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

Abstract

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.

When prey becomes killer: does a double lethal attack on a blue shark reveal a precise defensive strategy in young swordfish?

“…Clear proof of an interaction of a blue shark with two swordfish is reported for the first time in this paper (finding of two bill fragments impaled in the head of the shark), although Penadés-Suay et al (2019) hypothesized a similar event in a blue shark stranded on the beach of Vera (Spain). Indeed, these authors found a swordfish bill fragment inside the right nostril, referred to an old injury, and then to a previous not deadly interaction with the blue shark, but also a mortal wound anteriorly to the right eye, without an associated bill fragment and possibly related to another swordfish attack (Penadés-Suay et al, 2019). All swordfish attacks against blue sharks reported in the literature (see Appendix 1) seem to reveal a precise defensive strategy performed by juvenile swordfish, which aims to hit the shark by targeting the head region where vital organs are located.…”

“…The size and proportions of the bill fragments indicated that the perpetrators of these swordfish attacks were juvenile individuals, similarly to the other recent cases of swordfish attacks against the blue shark reported by Penadés-Suay et al . (2017, 2019). These authors firstly examined a large blue shark stranded on the Mediterranean Spanish coast of Valencia in September 2016 (Penadés-Suay et al ., 2017), and then reported four other cases of similar interactions (Penadés-Suay et al ., 2019).…”

“…(2017, 2019). These authors firstly examined a large blue shark stranded on the Mediterranean Spanish coast of Valencia in September 2016 (Penadés-Suay et al ., 2017), and then reported four other cases of similar interactions (Penadés-Suay et al ., 2019). Another analogous attack was observed in the past by Vacchi et al .…”

“…The hypothesis of accidental collisions between swordfish and sharks during feeding activity on the same prey school (Fierstine et al ., 1997; Penadés-Suay et al ., 2017), already proposed by some authors to also explain turtles' impalement (Frazier et al ., 1994), can be excluded for the contemporary finding of two swordfish bills impaled in the same shark. Moreover, both attacks were directed to the shark cranial zone, as in all the other cases reported in the literature (Vacchi et al ., 2000; Penadés-Suay et al ., 2017, 2019) (Appendix 1); Predator–prey relationship : data available in the literature show evidence of predation on juvenile swordfish by blue sharks (Vaske-Júnior et al ., 2009; Lopez et al ., 2010; Markaida & Sosa-Nishizaki, 2010). All records of impalement reported in Appendix 1 can be referred to an interaction between a juvenile swordfish (two in our study case) and a large shark (236–300 cm TL for P. glauca , 400 cm TL for A. superciliosus ).…”

“…Moreover, the diet of the blue shark also includes juvenile swordfish (Vaske-Júnior et al ., 2009; Lopez et al ., 2010; Markaida & Sosa-Nishizaki, 2010). The predator–prey interactions, competition and habitat use sometimes generate conflicts between swordfish and sharks, as already demonstrated by some documented swordfish attacks towards elasmobranchs (Starck, 1960; Vacchi et al ., 2000; Ellis, 2013; Penadés-Suay et al ., 2017, 2019). This swordfish aggressive behaviour has also been documented against other large pelagic animals (e.g.…”