharks as a defensive or territorial strategy that is likely to be far from
anecdotal. We suggest that no regular cases of these interactions are reported because they occur at high sea and evidence of them,
when available, can easily be overlooked.
Keywords: Xiphias gladius; Prionace glauca; lethal interaction; impalement; stranding; western Mediterranean.
Over the last decades, impalement of inanimate ob-
jects and marine organisms by swordfish (Xiphias glad-
ius L.) has been reported in a number of scientific pub-
lications and media news. There are reports of swordfish
driving its rostrum into inanimate objects, such as vessels
(Gudger 1940, Fierstine & Crimmen 1996), bales of rub-
ber (Smith 1956) and submarines (Zarudski & Haedrich
1974). There are also reports of impalement on animals,
such as large fishes (Starck 1960, Smith 1961), whales
(Jonsgard 1962), marine turtles (Frazier et al. 1994 and
references therein) and even people (e.g., Haddad & De
Figuereido, 2009; Georgiadou et al., 2010). Usually,
these interactions are detected by the presence of the dis-
tal segment of the bill embedded in the substratum, which
results from a transverse fracture of the bill due to the
stress of the impact.
The functional meaning of this spearing behaviour is
still under discussion. Carey & Robinson (1981) reported
evidence of intraspecific spearing events. In other bill-
fishes similar cases have been accounted for accidental
collisions. For instance, Fierstine et al. (1997) linked
the injuries in other billfish species to the hypothetical
collisions produced while feeding on the same bait ball.
Likewise, impalement of marine turtles would occur
when swordfish try to prey on the fishes that use shad-
ow beneath turtles as cover (Frazier et al. 1994). In other
cases, however, impalement has been interpreted as in-
tentional (Smith 1956, Ellis 2013, for a detailed historical
account). The clearest evidence of aggressive behaviour
comes from cases involving humans, particularly when
swordfish are provoked or disturbed (Georgiadou et al.
2010, and references therein; Romeo et al. 2017, and ref-
erences therein), which has sometimes resulted in fatal
spearing (Gooi et al. 2006). Descriptions of some of the
events indicate that swordfish usually try to pierce its ‘en-
emy’ with the bill (Romeo et al. 2017).
Medit. Mar. Sci., 20/2, 2019, 314-319
Penadés-Suay et al. (2017) recently reported the case
of a blue shark (Prionace glauca L.) apparently killed by
a swordfish. An adult female blue shark stranded alive
but died shortly afterwards, and the necropsy revealed a
fragment of the tip of a young swordfish’s rostrum that
caused significant lesions in the brain. Penadés-Suay et
al. (2017) raised the question whether or not impalements
by swordfish are anecdotal, and to what extent swordfish
uses spearing as defence behaviour. The answer to both
questions deemed difficult because the literature contains
mostly case studies, or reviews of ‘fait-acommpli’ re-
cords scattered over long periods.
In the present paper we report evidence of four addi-
tional cases of impalement of blue sharks by swordfish
that occurred between February 2017 and August 2018
in the western Mediterranean. Based on the new evidenc-
es, we suggest that impalement of blue sharks could be
far from anecdotal and might represent an overlooked
behaviour within the defensive or territorial strategy of
juvenile swordfish.
Materials and Methods
The four shark specimens included in the present study
were found dead on the beach at the moment of stranding
or shortly afterwards. The sharks were measured in situ to
the nearest cm and visually examined for external lesions.
In two cases, a standard necropsy was carried out on the
stranding beach (Figs. 1, 2). A detailed description of the
injuries provoked by the swordfish rostrum was performed,
and the rostrum fragment was collected, photographed and
measured to the nearest mm. In the third case (Fig. 3), the
intact head of the shark was carried out to the laboratory
for X-ray and computed tomography (CT). Diagnostic im-
aging studies included the following:
(1) Plain radiographic evaluation with a Philips Prac-
tix 400 unit (Philips Medical Systems) and a Kodak Di-
rect View Classic CR System (Carestream Health) with
35 × 43 cm Kodak cassettes (Kodak PQ Storage Phosphor
Screen Regular and 100 Microns, Carestream Health) in
dorsal-ventral (DV) and lateral-lateral (LL) projections.
