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. Author manuscript; available in PMC: 2021 Oct 11.
Published in final edited form as: Mediterr Mar Sci. 2021;22(1):169–179. doi: 10.12681/mms.24624

Using historical and citizen science data to improve knowledge about the occurrence of the elusive sandbar shark Carcharhinus plumbeus (Chondrichthyes – Carcharhinidae) in the Adriatic Sea

Patrick L Jambura 1,✉, Ilija Ćetković 2, Jürgen Kriwet 1, Julia Türtscher 1
PMCID: PMC7611800  EMSID: EMS135986  PMID: 34642564

Abstract

The Mediterranean Sea is an extinction hotspot for sharks, skates, and rays, with at least half of all species being threatened. The sandbar shark Carcharhinus plumbeus is among the most threatened species and is declared endangered (EN) in the Mediterranean according to the IUCN Red List of Threatened Species. The Adriatic Sea was previously proposed to be an important nursery area for this species based on sporadic sightings of neonates and juveniles. However, this species is generally considered to be rare in this region, and reports of its presence are scarce. Here, we present new records for the sandbar shark in the Adriatic Sea and review spatial and temporal data accumulated from literature, museum specimens, and citizen science initiatives in an attempt to assess the distribution and seasonality of this species. The majority of the records were from neonate and juvenile sandbar sharks, and three hotspots for their occurrence could be identified: (1) the Emilia-Romagna region, (2) the Veneto region, and (3) the Gulf of Trieste. Our data indicate a high concentration of sandbar sharks around the delta of the Po river, emphasising the need for scientific surveys in this confined area in order to confirm a possible nursery ground. Data collected by citizen science initiatives reveal that this species is still present in the Adriatic Sea and might be more abundant than previously thought, underlining the potential of citizen science as a valuable tool to monitor the distribution of rare and endangered species.

Keywords: Elasmobranchii, Estuaries, Mediterranean Sea, Migration, Neonates, Parturition

Introduction

The Mediterranean Sea has been exploited by intensive coastal fishing for millennia (Lotze et al., 2006) and is regarded as a key hotspot of extinction risk for sharks, skates, and rays (Dulvy et al., 2014). Many large shark species have been severely depleted, with reported declines of between 96 and 99.99% (Ferretti et al., 2008), and half of the chondrichthyan species in the Mediterranean Sea are threatened by extinction (Abdulla, 2004; Dulvy et al., 2016). Large-bodied species are at the greatest risk, especially those living in shallow waters, which makes them more exposed to fisheries and habitat degradation (Ferretti et al., 2008; Dulvy et al., 2014). The utilisation of inshore coastal areas as nursery grounds has become a particular liability for sharks, skates, and rays, as direct and indirect fishing pressures have intensified and coastal habitat loss and degradation have accelerated in the Mediterranean (Abdulla, 2004).

The sandbar shark Carcharhinus plumbeus (Nardo, 1827) is a large, bottom-dwelling, shallow coastal water species that is distributed worldwide in temperate and tropical seas. Sandbar sharks can grow to a length of 240 cm and possibly up to 300 cm (Soufi-Kechaou et al., 2018), with males and females reaching sexual maturity at 130–180 cm and 145–185 cm (Ebert et al., 2013) and at the age of 8 and 10 years, respectively (Romine et al., 2006). This coastal species’ slow growth, late maturation, and low fecundity of 1–14 (mean 6.5) pups every second year make it highly vulnerable to overfishing (Saïdi et al., 2005; McAuley et al., 2007; Ebert et al., 2013).

The sandbar shark used to be a common shark in the Mediterranean Sea, but it is currently regarded as being severely overfished (Dulvy et al., 2016) and has become locally extinct along the French coast (Capapé et al., 2000) and the Sea of Marmara (Kabasakal, 2015; 2019). Besides overfishing, habitat degradation of this species’ coastal nursery areas through coastal development and pollution poses a significant threat for sandbar sharks in the Mediterranean Sea. According to the IUCN Red List of Threatened Species, the sandbar shark is declared globally vulnerable (VU) and endangered (EN) in the Mediterranean Sea (Dulvy et al., 2016).

