Billfishes (swordfish, sailfish, spearfishes and marlins) are large highly migratory predators including 12 species and two families, i.e., Xiphiidae and Istiophoridae (Nakamura, 1985). They are considered important fishery resources in many countries (Collette et al., 2011; Pons et al., 2017), with populations impacted by both commercial and recreational fishing (Holland et al., 1998; Ditton and Stoll, 2003). Globally, less than half of billfish stocks exhibit healthy biomass levels and nearly a quarter are still experiencing overfishing (Pons et al., 2017). However there are cases (i.e. most spearfishes) where catch data are discontinuously reported and stocks have not yet been assessed (Punt et al., 2015; Pons et al., 2017). Technological advances have significantly increased our understanding of billfish biology and ecology, including through the use of electronic satellite tags to reveal horizontal and vertical movement behaviors (Braun et al., 2015). Previous satellite tagging studies have primarily focused on swordfish, sailfish, and marlins, while only a limited amount of research has been devoted to spearfishes (Tetrapturus spp.). Indeed, over 1000 satellite tags were deployed on billfish globally by 2014, however only 0.2% were on spearfish species (Braun et al., 2015), highlighting the need for more research on spearfishes.

Among spearfishes, Tetrapturus belone is endemic from the Mediterranean Sea (MED), where it is almost exclusively distributed (Nakamura, 1985; Collette and Graves, 2019). The status of this stock has not yet been assessed by ICCAT, due to the lack of sufficient data, although the species is assessed as Least Concern in the IUCN Red List of Threatened Species (Collette et al., 2023). Information on the distribution patterns and seasonal movements of T. belone in the MED is still incomplete and data on the occurrence of this species in the Levantine Basin (Bariche and Fricke, 2020; Gerovasileiou et al., 2020; Saad et al., 2024) and western MED (Robins and de Sylva, 1963) are often fragmentary. However, T. belone is distributed around the Italian peninsula, primarily found in the Strait of Messina, Tyrrhenian Sea and Strait of Sicily (Di Natale et al., 2005, 2011; Di Natale, 2012). Furthermore, many of the fisheries taking T. belone occurred (i.e. driftnets) or are occurring (i.e. longlines, harpoon, recreational fishing) mostly in spring, summer or early autumn. This temporal bias may limit our understanding of the species’ distribution during late autumn and winter. Additionally, there is a lack of information regarding the distribution of juvenile individuals.

T. belone is an important fish resource for some Mediterranean local markets, in particular in the area of the Strait of Messina where this species is caught and commercialized by local professional harpoon fishers (Romeo et al., 2009a, 2015; Malara et al., 2020). However, catch and landings data are often under-reported (Collette et al., 2023), thus contributing to the lack of information on this species. Most data available in the literature comes from catch series obtained from harpoon (e.g., Di Natale et al., 2005; Romeo et al., 2015), driftnets (currently banned) and longline fisheries (Di Natale et al., 2003). According to Collette et al. (2023), landings are largely unknown, but they have recently increased (above 100 t/year), despite only Italy, Spain and Portugal occasionally and partly providing catch data to ICCAT. Recently, some fragmentary additional data are also available from recreational fishing (Collette et al., 2023) and the European Union Marine Strategy Framework Directive.

According to the available knowledge, the Strait of Messina and surrounding areas play an important role in different life stages of the Mediterranean spearfish providing nursery, reproductive and foraging habitats (Spartà, 1960). Adult T. belone feed primarily on epipelagic fish and cephalopods (Romeo et al., 2009a, 2012), but can also act as a gelatinous plankton opportunistic consumer (Cardona et al., 2012). The regular presence of T. belone in the Strait of Messina during late spring and summer (mainly from May to July/August) is thought to be related to reproduction (Spartà, 1960; Potoschi, 2000) but may also be associated with food availability (Robins and de Sylva, 1963).

