|Scientific Name:||Prionace glauca (Linnaeus, 1758)|
Carcharhinus macki (Phillipps, 1935)
Carcharias glaucus (Linnaeus, 1758)
Carcharias gracilis Philippi, 1887
Carcharias hirundinaceus Valenciennes, 1835
Carcharias pugae Pérez Canto, 1886
Carcharias rondeletii (Risso, 1810)
Carcharinus glaucus (Linnaeus, 1758)
Galeus thalassinus Valenciennes, 1835
Glyphis glaucus (Linnaeus, 1758)
Hypoprion isodus Philippi, 1887
Isurus glaucus (Linnaeus, 1758)
Prionace mackiei Phillipps, 1935
Squalus glaucus Linnaeus, 1758
Squalus adscensionis Osbeck, 1765
Squalus caeruleus Blainville, 1816
Squalus rondeletii Risso, 1810
Thalassorhinus vulpecula Valenciennes, 1839
|Taxonomic Source(s):||Weigmann, S. 2016. Annotated checklist of the living sharks, batoids and chimaeras (Chondrichthyes) of the world, with a focus on biogeographical diversity. Journal of Fish Biology 88(3): 837-1037.|
|Red List Category & Criteria:||Near Threatened (Regional assessment) ver 3.1|
|Assessor(s):||Sims, D., Fowler, S.L., Ferretti, F. & Stevens, J.D.|
|Facilitator/Compiler(s):||Walls, R.H.L., Frazer, K. & Dulvy, N.K.|
European regional assessment: Near Threatened (NT)
Blue Shark (Prionace glauca) is found throughout the world’s oceans in temperate and tropical waters. It has been considered the most widespread and abundant chondrichthyan fish and is the most frequently caught shark species. It is relatively fast-growing and fecund, maturing in four to six years and producing average litters of 35 pups, one of the highest maximum annual rates of population increase of all commercially-fished oceanic pelagic shark species (at 29% per year, more than three times greater than the other species). Blue Shark is taken in large numbers by commercial fisheries (estimated at >10 million individuals annually; ca. 1.6 million in the North Atlantic), mainly as bycatch, but in more recent years in the Northeast Atlantic it is now targeted. Many catches are unreported and fishing effort is not declining; Blue Shark catch remains unregulated in the Northeast Atlantic. Declining catch rates over time in some northern regions of the Northeast Atlantic suggest a suspected range contraction. Although large data sets are available for this species, there are considerable difficulties reconciling the different spatial scales and relevance of the wide range of ecological and fisheries information to produce a coherent picture of fisheries or threat status. Most catch rates are available for the Northwest Atlantic and the Mediterranean Sea, but few are available for the Atlantic European waters. In particular, standardisation of catch rates for this shark is a major problem, with the International Commission for the Conservation of Atlantic Tuna (ICCAT) stock assessment being driven by the longer but less informative time series.
There is strong evidence for increased landings, particularly by Spain, in the past decade and a recent decline of Blue Shark catch rates from the Northwest Atlantic and the Mediterranean Sea. Declines ranged from 53% (over 13 years, more than one generation) to 88% (over 19 years, about two generations) in the Northwest Atlantic and >78-90% in the Mediterranean Sea over three generations. Given that North Atlantic Blue Sharks comprise a single subpopulation, we suspect that declines in the Northwest Atlantic may be representative of the whole North Atlantic subpopulation.
Further analysis and reconciliation of Northeast Atlantic data is required, to be able to confirm the full scale of declines across its European range. Its current regional assessment is Near Threatened, based on a decline of close to 30% over the last three generations (30 years). This status is assigned pending a more extensive, Atlantic-wide stock assessment, preferably in the near future. It is suspected that this status will be increased to Endangered based on the masking of greater declines than currently estimated. We consider the Mediterranean Sea to be a distinct subpopulation and on the basis of steep declines this subpopulation is inferred to have declined sufficiently to qualify as Critically Endangered in the Mediterranean Sea.