Focal distance was 1 m, using exposure values 75 kV
and 7.2 mAs based on animal size. Digital images were
processed afterwards with Kodak Acquisition Software
(Onyx-RAD Diagnostic Viewer) for better visualization
and image interpretation.
(2) A CT was done using a Somatom Volume Access
(Siemens). Acquisition parameters through head explora-
tion of the blue shark were 5 mm slice thickness and 5 mm
slice interval, with 0.5 mm retro-recon acquisition under
lung and mediastinal algorithms. Images were post-pro-
cessed with Osirix software version 3.3.1 (Pixmeo).
Rostrum fragments were identified as belonging to a
swordfish (X. gladius) based on the following combina-
tion of features (Fierstine & Voigt 1996, Penadés-Suay et
al. 2017): (1) flattened appearance in cross-section (i.e.,
depth less than half of width); (2) absence of denticles
on the surface, and (3) presence of central chambers in
cross-section. To estimate swordfish total body length
(TL), the ratio of maximum width to length of the frag-
ment (R) was calculated. Then, the rostrum of two sword-
fish specimens conserved at the Osteological Collection
of the Department of Zoology, University of Valencia,
were used to obtain the points in which the value of R
was equal to the value obtained in each case (Penadés-
Suay et al. 2017). Assuming an isometric relationship
in the growth of the snout, total snout length (TSL) was
Fig. 1: ‘A’: Blue shark (Prionace glauca L.) stranded in the coast of Garrucha (East Spain, western Mediterranean) with a sword-
fish (Xiphias gladius L.) rostrum in its skull. Scale indicates 50 cm. ‘B’: details of the bill stuck close to the left eye. Scale bar
indicates 5 cm. ‘C’: details of the fractured swordfish rostrum. Scale bar indicates 2 cm.
316 Medit. Mar. Sci., 20/2, 2019, 314-319
calculated for each case in relation to the distance from
the tip to the breaking point. To add further correction,
we observed the arrangement of the paired nutrient ca-
nals at the breaking point, as shown in cross sections, to
compare it with a swordfish analysed by Habegger et al.
(2015). The allometric regression between TSL and body
length (BL), from post operculum to tail fork, was ob-
tained following McGowan (1988).
Case 1: Garrucha (Spain)
On the 17th of February, 2017, a dead adult male
blue shark (TL: 236 cm) was found washed ashore on
the coast of Garrucha (Almería), Spain, 37.18833333º
N 1.81555556º W (Fig. 1A). The animal had an inci-
Fig. 2: ‘A’: blue shark (Prionace glauca L.) stranded in the coast of Ostia province (West Italy, western Mediterranean) with a
swordfish (Xiphias gladius L.) rostrum rostrum piercing its skull. its skull. Scale bar indicates 50 cm. ‘B’: details of the bill’s
incision close to the right eye. Scale bar indicates 5 cm. ‘C’: details of the fractured swordfish rostrum. Scale bar indicates 2 cm.
Fig. 3: ‘A’: blue shark (Prionace glauca L.) stranded in the coast of Vera (East Spain, western Mediterranean) with a swordfish
(Xiphias gladius L.) rostrum rostrum piercing its skull. its skull. Scale indicates 50 cm. ‘B’: details of the bill’s incision close to
the right eye. Scale bar indicates 5 cm. ‘C’: details of the fractured swordfish rostrum. Scale bar indicates 2 cm.