To date, there is minimal information about the movements, behavior, ethology, and ecology of T. belone (Robins and de Sylva, 1963; Braun et al., 2015). Only recently, one study investigated the horizontal and vertical movement behavior of T. belone in the MED collecting data on one tagged individual (Arostegui et al., 2019a) while another tagging study failed due to premature death and a subsequent scavenging event (Malara et al., 2020). Arostegui et al. (2019a) discovered that, on average, T. belone spent 93% of each 24-hr period above 30 m and preferred water layers between 26–28°C but occasionally descended into deeper, colder waters. In addition, the tagged fish exhibited a diel vertical migration spending more time in the near-surface at night and in deeper water layers during the day.

The aim of this paper is to help fill the knowledge gap on T. belone by investigating the movements and behavior of multiple individuals of this species in Mediterranean waters via satellite tagging. Specifically, we document the seasonal distribution and assess the depth and thermal preference of T. belone, relating the behavior to the available environmental data from tag-recorded parameters and to the diel cycle. These data will be useful as they increase knowledge on the species’ behavior and can help guide management decisions.

This research documented the seasonal variability in horizontal and vertical movements, as well as the thermal and depth occupation, of six T. belone in the MED using PSAT tags. Our data supplement and permit inference beyond the information from the first tagging experiment on this species (Arostegui et al., 2019a) which was restricted to a single individual.

In general, tagged spearfish dispersed from the release locations, moving to other areas of the Tyrrhenian Sea, Strait of Sicily, Ionian Sea and, in one case, towards Cypriot and Greek waters, where this species is considered less common (Akyol, 2020; Gerovasileiou et al., 2020). None of the tagged fish visited the western MED beyond Sardinia or to the Adriatic Sea, although, no data are available in spring, due to the premature detachment of the tags. T. belone showed significant mobility within the Mediterranean basin, with a mean straight-line distance from the tag area of 297.01 km, a mean total track distance of 1415.46 km, and mean daily displacement rate of 14.04 km/day. The only direct comparison could be done with the T. belone tagged in summer 2015 in the MED (Arostegui et al., 2019a), which travelled at a matching mean speed of 14.51 km/day. Similarly high extents of dispersal and degrees of mobility are known from other billfishes such as Kajikia audax, Istiompax indica and Makaira nigricans in other areas (Domeier, 2006; Rohner et al., 2020, 2021; Andrzejaczek et al., 2023).

Despite the variation in movement behavior among individuals, in most cases, fish remained within the Tyrrhenian Sea or around Sicily. Previous research suggests these areas are regularly used by T. belone as feeding (Romeo et al., 2009a, 2012) and reproductive grounds (Spartà, 1960; De Sylva, 1975). The spawning period of T. belone occurs between May and September, with eggs and larvae found from late spring to summer and juveniles in the fall in the Strait of Messina (Spartà, 1960). Therefore, our results support the hypothesis that the Strait of Messina and the surrounding seas are important areas for the ecology and life-history of T. belone. Previous studies have also identified the Strait of Messina and southern Tyrrhenian Sea as important spawning and/or nursery grounds for other large pelagic fishes such as swordfish and bluefin tuna (Nishida et al., 1998; Romeo et al., 2009b; Di Natale et al., 2018). The occurrence of juvenile spearfish as far east in the MED as the waters of Israel and Lebanon (Nakamura, 1985) and seasonal influxes of adult spearfish (some observed in pairs or aggregations seemingly engaged in courtship and/or spawning) as far west as Mallorca (Vince Riera, pers. comm.) and the southern coast of France (Bill François, pers. comm.) suggest the presence of spawning grounds throughout the basin. Yet, none of the tagged individuals in this study migrated to these longitudinally distant locations. Further long-term tagging to be carried out in more areas of the MED, is needed to elucidate the stock structure and potential connectivity or regional fidelity of Mediterranean spearfish. Furthermore, while our study provided evidence of the movements of T. belone within the Tyrrhenian Sea, around Sicily and in one case also in eastern MED, it is important to acknowledge that the species’ distribution extends beyond these regions. Additional literature suggests that T. belone is also present in various parts of the Mediterranean, including the Adriatic Sea and the Ligurian Sea (Dulcic and Soldo, 2004; Psomadakis et al., 2012). Moreover, the species is expected to inhabit the entire Mediterranean Sea, encompassing areas such as the Strait of Gibraltar and the near Atlantic region adjacent to the Strait (Nakamura, 1985; Collette and Graves, 2019). Therefore, future research efforts should aim to explore the movement patterns and distribution of T. belone across the broader Mediterranean basin to obtain a comprehensive understanding of its ecology and population dynamics.