|Range Description:||Blue Shark is one of the most wide-ranging of all sharks, found throughout tropical and temperate seas from latitudes of about 60°N, 50°S. It is found throughout the Atlantic and in the Mediterranean Sea. In the Northeast Atlantic, adult females, many of which are pregnant, are found around the Canary Islands and North African coast in winter (Stevens 1990). Adult males are found further north, mainly off Portugal, along with juveniles and sub-adult females, the latter group undertaking a summer migration into the western English Channel and Irish waters (Stevens 1976, Fitzmaurice et al. 2005). Adult males and juveniles are also found in offshore regions, particularly off the Azores (Litvinov 2006). Blue Shark occurring in the North Atlantic is widely considered a distinct stock from Southern Atlantic Blue Shark.|
Blue Shark occurring in the Mediterranean Sea is inferred to be a distinct subpopulation from the North Atlantic subpopulation on the basis of very limited exchange historically using ordinary tags and zero exchange based on recent satellite tracking. Data from tag-recapture programs and satellite tags suggest there was very little exchange of individuals between the North Atlantic Ocean and the Mediterranean Sea. Kohler and Turner (2008) analysed 39 years of tag-recapture data from 1962 to 2000. Of 90,752 individuals tagged in the North Atlantic, only two were recaptured in the Mediterranean Sea: one coming from the Northwest, and one from the Northeast Atlantic. Additionally, of the 698 sharks tagged in the Mediterranean Sea, 25 were recaptured there and only one was recovered in the Northeast Atlantic. Hence, there was, on average, less than one migrant per generation, with only two individuals crossing into the Mediterranean Sea and one leaving in four generation spans. A similar synthesis of tag-recapture data was conducted by Fitzmaurice et al. (2005). Of 16,719 specimens tagged and released by the Irish rod and reel fishery between 1970 and 2001, only two individuals were recaptured in the Mediterranean Sea (Fitzmaurice et al. 2005). Again this is less than the one individual per generation threshold. Similarly, Stevens (1990) reported tag-recapture locations of 2,585 Blue Sharks tagged between 1970 and 1981. All sharks were tagged off the United Kingdom in cooperation with the Shark Angling group of Great Britain. None of the tagged sharks were recaptured in the Mediterranean Sea.
Tag and recapture data need to be interpreted with caution because relocations depend on the range of fishing fleets and they do not provide information on the actual movements of the tagged sharks. Recent analyses of telemetry data obtained from satellite pop-off tags on Blue Sharks tagged off Portugal (47 individuals), and southeast England (six individuals) show larger clockwise movements throughout the Eastern Atlantic adjacent to the Gibraltar Straits, but no incursions into the Mediterranean Sea (Queroz et al. 2012, Vandeperre et al. 2014). Its depth range is zero to 1,160 m.
Native:Albania; Algeria; Belgium; Bosnia and Herzegovina; Croatia; Cyprus; Denmark; Egypt (Egypt (African part), Sinai); France (Corsica, France (mainland)); Greece (Greece (mainland), Kriti); Ireland; Israel; Italy (Italy (mainland), Sardegna, Sicilia); Lebanon; Libya; Malta; Montenegro; Morocco; Netherlands; Norway; Palestinian Territory, Occupied; Portugal (Azores, Madeira, Portugal (mainland), Selvagens); Slovenia; Spain (Baleares, Canary Is., Spain (mainland), Spanish North African Territories); Sweden; Tunisia; United Kingdom (Great Britain, Northern Ireland)
|FAO Marine Fishing Areas:|
Atlantic – northeast; Atlantic – eastern central; Mediterranean and Black Sea
Blue Sharks occurring in European waters are part of the wider North Atlantic stock that is exploited by fisheries from over 20 nations. Official Food and Agriculture Organization of the United Nations (FAO) statistics underestimate the true magnitude of catches: landings estimated from Blue Shark fin exports from the Atlantic Ocean alone are known to greatly exceed the reported catches from this area (Campana et al. 2006, ICCAT 2009). In the past decade since 2003, there has been a rise in catch landings reported to the FAO, particularly taken by Spain in the Northeast Atlantic. Catches of this shark increased by 62,000 tonnes globally with a large fraction of this taken from the Eastern Central Atlantic by Spain (Davidson et al. in press). This rise is likely to reflect increasing fishing pressure over and above increased reporting.
Demographic models, catch-rate analysis, age-structured models, food web and ecosystem models have been applied (or attempted) for this species, mainly in the North Atlantic and North and Central Pacific, and these yield a conflicting picture of Blue Shark status (West et al. 2004, ICCAT 2009). Increasing fin prices, further depletion of less productive sharks and lack of management (particularly on the high seas) will lead to greater pressure on Blue Shark stocks (Clarke et al. 2007) and for those population components occurring in European waters.
In the North Atlantic, different catch rate analyses of this species consistently show that it is declining but there is uncertainty as to the most likely decline rate. Analysis of logbook data from the U.S. pelagic longline fishery indicated that Blue Sharks declined by 60% between 1986 and 2000 (1.5 generation spans) (Baum et al. 2003), and Canadian standardised catch rate indices suggest a 5-6% decline per year since 1995 in the North Atlantic (Campana et al. 2006). Similarly, fishery-independent survey data indicate an 80% decline in males from the mid-1980s to early 1990s (Hueter and Simpfendorfer 2008). Cortes (2007) reports an 88% decline since 1986. Blue Shark standardized catch rates have decreased by 53% (CI: 38–64%) between 1992 and 2005 (Baum and Blanchard 2010). An analysis over a longer observation window (1950–2000) using multiple sources of data suggested that CPUE of Blue Shark declined by 30% (Aires-da-Silva et al. 2008). This overall 30% of decline comprised of two periods: an initial stage of stable abundance or even increase at the end of the 70s, and a second period of rapid decline. From recent catch patterns and these Western Atlantic surveys that these most recent declines have been the steepest. Note that these trend estimates end in the early 2000s prior to the recent increase in catches of Blue Sharks, particularly in the Eastern Central Atlantic (Davidson et al. in press).
