Lamna nasus (Northwest Atlantic subpopulation)
|Scientific Name:||Lamna nasus (Northwest Atlantic subpopulation)|
|Species Authority:||(Bonnaterre, 1788)|
See Lamna nasus
|Red List Category & Criteria:||Endangered A1abd 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.
|Previously published Red List assessments:|
|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 and 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 travelled by 143 porbeagle tagged in a US study ranged 4 to 1,005 nautical miles (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:Bermuda; Canada (Newfoundland I, Nova Scotia); Greenland; United States (Maine, Massachusetts, New Jersey, New York, Rhode Island)
|FAO Marine Fishing Areas:|
Atlantic – western central; Atlantic – northwest
|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 subpopulations 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 to 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 8–9 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).
Porbeagle fishing in the Northwest Atlantic started in 1961, when the fleet of Norwegian shark longliners began operating off the coast of New England and Newfoundland after the Northeast Atlantic stocks had been depleted. Catches increased rapidly from about 1,900 t in 1961 to more than 9,000 t in 1964. By 1965 many of the vessels had switched to other species or moved to other grounds because of the population decline. The fishery collapsed after only six years, landing less than 1,000 t in 1970. Smaller landings were also reported by Faeroese fishing vessels from around the same time and throughout the 1970s and 1980s. Norwegian and Faeroese fleets have been excluded from Canadian waters since the establishment of Canada’s EEZ in 1995. Canadian and US authorities reported all landings after 1995. Porbeagle sharks are now taken almost exclusively by a Canadian directed longline fishery. Bycatch in swordfish longline fisheries and various inshore fisheries are minimal, rarely exceeding 40 t in recent years (DFO 2001a).
Following the collapse of the fishery in the 1960s, it took ~25 years for recovery to ~30% of virgin biomass to take place. Three Canadian vessels entered the targeted Northwest Atlantic fishery in 1994. Catches of 1,000 to 2,000 t/year throughout much of the 1990s reduced population levels to a new low in under ten years: the average size of sharks and catch rates (number/hook) were the smallest on record in 1999 and 2000. By 2000, catch rates of mature sharks were reduced to 10% of the 1992 peak, and immature catch rates to 30% of the 1991 peak. The biomass in 2000 was estimated as 11 to 17% of virgin biomass and fully recruited F estimated as 0.26 (DFO 2001a). The current porbeagle population is seriously depleted and will require a greatly reduced fishing mortality if recovery is to occur (DFO 2001). The 2001 stock assessment by the Canadian Department of Fisheries states: ‘An annual catch of 200 to 250 t would correspond to fishing at about MSY and would allow population growth.’ Following this advice, the Canadian quota was reduced to 250 t for the period 2002 to 2007 to allow population growth and recovery. There is a small quota (92 t) for porbeagle in the US EEZ.
The 1995 Fisheries Management Plan for pelagic sharks in Atlantic Canada established precautionary catch levels of 1,500 t for porbeagle in the target shark fishery. License limitation, a ban on finning, restrictions on gear, area and seasons, bycatch limits and restrictions to recreational fishers permitting hook and release only were also implemented (Hurley 1998). The porbeagle TAC was reduced to 1,000 t in 1997 (O’Boyle et al. 1998), then to 1,700 t during the two years 2000–2001 while additional scientific information was collected (DFO 2001b). As a result of these studies, it was concluded that the population was seriously depleted (to 10–20% of virgin biomass) and would require a greatly decreased fishing mortality if recovery is to occur. An annual catch of 200 to 250 t would correspond to fishing at about MSY and would allow population growth. However, annual catch levels of about 1,000 t would be sustainable over the long term once the population has recovered. These DFO analyses, which are the basis for the regional assessment, are contested by COSEWIC (2004), who notes that the quota for 2002 to 2007 of 200 to 250 t represents a substantial reduction from catches in the mid-1990s, but even this amount now corresponds to a high exploitation rate because of the low population abundance. It is highly uncertain if this quota reduction will be sufficient to halt the porbeagle decline, and if so, to what extent the population will recover, given that there is uncertainty in estimating FMSY and the quota, the low number of mature animals remaining in the population, that at its current low abundance the population may experience depensation (Allee effects), and that reduction in fishing pressure is not always sufficient for population recovery (Hutchings 2001). COSEWIC (Committee on the status of endangered wildlife in Canada) designated porbeagle as Endangered in 2004 (COSEWIC 2004).
Since 1993, shark fisheries in Atlantic and Gulf of Mexico waters in the US have been managed under the Fishery Management Plan for Sharks of the Atlantic Ocean. The plan set commercial quotas in the US EEZ for 10 species of pelagic sharks at 580 t dressed weight annually, with recreational bag limits also applied. There is a small quota (92 t) for porbeagle. Commercial fishers require an annual shark permit, and finning is prohibited.
|Citation:||Stevens, J., Fowler, S.L., Soldo, A., McCord, M., Baum, J., Acuña, E. & Domingo, A. 2006. Lamna nasus (Northwest Atlantic subpopulation). The IUCN Red List of Threatened Species 2006: e.T39344A10210847.Downloaded on 28 July 2016.|