The Adriatic Sea is a highly productive area, but it is also strongly influenced by anthropogenic pressures, such as fisheries, nutrient inputs, and coastal development (Barausse et al., 2009; 2011). Excessive long-term exploitation and overfishing in this area led to a steady decline and, ultimately, the collapse of elasmobranch catches in the 1980s and 1990s. However, large-sized apex and mesopredatory sharks were already depleted before 1945 (Barausse et al., 2014). The sandbar shark was originally described from the Adriatic Sea as Squalus plumbeus (Nardo, 1827), but its presence in this region is solely documented by opportunistic reports, and it is therefore regarded to be a rare species (Kirinčić & Lepetić, 1955; Travaglini, 1982; Costantini & Affronte, 2003; Lipej et al., 2000; 2008; Dragičević et al., 2010; Fortuna et al., 2010; Ćetković, 2018). Captured neonate sandbar sharks with unhealed umbilical scars indicate that the Adriatic Sea, especially the northern basin, might serve as a nursery area (Costantini & Affronte, 2003; Dragičević et al., 2010). However, due to the scarcity of published records, little is known about the distribution, seasonality, and abundance of sandbar sharks in the Adriatic Sea, making effective conservation management for this species even more challenging in this highly productive but also heavily exploited Mediterranean sub-basin.

In the light of the data deficiency for rare and elusive species, several authors have proposed the use of alternative sources of information, such as local ecological knowledge (Stephenson et al., 2016) or citizen science (Giovos et al., 2019a). Citizen science –an approach where the general public participates in scientific data collection–has been proven to be a valuable tool for gathering information about rare and endangered species, e.g., abundance (Becerril-García et al., 2020), distribution (Araujo et al., 2020; McDavitt & Kyne, 2020), migration routes (Armstrong et al., 2019; 2020), behavioural patterns (Chin, 2014; Jambura et al., 2020), and critical habitats (Norton et al., 2012; Jiménez-Alvarado et al., 2020).

This study reviews the occurrence of the sandbar shark C. plumbeus in the Adriatic Sea and provides additional records, including the first reported occurrences off the Albanian coast. We are combining published, historical, and citizen science data to (1) evaluate the species’ distribution and (2) seasonal patterns of occurrences and (3) to find further evidence of possible nursery areas in the Adriatic Sea. Furthermore, we briefly discuss the role, weaknesses, and strengths of citizen science data in an increasingly digital and data-driven world.

Materials and Methods

The Adriatic Sea is an elongated basin located in the Central Mediterranean Sea between the Italian peninsula and the Balkans. The six countries bordering the Adriatic Sea in clockwise order are Italy, Slovenia, Croatia, Bosnia and Herzegovina, Montenegro, and Albania. The Adriatic Sea can be divided into three areas: the North Adriatic, the Central Adriatic, and the South Adriatic. The North Adriatic is the largest shelf area in the Mediterranean, with an average bottom depth of 35 m, rarely exceeding a depth of 100 m. This part is characterised by the largest riverine contributions into the basin, which induce intense phytoplankton development in autumn and winter (Cushman-Roisin et al., 2001; Kraus & Supić, 2011). The Central Adriatic has an average depth of 140 m and includes the Middle Adriatic Pit, which has a mean depth of 260 m. The South Adriatic is connected to the Ionian Sea by the Strait of Otranto and is characterised by a depression more than 1200 m deep, the deepest area in the Adriatic (Artegiani et al., 1997; Cushman-Roisin et al., 2001).