In contrast, fisheries agency analyses for international waters of the North Atlantic suggest little change (ICCAT 2009), although it is acknowledged that catches vary considerably across fleets and that reported catches over time are known to represent only a portion of total removals of the species (ICCAT 2009). The ICCAT standardised catch per unit effort (CPUE) series showed that the relative catch rate of Blue Shark increased from 1994 to 1998, but declined from 1998 to 2006. The ICCAT assessments suggest the Blue Shark is neither overfished nor is overfishing occurring, however, these assessments are deeply at odds with independent population trajectory data, catch levels, and trade demand patterns.
In the Northeast Atlantic, there are no population trends specifically for Blue Shark in European waters that are inferred from effort-corrected catch rates of commercial fisheries. Blue Shark occurring in Irish and United Kingdom (UK) waters are near the northern extent of their range in the Northeast Atlantic and are summer migrants from core population areas further south, such as the Azores and Atlantic Iberian waters (Queiroz et al. 2012). Although not considered a population trend, the catch per unit effort index from the Irish recreational fishery based on data from >16,000 individual sharks caught shows a current decreasing trend. CPUE (Blue Shark per boat per day) varies annually, but generally increased between 1980 and 1997 with a steady decline in catches between 1998 and 2006 to about 50% of mean catch made prior to 1997 (Green et al. 2009). Declining trends in recreational fisheries that catch northerly migrating Blue Sharks in summer may signify range contractions.
In the Mediterranean Sea, the population trends have been inferred from an index appropriate for the species. Since the mid-20th century Blue Shark has declined in abundance by three to four orders of magnitude. It was regularly caught in the tuna trap of Camogli in small numbers, but none have been caught in the past six decades (Ferretti et al. 2008). Recent observations from the same gear confirm that no individuals were taken from 2006 to the present day (Cattaneo-Vietti et al. 2014). It was the most abundant of the large predatory sharks taken in pelagic fisheries, but catch rates in the Mediterranean Sea have declined considerably. In the northern Ionian Sea, there were significant declines in abundance and biomass over 21 years (1978–1999). In Spanish waters, catch rates in biomass declined steeply in 25 years (1979–2004) as well. Pelagic fishing pressure remains high and catches unregulated. Taking into consideration other local trend estimates in abundance and biomass, a meta-analytical estimate of these trend analyses suggested that the abundance of Blue Shark has declined around 78–90% over the last 30 years (i.e. three generation period).
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||Blue Shark is oceanic and pelagic, found from the surface to at least 1,160 m depth (Queiroz et al. 2012). Occasionally, it occurs inshore where the continental shelf is narrow. It prefers temperatures of 12-20°C and is found at greater depths in tropical waters (Last and Stevens 1994).|
The species reaches a maximum size of about 380 cm total length (TL). About 50% of males in the Atlantic are sexually mature at 218 cm TL, although some may reach maturity as small as 182 cm TL. Females are sub-adult from 173-221 cm TL and fully mature from 221 cm TL (Pratt 1979).
This placental live-bearing shark produces litters averaging ~ 35 pups (maximum recorded 135 pups) after a gestation period of nine to 12 months. At birth, the pups are 35-50 cm TL. Reproduction has been reported as seasonal in most areas, with the young often born in spring or summer (Pratt 1979, Stevens 1984, Nakano 1994) although the periods of ovulation and parturition may be extended (Strasburg 1958, Hazin et al. 1994). Females can store sperm in their nidamental glands for extended periods, for later fertilisation (Pratt 1979). Ageing studies estimate longevity to be ~20 years, with males maturing at four to six and females at five to seven years (Stevens 1975, Cailliet et al. 1983, Nakano 1994). Smith et al. (1998) estimated the intrinsic rate of population increase at maximum sustainable yield (MSY) to be 0.061. The generation period (average age of mature females in an unfished population) is estimated as 10 years (Dulvy et al. 2008).
