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. 2026 Mar 20;109(1):594–598. doi: 10.1111/jfb.70420

First record of young‐of‐the‐year smooth hammerhead ( Sphyrna zygaena ) in coastal waters of Galicia, northwest Spain

Xabier Pin 1, Kenn Papadopoulo 2,3, Pablo Covelo 1, Gonzalo Mucientes 2,4,✉
PMCID: PMC13397273  PMID: 41860198

Abstract

The smooth hammerhead (Sphyrna zygaena) is typically considered an oceanic species and is only rarely observed in the cooler coastal waters of the northeastern Atlantic. On 20 December 2024, a juvenile S. zyganea was captured at 25 m depth in the inner Ría de Pontevedra (Rías Baixas, northwest Spain). This represents the first verified record of a young‐of‐the‐year individual in Galicia, and potentially across the entire Iberian Peninsula. This observation, together with four confirmed adult sightings in Galician waters in recent years, indicates the sporadic presence of the species far beyond its recognised nursery and distributional range. Such occurrences may reflect an emerging northward or inshore shift, potentially linked to climate‐driven changes in regional oceanographic conditions. Documenting these atypical events enhances our understanding of hammerhead shark ecology in temperate Atlantic systems and underscores the need to identify potential nursery habitats within European waters.

Keywords: Galicia, northeast Atlantic, nursery habitat, smooth hammerhead, Sphyrna zygaena, young‐of‐the‐year

Sharks occupy a wide range of habitats, from estuarine and marine environments to, in rare cases, freshwater systems (Ebert et al., 2021). As apex and mesopredators, they play pivotal roles in maintaining ecosystem balance (Dedman et al., 2024). Yet, over the past century, large‐bodied shark populations have declined sharply due to overexploitation, key habitat loss and shifting environmental conditions (Dulvy et al., 2024).

The smooth hammerhead (Sphyrna zygaena, Linnaeus 1758) is a cosmopolitan pelagic species found in temperate and tropical waters worldwide (Weigmann, 2016). Although frequently taken as bycatch in pelagic longline fisheries (Bezarra et al. 2016), it remains among the least studied of the large oceanic sharks (Coelho et al., 2011). Unlike its congeners, the scalloped (S. lewini) and great hammerhead (S. mokarran), which rely on tropical nursery grounds (Chiriboga‐Paredes et al., 2022; Cuevas‐Gómez et al., 2020; Duncan & Holland, 2006), S. zygaena is generally regarded as more oceanic (Santos & Coelho, 2018). Even so, juveniles are occasionally reported from shallow coastal and estuarine habitats before migrating offshore (Francis, 2016), though their Atlantic distribution remains poorly described.

Nursery grounds are crucial for the early life stages of hammerhead sharks, providing refuge from predators and access to abundant prey (Hlavin & Macdonald, 2025; Páez‐Rosas et al., 2024). In the eastern Atlantic, however, the location and characteristics of S. zygaena nursery areas remain largely unknown (Santos & Coelho, 2018). Along the Iberian Peninsula, observations are sporadic and typically involve large adults in offshore waters (Couto et al., 2018), with neonates virtually undocumented. The hammerhead species S. mokarran exhibit natal philopatry, with gravid females returning to coastal and estuarine zones to give birth (Guttridge et al., 2017). However, such behaviour has not been verified for S. zygaena either in the Atlantic Ocean or at a global scale, although studies have documented female philopatry and fidelity to specific birthing sites in the north Pacific Ocean (Félix‐López et al., 2019). Recent stable isotope studies revealed ecological connectivity between adult females and neonates in regions such as the Eastern Tropical Pacific, including the Galápagos Marine Reserve, suggesting site fidelity and localised reproductive strategies (Hlavin & Macdonald, 2025). In S. lewini, nursery populations connected by continuous coastlines exhibit high levels of connectivity, while oceanic dispersal by females appears to be rare (Duncan et al., 2006); additionally, ontogenetic shifts in diet and habitat use have been documented, with juveniles depending on coastal resources before moving offshore (Páez‐Rosas et al., 2024).

Here, we document the first confirmed record of a young‐of‐the‐year (YOY; individual born within the current reproductive year and still in its first year of life) S. zygaena captured within a sheltered ría system in northwestern Spain. The Galician Rías are drowned river valleys with estuarine circulation and high productivity influenced by seasonal coastal upwelling.

