Lamna nasus (Mediterranean subpopulation)
|Scientific Name:||Lamna nasus (Mediterranean subpopulation)|
See Lamna nasus
|Red List Category & Criteria:||Critically Endangered A2bd ver 3.1|
|Assessor(s):||Stevens, J., Fowler, S.L., Soldo, A., McCord, M., Baum, J., Acuña, E. & Domingo, A.|
|Reviewer(s):||Cavanagh, R.D., Heupel, M. & Simpfendorfer, C. (Shark Red List Authority)|
The porbeagle is a wide-ranging, coastal and oceanic shark, but with apparently little exchange between adjacent populations. Low reproductive capacity and high commercial value (in target and incidental fisheries) of mature and immature age classes makes this species highly vulnerable to over-exploitation and population depletion. This depletion, despite variations in availability of data and degree of depletion between the northern and southern hemispheres, is considered to meet Vulnerable criteria globally. The eastern and western North Atlantic populations have both been seriously over-exploited by directed longline fisheries. Collapse of the Northeast Atlantic population led to intensive target fishing in the well-documented Northwest Atlantic fishery in the 1960s, with most of the virgin biomass removed in just six years. Renewed target fishing in the 1990s led to a further population decline to ~11–17% of virgin biomass within the three generation period for this species. Recently improved management in the Northwest Atlantic should now help stocks to recover, however the Northeast Atlantic population has been subject to unrestricted fishing pressure ever since its earlier crash. Data are lacking, but stock depletion is considered to be much greater than in the Northwest Atlantic. Longline tuna and swordfish fleets in the southern hemisphere take a significant partially-utilised bycatch. Only limited trend data are available, including over 90% declines in landings by the Uruguayan longline fleet in the southwest Atlantic.
|Range Description:||The porbeagle shark is a wide-ranging coastal and oceanic species found in temperate and cold-temperate waters worldwide (1° to 18°C, 0 to 370 m), more common on continental shelves. Coastal and oceanic, amphitemperate, with centres of distribution in the North Atlantic and in a circumglobal band of temperate water of the southern Atlantic, southern Indian Ocean, southern Pacific and Antarctic Ocean (Compagno 2001). In the Southwest Atlantic Ocean it is found below 26°S (A. Domingo, pers. comm.), in the Southeastern Pacific, between 23 to 37°S (E. Acuña, pers. comm.). Tagging studies in the Northwest Atlantic have shown mainly short to moderate distances of up to 1,500 km along continental shelves (Francis et al. in press). Distances traveled by 143 porbeagle tagged in a US study ranged 4 to 1,005 nm, with a mean distance of 234 nm, with over 90% moving less than 500 nm from their original tagging location (Kohler et al. 2002). Porbeagles tagged off southern England have been recaptured off Spain, Denmark and Norway (2,370 km away), and a porbeagle tagged in Ireland travelled 4,260 km Kohler and Turner 2001), suggesting mixing throughout their range in the Northeast Atlantic (Stevens 1976, 1990). According to the Canadian Department of Fisheries and Oceans (DFO 2001a), mature porbeagle sharks are rarely seen in winter and spring, with monthly catches exhibiting a seasonal and sex-specific migration of mature sharks towards the southern Newfoundland mating grounds in spring. The birthing grounds for the porbeagle shark in the Northwest Atlantic remain unknown (DFO 2001a). There is little known about the porbeagle shark in subequatorial Africa. No information on mating or nursery grounds is available. Porbeagles in the Northwest Atlantic make annual migrations along the coast between the Gulf of Maine and Newfoundland (Campana et al. 1999, Campana and Joyce 2004).|
Native:Albania; Algeria; Bosnia and Herzegovina; Croatia; Cyprus; Egypt; France; Greece; Israel; Italy; Lebanon; Libya; Malta; Monaco; Montenegro; Morocco; Slovenia; Spain; Syrian Arab Republic; Tunisia; Turkey
|FAO Marine Fishing Areas:|
Mediterranean and Black Sea
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||North Atlantic tagging studies (DFO 1999, Kohler et al. 2002, Stevens 1990) indicate only one trans-Atlantic movement (Kohler and Turner 2001), implying that the two north Atlantic populations are distinct. There is no evidence of genetic exchange between the North Atlantic and the Southern Hemisphere population(s), which are separated by warm water. The number of sub-populations in the southern oceans is unknown.
Biomass of the Northwest Atlantic porbeagle population was estimated at 4,409 t (11% of virgin biomass) and female spawners estimated at 6,075 (10% of the virgin abundance) (Campana et al. 2001).
Populations studied in the North Atlantic and Mediterranean segregate by size and sex. Although adults are very rare in the Mediterranean, it appears to be a nursery ground.