Mediterranean spearfish in this study preferred the epipelagic environment as suggested in a previous study (Arostegui et al., 2019a), with more than 99% of time spent above 50 m and more than 70% at 0-10 m. Despite the MED being a temperate basin with unique oceanography, the epipelagic behavior was also observed in billfishes living in other oceanic areas (mostly tropical and equatorial) characterized by different physical-chemical features: T. angustirostris (Arostegui et al., 2019b, 2024), M. nigricans (Andrzejaczek et al., 2023), K. albida (Vaudo et al., 2017), T. pfluegeri (Kerstetter et al., 2009), I. indica and K. audax (Rohner et al., 2022). In addition, tagged individuals were observed to spend nearly 99% of their time above the 50-meter mark, suggesting a foraging strategy that focuses on prey within the epipelagic zone (Romeo et al., 2009a, 2012). However, some differences between night and daylight were observed, with the tendency to occupy deeper depths during the day than night. This was also observed in other billfishes such as T. pfluegeri (Kerstetter et al., 2009), M. nigricans (Andrzejaczek et al., 2023), Kajikia albida (Vaudo et al., 2017), and Istiophorus platypterus (Pohlot and Ehrhardt, 2017) but the reverse of the pattern characteristic of T. angustirostris (Arostegui et al., 2019b, 2024). Furthermore, recent study by Arostegui et al. (2019a) reported a maximum depth of 84 m reached by an individual equipped with a PSAT tag, while Nakamura (1985) supposed a vertical distribution not exceeding 200 m. However, we observed for the first time that this species is also able to perform deep excursions into mesopelagic waters, with recorded dives below 200 m and a maximum depth of 322.8 m.

Diving into the mesopelagic zone below 200 m may serve various ecological functions (e.g. predator avoidance, feeding activity; Braun et al., 2022) and has been observed in other billfish species (i.e., M. nigricans, I. indica, K. audax; Rohner et al., 2022; Andrzejaczek et al., 2023). We found that the depth range of T. belone extended to greater depths during fall and winter, reaching depths > 200 m. This behavior is likely due to the changes in the seasonal thermocline during fall and winter (Houpert et al., 2015). The seasonal variations in the depth of the mixed layer within the Mediterranean Sea significantly influence the vertical distribution of thermoclines, subsequently affecting the spatial distribution of prey species (Sabatés et al., 2009). Mediterranean spearfish, similar to other billfishes, are likely to modulate their vertical movements in response to these fluctuations (Braun et al., 2015), aiming to optimize feeding opportunities within the portion of the water column that is accessible to this species based on its thermal physiology (Arostegui et al., 2019a). These fluctuations in the MED mixed layer depth play a pivotal role in shaping the thermal structure of the water column, thereby potentially influencing the behavior and depth preferences of apex predators as documented elsewhere (Block et al., 2001). While we could not retrieve the thermocline depth and or mixed layers information from our tagging data, we were able to obtain sea surface temperature of the water column < 5m. Sea surface temperature is vital for influencing the mixing layer and thermocline depth (Houpert et al., 2015), and it is crucial during spawning seasons, enabling animals to spawn when the water temperature is optimal (Boyce et al., 2008). Indeed, environmental temperature affects metabolic rate (Fry, 1947; Clarke and Johnston, 1999) which, among other parameters, influences billfish distribution and behavior (e.g., Dale et al., 2022).