On 20 December 2024, a juvenile smooth hammerhead (Sphyrna zygaena) was incidentally captured by the gillnet of the artisanal fishing vessel Abuelo Sebas operating in the inner Ría de Pontevedra (Rías Baixas, northwest Spain) at 25 m depth, near Punta Festiñanzo (42.381°N, 8.774°W; Figure 1a,b). The specimen was retained and transported to the Coordinadora para o Estudo dos Mamíferos Mariños (a Galician non‐governmental organisation dedicated to the study and conservation of marine mammals and sea turtles) laboratory for morphometric and biological examination.

FIGURE 1.

FIGURE 1

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(a) Capture and sighting locations of five Sphyrna zygaena individuals along the Galician coast, including the young‐of‐the‐year specimen (location 4). (b) Locations within Europe. (c, d) Sphryna zygaena young‐of‐the‐year specimen (52 cm fork length, 66.5 cm total length) captured on 20 December 2024 in the inner Ría de Pontevedra (Galicia, northwest Spain). (e) Dorsoventral radiograph of the head showing cephalofoil morphology and absence of skeletal deformities and internal injuries.

Standard biometric data were recorded, including fork length (FL), precaudal length (PCL), total length (TL) and body weight (see Table S1 for full dataset). Species identification was confirmed based on diagnostic morphological features: absence of a median cephalofoil indentation, presence of a pronounced prenarial groove near the eye, a free rear tip of the second dorsal fin positioned well anterior to the origin of the upper caudal lobe and an anal‐fin base approximately equal in length to that of the second dorsal fin (Ebert et al., 2021). The specimen was inspected for external and internal parasites. Stomach content and reproductive analyses could not be performed due to advanced post‐mortem autolysis of the digestive and reproductive systems. Radiographic (X‐ray) imaging was conducted to detect potential skeletal malformations or internal injuries. In addition, we compiled verified reports of adult S. zygaena sightings along the Galician coast to contextualise this finding and evaluate potential northward range extensions of the species in temperate European waters.

The captured specimen was identified as a YOY female S. zygaena (Figure 1c,d), measuring 51.5 cm FL, 66.5 cm TL and 47.1 cm PCL, with a body weight of 1273 g. These values fall within the species' known size‐at‐birth range (maximum 55 cm TL; Vooren et al., 2005; Rosa et al., 2017), confirming its classification as a YOY individual.

External examination revealed no injuries, scars or ectoparasites. The oral cavity and dentition were intact. Internally, the liver (33 g) and central nervous system (9 g) exhibited advanced autolysis, precluding detailed anatomical assessment; kidneys (3 g), heart (2 g) and gallbladder were structurally normal. Radiographs showed no skeletal malformations and the absence of internal wounds caused by billfish (Figure 1e), for example swordfish‐shark interactions reported previously and explained by a potential defensive strategy (Romeo et al., 2020). Complete biometric and anatomical data are provided in Table S1.

This represents the first confirmed record of a YOY S. zygaena in Galician waters and potentially in the Iberian Peninsula. Its presence may reflect broader ecological pressures, climate change, oceanographic variability or shifts in prey distribution, allowing juveniles to occupy previously undocumented coastal habitats, suggesting a possible inshore expansion of nursery or foraging grounds. Comparable patterns have been observed with juvenile blue sharks (Prionace glauca) along the Galician coast since 2013 (Bañón et al., 2016; Mejuto et al., 2014; Mucientes et al., 2018), indicating that temperate upwelling zones may serve as recruitment or feeding grounds for pelagic sharks.

Hammerhead sharks are known to exhibit natal philopatry, with females returning to specific coastal zones to give birth (Barker et al., 2017; Zanella et al., 2019). This behaviour has been observed in Sphyrna mokarran along the nearshore waters of the eastern coast of North America (Barker et al., 2017). Additionally, S. mokarran shows limited migratory movements and a strong dependence on coastal and insular habitats (Boube et al., 2025), highlighting the ecological importance of coastal nursery habitats. The YOY record in Galicia might also indicate previously unrecognised reproductive behaviour or habitat use in the northeastern Atlantic.

In addition, four previously reported adult sightings in Galicia (Figure 1 and Table 1) were verified through photographs and direct observations, including the most recent record near the Cíes Islands, at the edge of the continental shelf, in September 2025. Adults exceeded 2 m TL and were observed within 100 m of the shoreline, further supporting potential shifts in spatial distribution, possibly linked to seasonal migrations or rising surface temperatures. In the south of Portugal, where a S. zygaena ‘hotspot’ was recently described near Sagres and Martinhal Bay and islands, the abundance was significantly associated with sea surface temperature (Couto et al., 2018). Similar seasonal and migratory behaviours have been reported in the Canary Islands, where juveniles and subadults are relatively frequent (Dippenaar et al., 2021; López‐Martínez et al., 2020).