Between July 23 and August 17, 2019, four sandbar sharks (C. plumbeus) were caught by recreational fishermen close to the Po delta in the Northern Adriatic Sea (44.975833°N, 12.559722°E) and reported to the citizen science initiative known as the MECO (Mediterranean Elasmobranch Citizen Observations) Project. Pictures of three specimens were taken prior to releasing the sharks alive back into the water (Fig. 1). The species was identified based on the following features: (1) stout body, greybrown or bronze in colour on the dorsal side and white on the ventral side; (2) no obvious markings on the tips and posterior edges of the fins; (3) moderately long rounded snout; (4) presence of an interdorsal ridge; (5) very large erect first dorsal fin with origin over or slightly ahead of the large pectoral fins insert (Ebert et al., 2013).

Fig. 1.

Fig. 1

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Sandbar sharks (Carcharhinus plumbeus) caught in the Po delta, Italy. Reports are inventoried in the MECO database under the following identification numbers: (A) MECO #DE2019_0039, (B) MECO #DE2019_0038, and (C) MECO #DE2019_0040. Detailed information about each observation number is shown in Table 1. Picture courtesy of Michael Kinzer & Robert Engel.

Additionally, a database of sandbar shark occurrences in the Adriatic Sea was assembled based on published data, sightings reported to the MECO Project, entries from the sharkPulse web platform (Ferretti et al., 2020), and historical data from museum collections. Carcharhinus plumbeus is the only confirmed species of this genus in the Adriatic Sea (Kovačić et al., 2020). Therefore, sharks reported as Carcharhinus sp. in the sharkPulse app were also included in our database, and if the image allowed identification on a species level, they were reassigned to C. plumbeus accordingly. All entries in the MECO database and the sharkPulse web platform are underlain with images and were validated by the authors. Following Saïdi et al. (2005), we distinguished between four life-history stages, which were determined based on the shark’s total length: (1) neonate (♀ 50–65 cm, ♂ 45–65 cm and/or with unhealed umbilical scar); (2) juvenile (♀ 65–164 cm, ♂ 65–138 cm); (3) subadult (♀ 164–171.5 cm, ♂ 138–160 cm); (4) adult (♀ > 171.5 cm, ♂ > 160 cm).

Results

The three sandbar shark specimens reported from the Po delta were all females. Images allowed the total lengths of two of them to be estimated at 140 and 160 cm, respectively. Therefore, they are within the reported size spectrum for juvenile sandbar sharks in the Mediterranean Sea (65–164 cm) but are close to the subadult stage. There are two additional records from the Po delta from sharkPulse: On July 25, 2017, and July 19, 2018, two sandbar sharks were caught and released by recreational fishermen. No data on size or sex are available for these two specimens (Fig. 2, Table 1).

Fig. 2.

Fig. 2

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Spatial distribution of different ontogenetic stages of the sandbar shark (Carcharhinus plumbeus) in the Adriatic Sea. Observations were gathered from historical, citizen science, and published data. Detailed information about each observation is shown in Table 1.

Table 1.

Records of sandbar sharks (Carcharhinus plumbeus) from the Adriatic Sea. Entries were rallied from published data, museum collections, and citizen science initiatives (MECO Project, sharkPulse). Data from the MECO database came from social media platforms or were communicated directly to scientists involved in the MECO Project. Abbreviations: MSNM, Museo Civico di Storia Naturale di Milano; MCSNT, Museo Civico di Storia Naturale di Trieste; MCSNV, Museo Civico di Storia Naturale, Venezia; MZF, Museo Zoologico “La Specola”, Firenze; NHMW, Naturhistorisches Museum Wien; PMD, Prirodoslovni Muzej Dubrovnik; PMR, Prirodoslovni Muzej Rijeka; PMS, Prirodoslovni Muzej Split.