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||This section is taken mainly from Stevens (in press) species assessment for Lamna nasus (in Fowler et al. in press).
The porbeagle reaches a maximum reported size of 355 cm TL (Francis et al. in press). Males mature at about 165 cm TL in the South Pacific and 195 cm TL in the North Atlantic. Females mature at about 195 cm TL in the South Pacific and 245 cm TL in the North Atlantic (Jensen et al. 2002, Francis and Duffy 2005, Francis et al. in press).
Reproduction is oophagous with litters of 1 to 5 pups (average four) produced, which are 68 to 78 cm TL at birth (Compagno 1984a, Gauld 1989, DFO 2001a, Francis and Stevens 2000, Francis et al. in press). Aasen (1963) estimated that the gestation period was about eight months in the North Atlantic and that individual females breed each year. However, Shann (1923) found two distinct size groups of embryos present in the December-February period and suggested that gestation may last 18 to 24 months. Gauld (1989) noted that a resting period may be present between parturition and fertilisation. Francis and Stevens (2000), Jensen et al. (2002) and Francis et al. (in press) estimate an 8 to 9 month gestation period. Birth occurs in spring off Europe, spring-summer off North America and winter in Australasia (Aasen 1963, Francis and Stevens 2000, Jensen et al. 2002) and the Eastern Pacific off Chile (Acuña et al. unpublished data).
Natanson et al. (2002) and Campana et al. (2002) examined age and growth in the North West Atlantic population and reported a maximum age of 26 years. However they estimated longevity might be as high as 46 years in an unfished population. Ages at 50% maturity for North Atlantic males and females are 8 and 13 years respectively (Jensen et al. 2002).
Porbeagles feed mostly on teleost fish, both pelagic and demersal species, and on cephalopods (Compagno 1984a). In the North West Atlantic, pelagic fish and squid are the main diet in deep water, and pelagic and demersal fish are important in their diet in shallow water (Joyce et al. 2002).
Like other mackerel sharks, the porbeagle is endothermic, maintaining its muscle and visceral temperatures above that of the surrounding seawater. It prefers temperatures below 18°C and has been caught in water temperatures as low as 2°C on the bottom (3°C on the surface) at high latitudes (Svetlov 1978). The preferred temperature range in the Northwest Atlantic is 5 to 10°C (Campana and Joyce 2004).
This species has virtually disappeared from Mediterranean records. In the North Tyrrhenian and Ligurian Sea Serena and Vacchi (1997) reported only 15 specimens of porbeagle during a few decades of observation. Soldo and Jardas reported only nine records of this species in the Eastern Adriatic since the end of 19th century until 2000. Recently two new records were reported (A. Soldo unpublished data). Orsi Relini and Garibaldi (2002) reported two newborn porbeagles were caught as bycatch of the swordfish longline fishery in the Western Ligurian Sea. A young porbeagle, considered to be very recently born was reported in the central Adriatic Sea (Orsi Relini and Garibaldi 2002). A young specimen was also caught in the central Adriatic during big-game fishing. On the basis of its length, it is suggested to be of an age 1–17 months (Marconi and De Maddalena 2001). These records indicate a possible nursery area in Central Mediterranean. During research of bycatch in the western Mediterranean swordfish longline fishery no porbeagles were caught (De La Serna et al. 2002). Only 15 specimens were caught during research conducted in 1998–1999 on bycatch of sharks in large pelagic fisheries: catches were reported only in the southern Adriatic and Ionian Sea, mainly by driftnets (Megalofonou et al. 2000). Official statistics for Mediterranean area show that the only landings in the Mediterranean were reported in 1996 by Malta – 1 t (FAO 2002). Porbeagles are also popular as recreational species (big game fishing) in some areas of Mediterranean.
The species is listed on Annex III, ‘Species whose exploitation is regulated’ of the Barcelona Convention Protocol concerning specially protected areas and biological diversity in the Mediterranean, signed in 1995 but not yet ratified. The Mediterranean population of this species was also added in 1997 to Appendix III of the Bern Convention (the Convention on the Conservation of European Wildlife and Natural Habitats) as a species whose exploitation must be regulated in order to keep its population out of danger. No management action has yet followed these listings. The draft action plan for the conservation of cartilaginous fishes (Chondrichthyans) in the Mediterranean Sea (Anonymous 2002) has identified porbeagle as a species that urgently needs development of a management programme for sustainable fisheries.
|Citation:||Stevens, J., Fowler, S.L., Soldo, A., McCord, M., Baum, J., Acuña, E. & Domingo, A. 2006. Lamna nasus (Mediterranean subpopulation). The IUCN Red List of Threatened Species 2006: e.T61420A12478828.Downloaded on 21 January 2018.|