The Biology and Ecology of the Porbeagle Shark, Lamna Nasus

Abstract

Information on the biology, ecology, and fi sheries of porbeagle sharks (Lamna nasus) is reviewed to assess biological and population parameters that are relevant to stock assessment, and to identify gaps in our knowledge. Separate porbeagle stocks occur in the Northwest and Northeast Atlantic, but stock identity is poorly understood in the Southern Hemisphere. Porbeagles are born at 58-67 cm fork length (FL). Length at maturity is lower for Southwest Pacifi c males and females (about 140-150 and 170-180 cm FL, respectively) than for North Atlantic males and females (166 and 208 cm FL). Ages at 50% maturity for North Atlantic males and females are 8 and 13 years, respectively. Porbeagles recruit to commercial fi sheries in both hemispheres during their fi rst year. North Atlantic males and females reach at least 253 and 302 cm FL, respectively, and longevity exceeds 26 years. Age at maturity and longevity in the Southwest Pacifi c are unknown. Growth is almost linear, and similar for both sexes, for about 8 years, after which females grow faster. The rate of natural mortality for the Northwest Atlantic is 0.10 for immature sharks, rising to 0.15-0.20 for mature sharks. The gestation period is 8-9 months and the length of the female reproductive cycle may be about 1 year. Mean litter size is 3.7-4.0 embryos, and the embryonic sex ratio is 1:1. Porbeagle sharks are vulnerable to overfi shing. Directed fi sheries in the Northwest Atlantic are now restricted by catch quotas, but stock biomass declined to about 10-20% of virgin biomass before restrictive quotas were introduced. In the Southern Hemisphere, porbeagles are taken mainly as bycatch in tuna longline fi sheries; no biomass estimates or stock assessments are available. The low productivity of porbeagles and their history of overfi shing indicate that sustainable yields will be low.

Larval parasite gene sequence data reveal cryptic trophic links in life cycles of porbeagle shark tapeworms

“…Linton 1922, Brown & Threlfall 1968, Pascual et al 1996; also reviewed by Hochberg 1990). Squid are an important food source for porbeagle sharks (Joyce et al 2002, Francis et al 2008. However, in the northwestern Atlantic, the number and frequency of occurrence of squid in their diet decreases in larger sharks (Joyce et al 2002).…”

“…This endo thermic shark is capable of maintaining its body temperature 7 to 10°C above ambient water temperatures (Carey & Teal 1969), thus allowing this species to inhabit cold temperate waters ranging between 1 and 18°C (Compagno et al 2005). Its diet consists primarily of teleosts and cephalopods (Ellis & Shackley 1995, Compagno 2001, Joyce et al 2002, Francis et al 2008. Because of its small litter size and long maturation time, this shark is susceptible to overfishing .…”

“…Accordingly, the International Union for Conservation of Nature (IUCN) lists this species as globally threatened (Stevens et al 2006), although regional assessments vary from endangered in the northwestern Atlantic to critically endangered in the northeastern Atlantic and Mediterranean Sea (Stevens et al 2006). Little is known about the stock status for this species in the Southern Hemisphere (Francis et al 2008).…”

Intrinsic Rates of Increase in Pelagic Elasmobranchs

“…Cailliet et al (1983); 6:Tanaka et al (1990); 7:Nakano and Seki (2002); 8:Aasen (1963); 9:Francis and Stevens (2000); 10:Francis et al (2008); 11:Tanaka (1980); 12:Goldman and Human (2005); 13: Bonfi l et al(1993); 14:Liu et al (1999); 15:Moreno and Morón (1992); 16:Chen et al (1997); 17:Liu et al (1998); 18: Pauly (1978); 19:Pauly (2002).…”

The Reproductive Biology of Pelagic Elasmobranchs

“…For the North Atlantic population, estimates are 200-250 cm (Aasen, 1961) and 236-259 cm (Jensen et al, 2002). In Australian and New Zealand waters, female maturity probably occurs at 185-202 cm, suggesting regional differences (Francis and Stevens, 2000;Francis et al, 2008). Bigelow and Schroeder (1948) reported pregnant females of L. nasus as small as 5 ft (152 cm), but no specifi c observations were reported so the signifi cance of this outlier cannot be assessed.…”