Country Region Date Size [cm] Weight
[kg]		 Sex Ontogeny Source
Albania Fier, Divjakё 30.06.2016 - - female (sub)adult sharkPulse source: alb365.com
Fier, Divjakё 04.05.2019 - - male - MECO #DE2019_0041
Croatia Dubrovnik-Neretva, Dubrovnik 1960 – 1979 92 - - juvenile PMD-6
Dubrovnik-Neretva, South Dubrovnik 28.05.1952 123 10.3 female juvenile Kirinčić & Lepetić (1955)
Split-Dalmatia, Brač Channel - - - - - PMS
Split-Dalmatia, Makarska Riviera - - - - - PMS
Split-Dalmatia, Split 04.1895 56 - male neonate NHMW-61349
Split-Dalmatia, Zaostrog 23.11.2009 78.5 3.65 female neonate+ Dragičević et al. (2010)
Zadar, Zadar 1903 78 - male juvenile PMR-04408
Italy Adriatic Sea - 174 - - adult MCSNT-ic2901
Abruzzo, Ortona 09.2017 - - - - sharkPulse source: sharkPulse*
Apulia, Frigole 25.05.2020 - - - - MECO #DE2020_0008
Apulia, Manfredonia 05.06.2014 150 100 - juvenile sharkPulse source: sharkyear.com
Emilia-Romagna, Porto Garibaldi 2006-2008 - - - - Fortuna etal. (2010)
Emilia-Romagna, Porto Garibaldi 22.08.2018 - - - - sharkPulse source: Fabio Reca
Emilia-Romagna, Ravenna 26.07.2017 - - - neonate/juvenile sharkPulse source: CESTHA
Emilia-Romagna, Ravenna 18.10.2017 - - - neonate/juvenile sharkPulse source: CESTHA
Emilia-Romagna, Ravenna 01.08.2018 50 - - neonate sharkPulse source: CESTHA
Emilia-Romagna, Ravenna 01.08.2018 50 - - neonate sharkPulse source: CESTHA
Emilia-Romagna, Riccione summer 1981 200 70 female adult Travaglini (1982)
Emilia-Romagna, Riccione 05.09.1999 46.5 0.46 male neonate+ Costantini & Affronte (2003)
Emilia-Romagna, Riccione 09.09.2000 60 1.36 male neonate+ Costantini & Affronte (2003)
Emilia-Romagna, Riccione 16.09.2000 52 0.94 male neonate+ Costantini & Affronte (2003)
Friuli Venezia Giulia, Gulf of Trieste - 49 - - neonate MCSNT-ic0071
Friuli Venezia Giulia, Gulf of Trieste 1955 80 - - juvenile MSNM-Pi 3389
Friuli Venezia Giulia, Marano Lagunare 03.09.1998 67.9 1.94 male neonate+ Costantini & Affronte (2003)
Friuli Venezia Giulia, Marano Lagunare 23.09.1998 68.8 2.04 female neonate+ Costantini & Affronte (2003)
Marche, Civitanova Marche 17.09.1884 - - male juvenile MZF-5872
Veneto, Caorle 21.08.1999 67.5 2.42 male neonate+ Costantini & Affronte (2003)
Veneto, Chioggia 29.07.1908 - - male - MCSNV-4863
Veneto, Po delta 23.07.2019 140 - female juvenile MECO #DE2019_0038
Veneto, Po delta 14.08.2019 - - female juvenile MECO #DE2019_0039
Veneto, Po delta 17.08.2019 160 - female juvenile MECO #DE2019_0040
Veneto, Porto Barricata 19.07.2018 - - - - sharkPulse source: Barricata Tuna Club
Veneto, Porto Levante 25.07.2017 - - - - sharkPulse source: Andrea Grassi
Veneto, Venice - 50 - female neonate MCSNV-2670
Veneto, Venice 02.1879 - - - adult MZF-6351
Veneto, Venice 04.1879 - - - subadult MZF-6330
Veneto, Venice 08.09.1898 51 - female neonate MCSNV-2665
Veneto, Venice - 152 - male subadult MCSNV-21342
Montenegro Budva, Budva 01.2009 - - - juvenile MECO #MNE-117
Ulcinj, Ada Bojana 14.05.2017 110 2.8 female juvenile Ćetković (2018)
Ulcinj, Ada Bojana 29.08.2020 150 35 male subadult MECO #MNE-135
Slovenia Piran, Gulf of Trieste 17.10.2000 71 2.6 female juvenile Lipej et al. (2000)
Piran, Gulf of Trieste 27.10.2000 81.5 3.6 - juvenile Lipej et al. (2000)
Piran, Gulf of Trieste 14.10.2007 70.5 2.34 male neonate+ Lipej et al. (2008)
Piran, Gulf of Trieste 25.12.2019 83 4 female juvenile MECO #DE2019_0042
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*