TABLE 1.

Records of smooth hammerhead (Sphyrna zygaena) in Galician waters.

ID Date Location, area Latitude, longitude Source/vessel Size (TL cm) Life stage
1 26/09/2022 Fisterra, A Coruña 42.858°N, 9.404°W Birdwatching/M. Rodríguez Esteban (observer) >300 Adult
2 13/09/2023 Praia da Marosa, Burela 43.677°N, 7.376°W Small scale fisheries/“Mariñeiro Dous” fishing vessel >300 Adult
3 23/09/2023 Continental shelf, Ría de Arousa 42.400°N, 9.133°W Birdwatching/“Chasula Birds” touristic vessel >200 Adult
4 20/12/2024 Punta Festiñanzo, Ría de Pontevedra 42.381°N, 8.774°W Small‐scale fisheries/“Abuelo Sebas” fishing vessel 66.5 Young‐of‐the‐year
5 15/09/2025 Edge of continental shelf, Ría de Vigo 42.228°N, 9.363°W Research survey/“Miguel Oliver” research vessel >200 Adult
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Note: Observations of adults and the present young‐of‐the‐year specimen.

Collectively, these observations underscore the ecological significance of S. zygaena in temperate coastal systems. These records provide critical baseline data for monitoring elasmobranch diversity, habitat use and potential range shifts in the northeastern Atlantic, enabling future detection of changes in the population size and reproductive activity of S. zygaena.

AUTHOR CONTRIBUTIONS

Kenn Papadopoulo and Gonzalo Mucientes compiled the sightings and wrote the manuscript. Xabier Pin performed the necropsy. Pablo Covelo provided the photographs of the observation conducted in the Ría de Arousa. All authors revised and approved the final version.

FUNDING INFORMATION

The Galician stranding network is supported by the Biodiversity Foundation of the Spanish Ministry for Ecological Transition, through the Revargal project, which is financed by the European Union—NextGenerationEU through the Recovery, Transformation and Resilience Plan; and also, by the Xunta de Galicia—Galician Ministry of the Environment. Both administrations also provide the legal permits for the handling and sample collection of protected species. K. Papadopoulo was supported by ‘Ayuda para Contratos Predoctorales’ Grant PREP2022‐000726 funded by MICIU/AEI/10.13039/501100011033 and by the ESF+. G. Mucientes was supported by the Juan de la Cierva Grant JDC2023‐050436‐I funded by MICIU/AEI/10.13039/501100011033 and by the ESF+.

CONFLICT OF INTEREST STATEMENT

The authors declare no competing interests.

Supporting information

TABLE S1. Complete biometric dataset obtained during the necropsy of a young‐of‐the‐year Sphyrna zyganea specimen recovered in the Ría de Pontevedra (Galicia, northwest Spain) on 20 December 2024. Biometric reference and morphological measures extracted from Ebert et al. (2020).

JFB-109-594-s001.docx (29.1KB, docx)

ACKNOWLEDGEMENTS

We would like to thank the skipper of the vessel Abuelo Sebas, Iago Tomé Blanco, and his son, Iago Tomé Silva. We also extend our gratitude to Juan Chapela Portela, on‐board observer for the Unidade Técnica de Pesca de Baixura, Dirección Xeral de Desenvolvemento Pesqueiro, Consellería do Mar de la Xunta de Galicia, for notifying us and allowing us to collect the specimen. We thank Julio Valeiras for providing photographs and data collected during the scientific survey DESCARSEL 2025. Finally, we thank the ESVET veterinary clinic for allowing us to perform the radiographs.

DATA AVAILABILITY STATEMENT

The data that supports the findings of this study are available in the supplementary material of this article.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

TABLE S1. Complete biometric dataset obtained during the necropsy of a young‐of‐the‐year Sphyrna zyganea specimen recovered in the Ría de Pontevedra (Galicia, northwest Spain) on 20 December 2024. Biometric reference and morphological measures extracted from Ebert et al. (2020).

JFB-109-594-s001.docx (29.1KB, docx)

Data Availability Statement

The data that supports the findings of this study are available in the supplementary material of this article.

Articles from Journal of Fish Biology are provided here courtesy of Wiley

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