Medit. Mar. Sci., 20/2, 2019, 314-319
sive injury between the left eye and the snout. During
the necropsy, the broken tip of a swordfish bill, measur-
ing 180 mm long and 20 mm wide at the breaking point,
was found inside the cranium (Fig. 1B). The bill entered
the skull at 30º angle from the anteroposterior axis. The
lesion was assumed to be the cause of death due to the
associated brain damage. The broken tip found (Fig. 1C)
could be part of a snout ca. 404 mm long, corresponding
to a juvenile swordfish.
Case 2: Ostia (Italy)
On the 8th of March, 2017, an adult female blue shark
(TL: 260 cm) was found stranded on the coast of Ostia,
Italy, 41.70861111º N 12.32638889º E (Fig. 2A). The an-
imal was alive when found, but died shortly afterwards.
The shark showed an advanced state of pregnancy, with
five developed embryos inside her uterus, and no sign of
starvation or disease. An incision was visible between the
first gill slit and the right eye. The necropsy revealed the
presence of a swordfish bill penetrating the flesh between
the optic capsule and the post-orbital process (Fig. 2B).
Apparently, the incision did not damage any sensory or-
gan. The swordfish fragment (Fig. 2C) was 250 mm long
and 30 mm wide at its proximal end, corresponding also
to a juvenile swordfish (snout ca. 505 mm long).
Case 3: Vera (Spain)
On the 28th of February, 2018, an adult female blue
shark (TL: 260 cm) was found dead stranded in the coast
of Vera (Almería), Spain, 37.19388889º N 1.81361111º
W (Fig. 3A, 3B). An incision close to the right eye was
found, entering the head at 20º angle from the antero-
posterior axis. No fragment of bill was found by visual
inspection. Both X-ray (Fig. 4A) and CT (Fig. 4B) also
failed to find fragments of bill inside the incision but,
surprisingly, revealed the presence of a distal fragment
of swordfish bill inside the right nostril, anterior to the
firstly detected injury. No external sign of injury was ap-
parent in this case. The cavity of the nostril was surround-
ed by scarred tissue (Fig. 5). The broken tip of the bill
(Fig. 3C) measured 53 mm long and 12 mm wide at its
proximal end, corresponding also to a juvenile swordfish
(snout ca. 242 mm long).
Case 4: Manacor (Spain)
On the 2nd of August, 2018, an adult male blue shark
(TL: 293 cm) was found stranded and returned to the
water by bystanders several times within an hour in the
coast of Manacor (Balearic Islands), Spain, 39.457296º N
3.277984º W (Fig. 5A). By the time the rescue team ar-
rived, the shark was motionless in lateral decubitus close
to the shore, half meter deep in the water, without reacting
to any stimulus. After euthanasia, the necropsy revealed
signs of starvation and the presence of a distal fragment of
a swordfish bill embedded in the snout (Fig. 5B). The frag-
ment (Fig. 5C), measuring 68 mm long and 15 mm wide at
its proximal end, had entered the centre of the snout from
the right posterior side in a close angle to the anteroposte-
rior axis of the shark and was identified as belonging to a
juvenile swordfish (snout ca. 303 mm long).
This survey reports four cases of impalement of blue
sharks by juvenile swordfish in the western Mediterra-
nean in less than two years. The question that arises is
the extent to which such events are instances of acciden-
tal collision between both species or of purposeful be-
haviour. Although we cannot rule out the first possibility,
there are some lines of evidence that conform to the hy-
pothesis that swordfish could actually have intentionally
hit sharks. Firstly, all cases here examined involved only
small swordfish and adult blue sharks. There is the pos-
sibility that juvenile swordfish perceive large blue sharks
as a threat, given that blue sharks are opportunistic preda-
tors that feed on a wide variety of cephalopods and fishes
(Vaske-Júnior et al. 2009; Markaida & Sosa-Nishizaki
2010), and some studies of their diet included swordfish,
although the size of swordfish consumed (ca. 1 kg, see
Vaske-Júnior et al. 2009) seems to be smaller than that
estimated for individuals in the present study. As noted
Fig. 4: Pictures showing the injury inflicted by a swordfish’s
bill to a blue shark stranded in Vera (Spain): ‘A’ ventral view
of the injury with X-rays, showing the fractured bill allocated
inside the nostril. ‘B’ lateral view of the wound using Comput-
ed Tomography, showing the bill perforating the nostril. Red
arrows indicate the location of the swordfish’ fractured bill in
both pictures.