During winter months in the MED, heightened wind activity and cooler temperatures lead to increased mixing of the water column and the formation of deeper mixed layers (Heslop et al., 2012). Similarly, our tags recorded SST in winter exhibited a significant region-specific temperature decrease ( Table 4 ). The coolest temperatures were recorded in the Tyrrhenian Sea, followed by the Sicilian Channel and the Aegean Sea. These cooler temperatures are characteristic of winter conditions and are potentially influenced by seasonal currents or regional weather patterns (Zveryaev, 2015). Conversely, in the summer months, the MED experiences shallower mixed layers due to warmer temperatures and reduced wind activity (Heslop et al., 2012). Tag-based SST reached its peak, with the Ionian Sea recording the highest average temperature, likely due to exposure to solar radiation contributing to its warmer temperatures. The Tyrrhenian Sea, although slightly cooler than the Ionian Sea, still maintained warm temperatures conducive to fish activity ( Table 4 ). In response, large pelagic organisms may seasonally shift the distribution of their vertical movements to continually exploit prey associated with this thermal gradient, promoting increased metabolic rates and feeding activity among fish populations. In the fall, the Mediterranean Sea experiences changes in water thermocline and mixing, affecting large pelagic animals. Cooling temperatures and shifting wind patterns lead to a shallowing of the thermocline and increased water column mixing (Houpert et al., 2015). SST exhibited notable variation across different regions. The Aegean Sea and Tyrrhenian Sea recorded moderate temperatures, while the Ionian Sea experienced slightly cooler conditions. On the contrary, the Sicilian Channel exhibited significantly warmer temperatures, likely influenced by local environmental factors. These changes can influence spawning behaviors, migration patterns, and foraging strategies of large pelagic animals as they adapt to shifting environmental conditions in pursuit of prey and suitable spawning grounds.

The sea surface temperatures discussed here closely align with the mean seasonal SST observed in the MED across all regions (Skliris et al., 2011; Shaltout and Omstedt, 2014; Zveryaev, 2015; García-Monteiro et al., 2022). The rising sea surface temperatures in the Mediterranean Sea significantly impact marine ecosystems (Calvo et al., 2011; Lange, 2020), affecting billfish populations (Dell’Apa et al., 2018). Satellite data confirms this rise (Calvo et al., 2011; Lange, 2020), potentially altering billfish distribution, behavior, and health (Alix et al., 2020). This could lead to shifts in habitat range and migratory patterns (Alix et al., 2020). Additionally, higher sea temperatures may decrease billfish growth rates, reproductive success, and increase susceptibility to diseases (Alix et al., 2020). Understanding these impacts is crucial for conserving marine ecosystems, especially for apex predators.

The results of this paper improve the current understanding of the ecology, migratory behavior, and habitat preference of T. belone, as well as help in identifying crucial knowledge gaps that might help understand the distribution of the species within its range of occurrence. In particular, to better understand the migratory patterns of this species it is necessary to increase the tagging efforts, expanding to a broader range of the Mediterranean. In concert with approaches from other disciplines, such as genetics, additional tagging data would help resolve whether there is a single panmictic stock or multiple differentiated stocks within the basin, which is paramount to ensuring appropriate management strategies for this fish resource. Furthermore, additional research must also be conducted to collect yearly/seasonal catch data (landings by country, season, size composition) as well as life history information that are key factors for future stock assessments of this fishery resource.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The animal study was approved by SZN animal welfare body “case 04/2020/ec AWB-SZN -27 June 2020 – ricerca in natura fuori applicazione DLgs 26/2014”. The study was conducted in accordance with the local legislation and institutional requirements.

DM: Conceptualization, Data curation, Formal Analysis, Investigation, Methodology, Validation, Visualization, Writing – original draft, Writing – review & editing. PB: Conceptualization, Investigation, Methodology, Supervision, Writing – original draft, Writing – review & editing. MA: Formal Analysis, Writing – original draft, Writing – review & editing, Data curation. JD: Writing – original draft, Writing – review & editing. BB: Writing – original draft, Writing – review & editing. CB: Formal Analysis, Writing – original draft, Writing – review & editing. MB: Writing – original draft, Writing – review & editing, Investigation. SG: Funding acquisition, Writing – original draft, Writing – review & editing, Supervision. TR: Conceptualization, Funding acquisition, Supervision, Writing – original draft, Writing – review & editing, Project administration.