Carcharhinus sp.

+

visible umbilical scar reported

The established database incorporates 47 records of sandbar sharks in the Adriatic Sea from 1879 to 2020. The majority of the entries arose from citizen science reports (38.3%; n=18), while 31.9% (n=15) were from museum collections, and 29.8% (n=14) were from published data. In the period between 2000 and 2020, 26 records were collected; 69.2% (n=18) were from citizen science initiatives, 30.8% (n=8) were published, and none came from museum collections (Fig. 2, Table 1).

Information about ontogenetic stages and sex are available for 34 and 26 specimens, respectively. The vast majority (82.4%) were neonates and juveniles (41.2%, n=14 each), while subadults and adults only constituted 8.8% (n=3) each.

Most records came from the Northern Adriatic Sea (67.4%, n=31), followed by the Southern Adriatic Sea (21.7%, n=10) and the Central Adriatic Sea (10.9%, n=5). One historical record of an adult sandbar shark, deposited in the Natural History Museum Trieste (MCSNT-ic2901), is labelled as being caught in the Adriatic Sea but without further information about the locality.

Sandbar sharks were present in the Adriatic Sea throughout the whole year, with March being the only month without any record. Thirty-six records provided detailed temporal information (month of capture/sighting), of which the majority were encountered between April and October (88.9%, n=32). Occurrences peaked in May and June in the Southern Adriatic Sea (n=5) and from July to October in the Northern Adriatic (n=21). Only three records in the Central Adriatic Sea had temporal information: a juvenile caught near Split, Croatia, in April and two sandbar sharks caught near Ortona and Civitanova Marche, Italy, in September (Fig. 3, Table 1).

Fig. 3.

Fig. 3

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Temporal distribution of sandbar sharks (Carcharhinus plumbeus) observed in the Adriatic Sea. The number of observations is low during the winter months and starts to increase in spring, reaching a peak in summer/autumn, and it rapidly decreases at the end of autumn.

Records of the sandbar shark Carcharhinus plumbeus in the Adriatic Sea by country

Albania

Two sandbar sharks were reported from Albania. One was from a local newspaper article shared on social media, while the second one came from sharkPulse. To our knowledge, these very recent reports (2016 and 2019) are the only known records from the Albanian coast. Both sharks were caught by recreational fishermen near the town of Divjakё. Unfortunately, no measurements were taken. However, the record from sharkPulse indicates that the shark was similar in size to the fisherman and, therefore, represents either a subadult or adult female (Table 1).

Bosnia and Herzegovina

There are no records available from the 20-km-long coastline of Bosnia and Herzegovina.

Croatia

Seven records were collected from Croatia between 1895 and 2009. Most of them came from museum collections (n=5), while only two sightings, one from Dubrovnik (Kirinčić & Lepetić, 1955) and one from Zaostrog (Dragičević et al., 2010), were published. No citizen science data are available for this region. Three specimens were juveniles, two were neonates, and two did not have a known ontogenetic stage. The northernmost occurrence was recorded in Zadar, while the remaining occurrences were from Split-Dalmatia (n=4) and Dubrovnik (n=2) (Table 1).