Blue Sharks are highly migratory with complex movement patterns and spatial structure related to reproduction and the distribution of prey. There tends to be a seasonal shift in population abundance to higher latitudes associated with oceanic convergence or boundary zones, as these are areas of higher productivity. In the North Atlantic, mark-recapture studies indicate that different segments of the population (i.e., juveniles and adults, males and females) exhibit disparate movement patterns (Stevens 1976, 1990; Casey 1985). In the western Atlantic, the population mainly comprises juveniles of both sexes and adult males. As adults of both sexes are found in the Northeast Atlantic during spring and summer, it is thought that mating occurs at this time (Stevens 1990). Adult females, many of which are pregnant, are found around the Canary Islands and North Africa in winter, while mature males are found further north off Portugal, along with juveniles and sub-adult females. The presence of young-of-the-year Blue Shark indicates that parturition occurs in nurseries off the Iberian Peninsula, particularly off Portugal but also in the Bay of Biscay (Stevens 1990, Litvinov 2006). As juveniles are not known from the western Atlantic, it seems likely that the eastern Atlantic is an important breeding area for the North Atlantic Blue Shark population (Stevens 1990, Kohler et al. 2002). Some individuals have shown trans-Atlantic movements of up to 7,176 km (Casey 1985, Stevens 1990). In addition, seasonal north-south movements, such as the summer migration into the western English Channel and Irish waters (Stevens 1976, Fitzmaurice et al. 2005) from the Iberian Peninsula and Gulf of Cadiz (Queiroz et al. 2012), and the existence of trans-equatorial movements linking the northern and southern hemispheres of the Atlantic Ocean, have also been observed (Stevens 1990).
Satellite tagging studies in the Northeast Atlantic confirm wide-ranging southward movements away from summer tagging areas (English Channel, southern Portugal and the Azores Islands) as waters cool seasonally, with sharks covering minimum distances of 268-2,789 km in 13-70 days (Queiroz et al. 2010). Vertical movements ranged from the surface to a maximum depth of 1,160 m, and water temperatures varied from 10.6 to 24.6°C. Tracked sharks exhibited pronounced site fidelity correlated with localised productive frontal areas, including tidal and upwelling fronts (Queiroz et al. 2012). Fishing-induced mortality of tagged sharks was approximately 10% up to 70 days post-tagging (Queiroz et al. 2010).
|Generation Length (years):||10|
|Use and Trade:||Blue Shark fins are the most prevalent on the Hong Kong shark fin market (Clarke et al. 2006a) and comprise at least 17% of the overall market, and an estimated 10.7 million individuals (0.36 million tonnes) are killed for the global fin trade each year (Clarke et al. 2006a, 2006b).|
Blue Shark is a commonly used bycatch of longline (and formerly driftnet) fisheries, particularly from nations with high-seas fleets, and in the North Atlantic longline fisheries. They are now regarded as targeted bycatch. Much of this bycatch is often unrecorded or under-reported (Camhi et al. 2008). In the North Atlantic, an estimated 61,845 tonnes (t) of Blue Shark were taken for the shark fin trade in 2006 (ICCAT 2009, Table 8b). Assuming a mean individual biomass of 38 kg, it is estimated that ~1.63 million individuals are taken from the North Atlantic every year. Fishing effort is not declining and so these trends are likely to continue.
This species is taken in relatively small numbers by sport fishermen, particularly in the United States, Europe and Australia.
The limited fishery assessments carried out to date have shown no evidence of a declining population trend in catch rates of Blue Shark in the Atlantic Ocean. However, the stock assessment results could not be evaluated for their sensitivity to a range of assumptions in the latest assessment. Nevertheless, ICCAT states that the weight of evidence does not indicate depletion (ICCAT 2009).
Blue Shark is the bycatch of pelagic longline fisheries throughout the Mediterranean. This species was the target of big game tournaments in the Adriatic Sea and is an incidental catch of tuna traps.
There are currently no species-specific catch limits or other protections in place in international waters for this species. Furthermore, the trade in their meat and fins is not monitored, despite their prominence in international trade (Rose 1996, Clarke et al. 2006a, 2006b). The species’ trans-oceanic migrations strongly support arguments for regional and international management (Camhi et al. 2009).
In 2013, the European Union (EU) banned removal of shark fins on board vessels (Regulation No. 605/2013), in line with advice from the International Union for the Conservation of Nature’s Shark Specialist Group and other shark fishery experts, in order to enhance enforcement of the 2003 EU ban on shark finning (Regulation No. 1185/2003) and facilitate improved shark fishery data collection. Since this species has been subject to high rates of finning in the past, well-enforced shark finning bans could help to reduce mortality.
With evidence of declines in the Mediterranean and in the Northwest Atlantic, but very limited information on the status of the population in the Northeast Atlantic, there is an urgent need for resources to compile, fully analyse and discuss available data from the Northeast Atlantic in order to confirm the full scale of decline of this species across the European par of its range.
|Citation:||Sims, D., Fowler, S.L., Ferretti, F. & Stevens, J.D. 2015. Prionace glauca. The IUCN Red List of Threatened Species 2015: e.T39381A48924261.Downloaded on 24 March 2018.|
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