The Accidental Capture of a Newborn Shortfin Mako Shark Isurus oxyrinchus (Rafinesque, 1810) in the Northwestern Mediterranean Sea off the Coast of Marseille, France

Abstract

A small-sized shark was accidentally caught in the coastal waters of southeastern France and abandoned in the port of L’Estaque (Marseille) in May 2024. An autopsy revealed that it was a young female shortfin mako, Isurus oxyrinchus (Rafinesque, 1810). Numerous superficial wounds on the head, back, and flanks, in the form of parallel striations resulting from heavy friction, indicate meshing in a gillnet that provoked the animal’s death by asphyxiation. Its very small size (64.77 cm TL) suggests that this is a newborn individual, leading to the hypothesis that the waters off Marseille could be a nursery area for this species. This observation alone is not enough to prove this point, but it does suggest the need for closer monitoring to validate it. Should the area be proven to play this key role for this critically endangered species, measures to limit fishing pressure would be essential, particularly during the parturition period between February and May.

1. Introduction

The shortfin mako shark, Isurus oxyrinchus (Rafinesque, 1810), belongs to the Lamnoïds, a family characterized by an ogive-shaped cylindrical body, pointed snout, and homocercal tail. Adults can reach >400 cm in total length (TL) and are generally 150–200 cm in TL [1,2]. This species frequents tropical or temperate waters, evolving mainly offshore, and is present in the Mediterranean, where it is the victim not only of by-catches due to industrial fishing [3], particularly for tuna or swordfish, but also recreational fishing [4]. In the Mediterranean, several documented observations [5,6,7,8,9] suggest higher densities in the eastern basin compared to the western one, where it is more rarely observed [4,10,11,12,13].

Like most elasmobranch species, the resilience of this species is penalized by its low reproductive efficiency, with relatively late sexual maturity and a litter size of between 4 and 25 pups (with an average of 12), each measuring between 60 and 70 cm in total length (TL) [14]. Population levels in the Mediterranean have fluctuated sharply downwards, by as much as >99.99% in the early 2000s [15], and this inexorable downward trend has recently been confirmed in the western part of the Mediterranean [4]. Considered ‘near threatened’ at a global level, this species is classified in the Mediterranean as ‘critically endangered’ in the IUCN Red List Conventions [16] and listed in Annex A3 and Annex A2 of the Bonn and Berna Conventions, respectively. Given the interactions with industrial fishing, the species’ conservation is a critical issue [17], and any information that can contribute to decision-making in favor of protecting this species appears all the more relevant.

In this study, we describe the accidental capture of a shortfin mako shark off Marseille (southeastern France), whose size suggests that it was a newborn, leading us to hypothesize that the area where it was caught could be an important breeding ground for the species.

2. Materials and Methods

The ‘Port de l’Estaque’ (Marseille, southeastern France) has a boat capacity of >1000 but is mainly oriented towards recreative boating, with only 3 or 4 professional fishing boats left using gillnets, among other inshore fishing techniques.

On the afternoon of 21 May 2024, a small-sized shark was collected while it was floating on the surface of the harbor. Kept on site in cold storage, an autopsy was carried out on the morning of 22 May, which allowed (i) a detailed external observation of the shark’s body and (ii) a detailed internal exploration, including organ separation and investigation of internal cavities before (iii) taking histological samples. The sex of the animal was assessed through the presence/absence of external claspers (pterygopodia) [18].

The occurrence of the capture of sharks in French waters, particularly juveniles with a TL < 80 cm, was explored by consulting the Elasmed database [19] and interviewing key persons such as fishermen and marine scientists working in the area.

3. Results

3.1. Species, Gender Identification, and Ontogenic Status

The coloration of the lower body and belly of the shark was whitish, with a pale blue metallic sheen on the upper part (Figure 1). Examination of the specimen revealed a female individual (in the absence of claspers), with a slender body with a distinctly conical snout, large dark eyes, and long blade-like teeth without any serration (Figure 1A). The dorsal fin was shorter than it was wide, with a rounded apex, and both the anal and second dorsal fins were very small. The pectoral fins were smaller than the head. The tail was crescent-shaped and homocercal (upper and lower lobes of equal length) with a caudal keel. All these elements were consistent in identifying a shortfin mako Isurus oxyrinchus [1]. A total length (TL) of 64.5 cm, below the threshold for the average lowest size of 70 cm (TL) at birth [2], led to its identification as a newborn individual. The details of the necropsy are compiled in the ESM (Supplementary Material); here, we present the most interesting findings.