Italy

With 31 records, more than half (66%) of all sandbar shark records in the Adriatic Sea came from Italy. The database entries cover the years from 1879 to 2020. Most of the entries came from citizen science initiatives (41.9%, n=13), followed by museum collections (32.3%, n=10) and published data (25.8%, n=8). The vast majority of all individuals with a known ontogenetic stage were either neonates or juveniles (79.2%; nneo =11, njuv =6, nneo/juv =2), while two were subadults, three were adults, and seven did not have a known ontogenetic stage. One of the adults was reported to be a pregnant female (Travaglini, 1982), while there are no data on the sex of the other two specimens. Three main hotspots for sandbar shark occurrences could be detected, all of which are situated in the northern part of Italy: (1) the Emilia-Romagna region, (2) the Veneto region, and (3) the Gulf of Trieste (Table 1).

Montenegro

Three records were collected from Montenegro between 2009 and 2020. Two specimens were juveniles caught in gillnets in depths between 20–40 m, while the third one was a male subadult caught close to the Albanian border (Table 1). This species is considered rare in Montenegrin waters (Ćetković, 2018).

Slovenia

Four specimens caught in the Gulf of Trieste were landed in Piran, Slovenia. Three records came from published data (Lipej et al., 2000; 2008), and one came from a local newspaper article in 2019. Three of the specimens were described as juveniles and one as a large neonate (70.5 cm) with an unhealed umbilical scar.

Discussion

The presence of the sandbar shark in the Adriatic Sea has been well known since the 19th century (Nardo, 1827; Faber, 1883; Brusina, 1888). Paradoxically, published data on its occurrence in this region have remained scarce, probably due to misidentification with other shark species (Lipej et al., 2000). The present study reports new records of the sandbar shark C. plumbeus in the Adriatic Sea, including the first occurrences off Albania, and gives a thorough revision of this species’ distribution in this area. Sandbar sharks were previously regarded to be rare in the Adriatic Sea (Faber, 1883; Brusina, 1888; Kirinčić & Lepetić, 1955; Lipej et al., 2009), even though this was questioned by various other authors (Lipej et al., 2004; Lipej et al., 2008; Dragičević et al., 2010). Our results confirm the continuous presence of the sandbar shark in the Adriatic Sea and add 18 new records since 2009, 16 of which were from the last five years (Fig. 4).

Fig. 4.

Fig. 4

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Records of sandbar sharks (Carcharhinus plumbeus) in the Adriatic Sea gathered from specimens deposited in museum collections, published data, and citizen science initiatives from the last (A) 150 years; (B) 20 years.

Spatial patterns and occurrence hotspots

The vast majority of the records came from the North Adriatic basin and could be assigned to three hotspots along the Italian-Slovenian coast: (1) the Emilia-Romagna region, (2) the Veneto region, and (3) the Gulf of Trieste. These regions held a high concentration of neonate and juvenile sandbar sharks, and only two neonates were caught outside these hotspots; Dragičević et al. (2010) reported the capture of a female neonate sandbar shark near Zaostrog (Split-Dalmatia) in autumn. With a total length of 78.5 cm, this specimen was well outside the usual size range for neonates (45–65 cm). However, an unhealed umbilical scar was still visible, and therefore, we kept its original assignment as a neonate. The second neonate specimen recorded outside the North Adriatic basin was significantly smaller (56 cm) and was caught in Split in spring 1895. Whether these specimens migrated there from the North Adriatic or this region represents another critical area for this species in the Adriatic Sea cannot be answered yet. Lipej et al. (2000) mentioned that sandbar sharks are commonly found at fish markets in different Dalmatian towns in the Middle Eastern Adriatic but are often misidentified and rarely published. This is also indicated by our data, which clearly demonstrate the presence of this species along the Croatian coast but are based mainly on historical data from museum collections. Recent occurrences are rare in this region (Kirinčić & Lepetić, 1955; Dragičević et al., 2010), and therefore, we urge for a thorough revision of the presence of this species in this area, including strengthening the efforts of citizen science to find critical habitats for this species along the eastern Adriatic coast.