318 Medit. Mar. Sci., 20/2, 2019, 314-319
above, swordfish frequently displays pugnacious be-
haviour, even towards prima facie non-threatening spe-
cies, including humans (Smith 1956, Georgiadou et al.
2010, Ellis 2013). Secondly, in all cases of impalement
on blue sharks reported so far (Penadés-Suay et al. 2017;
present study), the angle of piercing indicates a nearly
horizontal strike with respect to the sharks’ anteroposteri-
or axis. Although there is no reason to exclude the possi-
bility of an accidental collision during an act of predation
to the same prey, a voluntary offense to the blue shark
by juvenile swordfish is to be considered, given that the
type of impalement described could hardly result from
random collisions. Finally, at least the shark stranded in
the coast of Vera showed signs of two diverse interactions
with swordfish happened at different times, which would
refer to two swordfish interactions.
In this context, one may wonder if swordfish use
spearing as a typical defence strategy/territoriality ag-
gression or only as a last resort behaviour against blue
sharks. This is difficult to ascertain. On one hand, sever-
al studies suggest that the swordfish’ bill has important
hydrodynamic and feeding-related functions (Habegger
et al. 2015, Videler et al. 2016) thus bill breakage could
be seriously disadvantageous for swordfish. On the oth-
er hand, at least in billfishes, there are several reports of
apparently healthy individuals with damaged, malformed
or even missing rostra (Frazier et al. 1994). This raises
the possibility that juvenile swordfish may experience a
trade-off between the benefits of repelling a perceived
serious threat and the potential impairment of swimming
performance if the bill eventually breaks.
A third important question concerns the frequency of
such intentional impalements. The present study, and that
by Penadés-Suay et al. (2017), report on stabbings that
were inflicted on the head and, in some cases, resulted
in the death of the victim. However, blue sharks are oce-
anic predators that only exceptionally are washed ashore
when moribund or dead (hence the lack of studies report-
ing these events, see Neto et al. 2013). In general, most
carcasses of large marine vertebrates disappear at sea
(Carretta et al. 2016 and references therein). Hence, there
is the possibility that many cases of fatal impalement
of blue sharks have gone undetected because carcasses
were consumed by scavengers or sank before they could
be discovered. On the other hand, non-fatal impalement
lesions could easily be overlooked. Blue sharks are target
species of long-line fishery and, therefore, many individ-
uals are visually exposed in fish markets. Also, the fact
that only fragments from juvenile swordfish were found
leaves the question on whether they are more prone to
breakage than those of adults, giving then an added diffi-
culty to finding evidence of the collisions in those cases.
However, our study shows that not all impalements result
in a breakage of the swordfish’ bills, thus leaving only a
small external incision that is difficult to draw attention
or even to be noticed unless it is a specific target for ex-
amination. Moreover, old lesions are not visible external-
ly, even when bill fragments remain in the scarred tissue,
unless sophisticated technologies (X-rays, CT) are used,
and only a deep, thorough necropsy could reveal the evi-
dence of such past interactions. In fact, the finding of an
old impalement in shark 3 in our study was serendipitous.
In conclusion, our study reports four cases within less
than two years and only in the Western Mediterranean ba-
Fig. 5: ‘A’: blue shark (P. glauca L.) stranded on the coast of Manacor (Balearic islands, western Mediterranean) with a swordfish
(Xiphias gladius L.) rostrum piercing its snout. Scale indicates 1 m. ‘B’: details of the bill’s incision close to the centre of the snout.