3.2. Presence of Multiple External Wounds

The main anomaly found on the animal’s body consisted of multiple superficial wounds made up of lacerations of varying lengths (<5 cm) and were generally parallel to each other, mainly located in three distinct places. These lacerations were apparently caused by friction. The first, covering an area approximately 5 cm high and 3 cm wide on average, was located on the head at eye level, with another behind it on the right side of the animal (absent on the left) (Figure 1D). The second markings were in the form of bands of parallel lacerations, two of which, each about 15 cm long and 3 cm wide, were arranged in a V-shape, straddling the animal’s back, positioned slightly behind the gills and the leading edge of the pectoral fins (Figure 1C); the other bands, including one about 20 cm long, were on the animal’s right flank (Figure 1E,E’). The other set of markings was located on the dorsal fin in the form of longer lacerations but without any particular pattern, particularly on the right side (Figure 1B). Two deep slashes, resembling deeper cuts rather than friction-based lacerations as described above, were observed on the ventral side of the left pelvic fin (Figure 1F). On the left flank, above the pelvic fins, several small punctures were observed arranged in a slightly curved arc, likely tooth imprints (Figure 1F’).

3.3. Assessment of Internal Organs

The exploration of the internal abdominal cavity and internal organs was characterized by the overall absence of any major abnormality in terms of size, appearance, or coloration (Figure 2). The spiral intestine was completely empty (Figure 2A). Only the gills (Figure 2B) looked abnormally pale. The heart (Figure 2C) and the liver (Figure 2D) showed no abnormalities. The exploration of the fundic stomach revealed the presence of a blood clot containing several squid beaks (Figure 2E,E’). The ovaries (Figure 2F) showed no abnormalities.

4. Discussion

4.1. Death Likely Caused by an Accidental Capture in a Net

The presence of multiple superficial lacerations, mostly arranged in parallel, leaves little doubt as to their origin, namely the imprint of the symmetrical meshes of a gillnet in which the animal was entangled. This assumption is consistent with the mesh size (approximately 50 mm) of the gillnets used in this part of the Mediterranean. These marks probably result from the shark’s efforts to free itself from this restraint, potentially accentuated by the pressures inherent in hauling up and handling the net by the fishermen. The most likely hypothesis is that the shark died from chronic asphyxiation at sea and was subsequently transported and then thrown overboard inside the harbor. The multiplicity of skin aftermarks suggests that it struggled and agonized for some time. This hypothesis is reinforced by the presence of parallel lacerations (in the form of cuts) on the left pelvic fin and marks arranged in an arc of a circle, facing the flank, which are probably the result of a predator’s bite, probably linked to an attempt at depredation. The characteristics of these marks point to another shark, bearing in mind that intra- and inter-specific depredation among sharks is a frequent and proven occurrence [20]. This depredation, potentially carried out by a conspecific, as suggested by the characteristics of the distant teeth imprints, one from each other, typical of mako jaws, was unsuccessful, as the shark likely struggled and deterred its attacker.

4.2. Hypothesis of a Nursery off Marseille (Southeastern France)

The place at sea where the shark was caught is impossible to determine within the scope of our investigation, but it was likely close to the port of Estaque, in the waters around Marseille, which hosts several fishing boats using gillnets. As the birth size of this species varies between 60 and 70 cm TL [2,9], the reduced size of the individual (<65 cm) suggests that it was not a Young-Of-the-Year (YOY) shark but more likely a newborn. Although the lack of adequate photographic details does not allow full confirmation, the first and second tooth files seem to differ significantly in size (Figure 1A), suggesting that this may be one of the earliest tooth sets of this individual (see the CT scan of a late embryo of I. oxyrinchus in [21]); additionally, the squids found in the stomach (Figure 2B) could be one of its first meals. These findings suggest that the area in question could be a nursery for shortfin mako sharks. The fact that the individual is a newborn makes it highly improbable that this animal made a long journey after birth to end up in this area, confirming its potential nursery status. The month of May also fits with the known parturition period of this species, which spans from late winter (February) until mid-spring in the Eastern Mediterranean [22]. However, the literature definition of a nursery [23] requires compliance with three criteria: (1) higher-than-average density of YOY sharks in the area, (2) tendency for YOY sharks to remain or return to the area for extended periods, and (3) the area being used repeatedly over the years. The identification of this individual is therefore insufficient to conclude that this area is a nursery, as has been established for Turkey [24,25], the Ionian coast of Calabria [26], and the Strait of Sicily [27]. So far, the ELASMED database in France includes very few (n = 4) validated sightings of this species, including one in the area of Marseille [19]; as far as we know, none of them were newborn individuals. However, the testimony of a professional artisanal fisherman (G. Carrodano, Pers. Comm.) from La Ciotat (30 km east of Marseille) reports very rare but multiple catches of young mako individuals (<1 m TL) in the waters off Marseille in April, consistent with our findings. This context underscores the importance of the information provided in this study regarding a potential nursery that could play a critical role in improving the conservation of a critically endangered species in the Mediterranean Sea. As recently demonstrated in the Southeast Pacific, the detection of a breeding area, vital for the survival of this specific endangered species, should lead to the lifting of fishing pressure in this area [28]. Based on this scarce and promising observation of a newborn mako shark in this area of the Mediterranean Sea, assiduous monitoring over the coming years, accompanied by appropriate information for fishermen on the importance of revealing the presence of this species, which may interact with their activity, particularly YOY individuals <80 cm TL, could validate our hypothesis of the existence of this nursery, as has recently occurred in the Eastern Mediterranean and the Strait of Sicily (see above for references).