Records of subadult and adult specimens were generally scarce in the Adriatic Sea and restricted to the South Adriatic basin off Albania and the western shores of the North Adriatic basin. Only one report came from this region, which was of a 2-m-long pregnant female sandbar shark that was caught 15 km off Riccione (Italy, North Adriatic) and was carrying nine still-living embryos (Travaglini, 1982). Although the prevalence of neonate and juvenile specimens indicate the presence of a possible nursery area in this region, an explanation for this apparent lack of pregnant females in this heavily fished area is still pending.

Temporal trends

The presence of sandbar sharks in the Adriatic Sea was reported throughout the whole year. Sightings increased between May and June in the southern part of the Adriatic Sea and from July to October in the northern part. Most sightings of neonate and juvenile sandbar sharks occurred in the western part of the Northern Adriatic Sea during the summer months (July to September) and later in October in the Gulf of Trieste in the east. Whether neonates stayed there or migrated further south cannot be answered with certainty yet due to the rare reports in winter. We cannot exclude that the low number of records during the winter months is an artefact, as the Adriatic coast is more crowded during summer, making sightings by citizen scientists more likely during this time. Thus, a year-round survey would be necessary to confirm such an increase during the summer months. However, a similar timing for the occurrence of neonate sandbar sharks was reported in the Gulf of Gabès (Tunisia, Central Mediterranean; Bradaï et al., 2005) and Yumurtalık Bight (Turkey, Levantine Sea; Başusta et al., 2021), indicating a true trend rather than an observational bias. Both regions are considered to be nursery areas, where pregnant females migrate to give birth in late spring to early summer and neonate sandbar sharks appear between July and October (Bradaï et al., 2005; Enajjar et al., 2015), coinciding with the emergence of neonate sandbar sharks in the Northern Adriatic Sea.

The Northern Adriatic Sea as a possible nursery area

Sporadic reports of predominantly neonate and juvenile sharks in the shallow North Adriatic basin, led several authors to suggest that it is an important nursery area for this species (Costantini & Affronte, 2003; Lipej et al., 2008). Heupel et al. (2007) emphasised that the mere occurrence of juvenile sharks in an area is insufficient evidence to proclaim a nursery area, as such a liberal definition would lead to potentially identifying all coastal waters as nurseries and thus undermine the importance of protecting essential fish habitats. To avoid such a dilution of the term nursery, they defined shark nurseries based on three criteria: (1) Neonate and juvenile sharks are more commonly encountered in the area than in other areas, (2) immature sharks have a tendency to remain in or return to the area for extended periods, and (3) the area is repeatedly used by immature sharks over a period of years. Our accumulated dataset on sandbar shark records in the Adriatic Sea revealed that the presence of juvenile specimens in this region has been known since at least 1884. Data collected by citizen science initiatives that have been established in recent years, like the MECO Project and sharkPulse (Ferretti et al., 2020), indicate an annual occurrence of neonate and juvenile sandbar sharks in the Northern Adriatic Sea. Additionally, immature individuals of different ages (46.5–160 cm) are present in this area, and therefore, all three criteria established for defining shark nurseries are fulfilled by the Northern Adriatic Sea. The absence of adult males is another indicator that this area represents a nursery ground (Castro, 1987; 1993). It must be noted that records of adult sandbar sharks in the Adriatic Sea are generally rare, indicating that they only remain there for a short time (e.g., for parturition) and leave the area quickly after, while immature individuals stay for an extended period. The recurrence of neonate specimens is confined to the Northern Adriatic Sea, while older juveniles are also found further south. This leads us to the conclusion that parturition most likely takes place in the northern basin, as suggested in previous studies (Costantini & Affronte, 2003; Lipej et al., 2008; Dragičević et al., 2010).