Scale bar indicates 5 cm. ‘C’: details of the fractured swordfish rostrum. Scale bar indicates 1 cm.
Medit. Mar. Sci., 20/2, 2019, 314-319
sin, suggesting the relevance of this phenomenon. As an
additional example,, there is one short report describing
putative wounds in a bigeye thresher Alopias supercil-
iosus (Vacchi et al. 2000), similar to the case of a blue
shark reported in the same region by Penadés-Suay et al.
(2017). More attention on this matter is needed, looking
for evidences in all necropsy protocols and investigating
this kind of interactions in other shark species.
We would like to thank Equinac for all their field
work and assistance with the investigation in the first two
cases from Spain, MedSharks (especially Aldo Marinel-
li, Simona Clò and Eleonora de Sabata) for their report
and information from the case in Italy and Guillem Félix
along with the team from Palma Aquarium and the Bale-
aric Islands Government (Direcció General de Pesca de
la Conselleria de Medi Ambient, Agricultura i Pesca de
les Illes Balears) for their report on the last case. Special
acknowledgement goes to the staff of the Veterinary De-
partment of the Oceanogràfic Aquarium in Valencia for
letting us use their equipments to take some of the im-
ages shown in this paper and, by doing so, allowing us
to add further insight into the discussion. FJA and JT are
supported by project PROMETEO III of the Generalitat
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... Impalements caused by billfishes on different taxonomic groups such as whales (Jonsgard 1962;Machida 1970;Oshumi 1973;Major 1979), sharks (Fierstine et al. 1997Penadés-Suay et al. 2017, 2019Jambura et al. 2020;Romeo et al. 2020), even humans (Carvajal et al. 2002Gooi et al. 2007;Mendonça-Caridad et al. 2008;Haddad Jr and Figueiredo 2009;Georgiadou et al. 2010;Galarza et al. 2016), and diverse inanimate objects, such as submarines (Zarudzki and Haedrich 1974) and ships (Gudger 1940;Romeo et al. 2017) have been recorded around the world. In contrast, records of turtles impaled by billfishes are rare (Frazier et al. 1994), even today, when images can be quickly captured using mobile phones. ...
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Sea turtles are susceptible to a wide range of impacts. In Brazil, the loggerhead turtle Caretta caretta (Linnaeus, 1758) is the main sea turtle species caught incidentally by longline fishing fleets that target the blue shark Prionace glauca (Linnaeus, 1758) and swordfish Xiphias gladius (Linnaeus, 1758). The latter is well known for its predation strategy, which consists of using the rostrum to injure and catch prey. In this study, we recorded for the first time the impalement of a juvenile loggerhead sea turtle by a swordfish during a fishing operation of a Brazilian longline vessel in 2018. Two videos of this interaction were recorded by the shipmaster around 260 km from the Brazilian coast. The sea turtle was incidentally caught with a hook and subsequently attacked by the swordfish. The rostrum initially pierced the anterior left shoulder of the turtle and passed through the body, exiting from the right posterior carapace. Given the position of the attack, considerable internal damage was most certainly done to the turtle. Although the turtle was hauled aboard to remove the hook and then returned to the sea alive, it probably died due to the extent of the injuries caused by the rostrum having passed through the center of the body. Since both swordfish and turtle are pelagic and these interactions are poorly recorded, such encounters may be relatively common. Therefore, this type of information should be included in the data collection protocols of fishery monitoring programs to better understand and scale the bill-stab phenomenon, not only for sea turtles but also in relation to other groups of fauna.
... sharks) and the potential impairment of high-speed locomotion. The shark species involved in these incidents are usually either the blue shark Prionace glauca (Penadés-Suay et al. 2017, 2019 or the mako shark Isurus oxyrinchus (Cliff et al. 1990;Fierstine et al. 1997), both of which are known to opportunistically prey on fish, including swordfish (e.g. Stillwell and Kohler 1982;Maia et al. 2006;Vaske Júnior et al. 2009). ...
In April 2020, three citizen scientists reported the stranding of an adult female bigeye thresher shark Alopias superciliosus (TL 445cm) on the coast of Libya. The shark had a deep penetrating trauma on the dorsal surface, between head and first dorsal fin, with the distal segment of a swordfish rostrum embedded in it. The swordfish involved in this interaction had an estimated TL of 285cm, corresponding to an adult individual. This fatal injury might have been the result of an accidental collision, e.g., while feeding on the same prey, however, an intentional attack against the shark cannot be excluded either.
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We describe the interaction between harpoon fishing activity and sharks, which opportunistically depredated harpoon catches in the Strait of Messina. Shark bite marks were observed on harpooned swordfish during the period 2014–2020, with different damages to the catches. Most of the depredation events have focused on large swordfish, generally weighing more than 60 kg. Data on direct observations were implemented by interviews and questionnaires to fishermen aimed to recover the information on their local fishing and ecological knowledge. Fishermen provided additional data on sharkharpoon fishing interactions also supplying information on by-catch species (i.e., bluefin tuna). Therefore, these results suggest that sharks migrating through the Strait of Messina are occasionally attracted by injured prey, due to their ability to detect chemical cues, fish distress stimuli and body fluids (i.e. blood) in the water. In addition, our investigations showed an increase in shark attacks on harpooned fish over time, likely due to an increase in harpoon swordfish catches. This may be related to the effects of the driftnets’ ban enforced by European Regulations in the last decades.
In this paper a rare case of a double swordfish mortal attack against an adult blue shark ( Prionace glauca ) is reported. A female blue shark, with a total length of 3 m, was found stranded along the southern Sicilian coast (Strait of Sicily, Mediterranean Sea) on 30 May 2018. The analysis of this carcass revealed the presence of two swordfish bill fragments, impaled in the shark head; the former on the snout, the latter near the eye. The results of anatomical and computed tomography scanning analysis on the head of the blue shark showed that the larger bill fragment (19.7 cm) probably determined the death of this animal, having been impaled in a vital point, just behind the right eye. The analysis of both these events and other similar swordfish-shark interactions reported in the literature makes possible the hypothesis that young swordfish specimens put in place a precise defensive strategy against their potential predators or competitors, aimed at hitting vulnerable and vital points and delivering a mortal blow.
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In September, 2016, an adult female blue shark (Prionace glauca) 247 cm long stranded alive on the coast of Valencia (Spain, Western Mediterranean) but died shortly afterwards. The necropsy revealed ongoing pregnancy, with 65 embryos in early stage of development, and a healthy condition with no signs of starvation. Two fishing hooks surrounded by scarred tissue were detected in the mandible, indicating past interaction with fisheries. In addition, a fragment of the tip of a swordfish (Xiphias gladius) rostrum (length: 18 cm long, width: 0.5 cm (distal) and 3 cm (proximal)) was removed from the animal. The fragment had pierced the head producing an incision of 3.5 cm close to the left eye, crossing the head anterior to the pre-orbital process. No apparent damage was observed in the olfactory capsule or the eye, but the fragment had penetrated both sides of the skull causing extensive lesions in the brain, which were inferred to be the cause of death. Allometric analysis suggested that the swordfish was ca. 110 cm long, corresponding to a juvenile 1-2 yrs old. Swordfish had previously been reported driving their rostrum into pelagic sharks, allegedly as a defensive strategy. However, this is the first report of impalement as the direct cause of death in blue sharks.