Our data indicate an unusually high concentration of sandbar sharks in the Venetian region around the delta of the Po river. The mouths of rivers are productive ecosystems that provide abundant food sources, which can facilitate rapid early growth in neonates (Castro, 1987) and, therefore, act as important nursery areas for a number of requiem sharks, including the sandbar shark (Castro, 1993; Grubbs et al., 2007; Ulrich et al., 2007; Bangley et al., 2018). The nutrient-rich freshwater outflow of the Po river is the major contributor to the high production rates in the Northern Adriatic Sea (Degobbis et al., 2000), which is especially pronounced in its western part close to the delta (Kraus & Supic 2011; Giani et al., 2012) and coincides with the spatial distribution of neonates and juveniles between July and September. Both neonates and large juveniles were reported in this area not just historically, but also very recently. Consequently, we consider this region around the Po delta to represent a very promising study area for conducting systematic surveys to fully resolve the abundance, demographic composition, and migration of sandbar sharks in the Northern Adriatic Sea and to prove the existence of a nursery area in this region.

Impact, strengths, and weaknesses of citizen science

Although the citizen science data were collected only opportunistically, they constitute the majority of all sightings since 1879 and suggest that this species is more abundant in this area than previously thought. Citizen science programs are growing in number around the world and provide valuable data on species occurrences, changes in distribution, phenology, invasive species, and threatened species and can thus make tangible conservation contributions (Giovos et al., 2016; 2019a, b; 2020; Chin & Pecl, 2018; Taklis et al., 2020). The strength of citizen science in detecting even very elusive species was also demonstrated in previous studies on the rare river sharks Glyphis spp. in Papua New Guinea (White et al., 2015) and, in the case of the smalltooth sawfish Pristis pectinata in the southern United States, even led to the detection of critical habitats for this species, which are now protected (Norton et al., 2012). However, this approach also comes with drawbacks: Although in most cases, images from citizen scientists allowed the approximate size to be estimated and, therefore, an ontogenetic stage to be allocated, exact measurements could not be conducted. This was also an issue in previous studies incorporating citizen science data (e.g., White et al., 2015; Panayiotou et al., 2020) and could be overcome by training citizen scientists (e.g., teaching them how to take pictures properly and with a scale for better size estimates) before systematically letting them collect data in the field. Nonetheless, this study shows that the strength of citizen science does not simply lie in its ability to engage the public in research and conservation but also in that it can be an invaluable source of data collection and monitoring species distribution. Nevertheless, systematic scientific surveys should accompany these citizen science initiatives in the future to augment our understanding of the abundance and spatial and seasonal distributions of this species in the Adriatic Sea and to help confirm a possible nursery area in the northern basin. Identifying these critical habitats is crucial to improve the management of fisheries in this area and ensure the survival of the sandbar shark C. plumbeus in the Mediterranean Sea.

Acknowledgements

We want to thank Michael Kinzer, Robert Engel, and Raymond Salathe for reporting catches and providing data on sandbar sharks in the Po delta. Additionally, we would like to thank a number of curators and collection managers from various institutions, including Bettina Riedel (Natural History Museum Vienna), Jadranka Sulić Šprem (Natural History Museum Dubrovnik), Marcelo Kovačić (Natural History Museum Rijeka), Bressi Nicola (Natural History Museum Trieste), Giorgio Bardelli (Milan Natural History Museum), Paolo Agnelli (Museum of Zoology and Natural History “La Specola”, Firenze), and Luca Mizzan (Natural History Museum of Venice), for providing information on museum specimens. We would also like to thank Jürgen Pollerspöck from the Shark-References database, Anita Marušić (technical editor of Acta Adriatica), Stefano Donca Aicardi (University of Genoa), and Tadej Gračner (National Institute of Biology, Piran) for assisting with literature essential for this work. We are grateful to three anonymous reviewers for their comments on a previous version of this manuscript.

This study was supported by a grant of the Austrian Science Fund (FWF): P 33820 to Jürgen Kriwet.

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