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The swordfish is reputedly the fastest swimmer on Earth. The concave head and iconic sword are unique characteristics, but how they contribute to its speed is still unknown. Recent computed tomography scans revealed a poorly mineralised area near the base of the rostrum. Here we report, using magnetic resonance imaging and electron microscopy scanning, the discovery of a complex organ consisting of an oil-producing gland connected to capillaries that communicate with oil-excreting pores in the skin of the head. The capillary vessels transport oil to abundant tiny circular pores that are surrounded by denticles. The oil is distributed from the pores over the front part of the head. The oil inside the gland is identical to that found on the skin and is a mixture of methyl esters. We hypothesize that the oil layer, in combination with the denticles, creates a super-hydrophobic layer that reduces streamwise friction drag and increases swimming efficiency.
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Perhaps the most striking feature of billfishes is the extreme elongation of the premaxillary bones forming their rostra. Surprisingly, the exact role of this structure in feeding is still controversial. The goal of this study is to investigate the use of the rostrum from a functional, biomechanical, and morphological standpoint to ultimately infer its possible role during feeding. Using beam theory, experimental and theoretical loading tests were performed on the rostra from two morphologically different billfish, the blue marlin (Makaira nigricans) and the swordfish (Xiphias gladius). Two loading regimes were applied (dorso-ventral and lateral) to simulate possible striking behaviors. Histological samples and material properties of the rostra were obtained along their lengths to further characterize structure and mechanical performance. Intraspecific results show similar stress distributions for most regions of the rostra, suggesting this structure may be designed to withstand continuous loadings with no particular region of stress concentration. Although material stiffness increased distally, flexural stiffness increased proximally owing to higher second moment of area. The blue marlin rostrum was stiffer and resisted considerably higher loads for both loading planes as compared to that of the swordfish. However, when a continuous load along the rostrum was considered, simulating the rostrum swinging through the water, swordfish exhibited lower stress and drag during lateral loading. Our combined results suggest the swordfish rostrum is suited for lateral swiping to incapacitate their prey, whereas the blue marlin rostrum is better suited to strike prey from a wider variety of directions.
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Strandings of oceanic-pelagic elasmobranchs in the southeastern Brazil are reported. Data comes from animals observed in the coast of São Paulo state, between 1999 and 2012. Nine individuals of two species were recorded: Pteroplatytrygon violacea (n = 5; mostly during the winter) and Isurus oxyrinchus (n = 4; two in the winter and two in the summer). For P. violacea the strandings restricted to the austral winter suggest that the species follows the intrusion of high temperatures water masses recorded in southeastern Brazil during this season, bringing some individuals to shallow waters. For I. oxyrinchus is possible that individuals escaped from hooks of the commercial pelagic longline fishery and suffered injuries in the esophagus and in the gastric wall, stranding due to difficulties in locomotion and feeding. As these stranded sharks were not necropsied and only two animals were observed during the austral summer, we cannot exclude other causes of beaching such diseases or the intrusion of cold water masses in the continental shelf during this season. Keywords: Stranded animals; mortality; Pteroplatytrygon violacea; Isurus oxyrinchus; Atlantic Ocean.
Recent cases of swordfish attacks on harpoon fishing vessels of the Sicilian fleet were examined and the particular behavior adopted by this fish during the approach to fishermen was described. The information was collected in the period between 1999 and 2014 in Sicilian waters (Mediterranean Sea) through interviews with harpoon fishermen and through direct observations during research activities. Additional cases of the attacks from the previous years are also reported. Overall, 29 cases of swordfish aggressive behavior against fishing boats were documented. Swordfish usually attacked when provoked or when wanted to defend its mate and in several cases, it tried to pierce the boat by the bill. Some attacks culminated with a damage to the target, whereas no injuries to humans were recorded. © 2017, Institute of Oceanography and Fisheries. All rights reserved.
Recovery of cetacean carcasses provides data on levels of human-caused mortality, but represents only a minimum count of impacts. Counts of stranded carcasses are negatively biased by factors that include at-sea scavenging, sinking, drift away from land, stranding in locations where detection is unlikely, and natural removal from beaches due to wave and tidal action prior to detection. We estimate the fraction of carcasses recovered for a population of coastal bottlenose dolphins (Tursiops truncatus), using abundance and survival rate data to estimate annual deaths in the population. Observed stranding numbers are compared to expected deaths to estimate the fraction of carcasses recovered. For the California coastal population of bottlenose dolphins, we estimate the fraction of carcasses recovered to be 0.25 (95% CI = 0.20–0.33). During a 12 yr period, 327 animals (95% CI = 253–413) were expected to have died and been available for recovery, but only 83 carcasses attributed to this population were documented. Given the coastal habits of California coastal bottlenose dolphins, it is likely that carcass recovery rates of this population greatly exceed recovery rates of more pelagic dolphin species in the region.
Two of 5 Pacific fish showed a clear daily cycle of movement between an inshore bank during the day and deep water offshore at night. All of the swordfish responded to light, swimming deep during the day and coming near the surface at night. In the Pacific depth during daylight appeared to be limited to c.100 m by the oxygen-minimum layer, but in well-oxygenated waters of the Atlantic, a midday depth of greater than 600 m was recorded and the fish appeared to follow an isolume. Depth of the Atlantic fish in daylight was related to changes in light caused by variation in water transparency. Vertical movements were associated with temperature changes of as much as 19oC within 2 hr.-from Authors
Seven species of the family Istiophoridae and Xiphias gladius were identified using only features of their rostrum. In the Istiophoridae, two rostral regions were emphasized, one-fourth and one-half the distance between the distal tip and the orbital margin of the lateral ethmoid bone. Characters studied in each region were the depth and width of rostrum and height, width, and position of nutrient canals (as seen in cross-section). Characters studied without reference to region were the distribution of denticles on both dorsal and ventral surfaces of the bill and position of the prenasal bone. In the Xiphiidae, the only characters studied were the depth and width of the rostrum at the level of the dermethmoid bone and the presence and placement of central chambers as seen in radiographs. A total of 32 characters were analyzed as ratios using both multivariate and univariate statistics. The rostrum of X. gladius was separated from the Istiophoridae by its fiat shape, Tetrapturus angustirostris from all other istiophorids by its widely separated nutrient canals, and the complex of T. audax/T. pfluegeri/Makaira nigricans/M. indica from the complex of Istiophorus platypterus/T. al bidus by having a smaller area of denticles on the dorsal surface. Tetrapturus pfluegeri was separated from T. audax, M. nigricans, and M. indica by having a longer denticle-free midline on the ventral surface of the rostrum. Tetrapturus audax was separated from M. nigricans and M. indica by the location of its nutrient canals. The complexes of Makaira nigricans/M. indica and L platypterus/T. albidus were each separated using multivariate discriminant analysis. We show the study has application in identifying rostral fragments found as fossils and impaled in animate and inanimate objects such as marine turtles and wooden ships and should have application wherever rostral fragments are found.
Billfishes (Istiophoridae and Xiphiidae) are notorious for driving their rostra into animate and inanimate objects, a behavior usually resulting in transverse fracture of the bill and leaving the distal segment embedded (Gudger, 1940; Frazier et aI., 1994). Some billfishes recover from this loss because there are records ofapparently healthy fish with missing rostra (Frazier et aI., 1994). Generally only one rostral fragment is found in each object, but multiple stabbings have been reported. For example, fragments of three swordfish bills were discovered in a whale during flensing (Jonsgard, 1962), several "marlin" spears were found impaled in bales of rubber that were floating at sea (Smith, 1956), and two istiophorid rostra were identified in the timber of a vessel that was brought in for repair (Gudger, 1940; Fierstine and Crimmen, 1996). The following is a detailed account of a large Atlantic blue marlin with two rostral fragments embedded in its head and is the first record of a fish with multiple wounds. I briefly discuss whether impalement was the result of a predator-prey interaction, if embedded rostra aid in understanding migration patterns in both prey and predator, and the effect of impalement on a predator.