42 



Fishery Bulletin 98(1) 



low and Schroeder, 1948; Aasen, 1963; Gauld, 1989). 

 Thus, despite the early discovery of oophagy in por- 

 beagles, little is known about their reproduction. Most 

 parameter estimates are imprecise, and several are 

 speculative or conflicting. 



Few pregnant females have been reported from the 

 Southern Hemisphere, and few details have been pro- 

 vided for any of them. Graham (1939, 1956) reported 

 one caught at Otago Heads, New Zealand, in 1933. It 

 had three embryos that were approaching full term 

 and weighed 3.4-4.3 kg each. Graham (1956) also 

 examined several other pregnant females but he re- 

 ported few details. Duhamel and Ozouf-Costaz (1982) 

 found four small embryos in a female caught in 1981 

 near Kerguelen Island in the southern Indian Ocean 

 (51°S, 70°E). 



Growth curves are available for northwest Atlantic 

 porbeagles, based on modal analysis of length-fre- 

 quency distributions, and back-calculation of length- 

 at-age from bands on a vertebra (Aasen, 1963). They 

 suggest that growth is relatively fast, at least in the 

 first few years, and that longevity is 20-30 years. 

 No growth information is available for the Southern 

 Hemisphere. 



Porbeagles have been exploited for their flesh for 

 many decades, and have proven to be vulnerable to 

 overfishing. A target longline fishery in the northwest 

 Atlantic in the 1960s lasted only six years before col- 

 lapsing (Anderson, 1990; Pratt and Casey, 1990). In 

 the Southern Hemisphere, porbeagles have not been 

 targeted, but they are frequently taken as bycatch in 

 tuna fisheries, especially the pelagic driftnet fishery 

 for albacore (Thunnus alalunga) during 1982-91 in 

 the South Pacific (Murray, 1994; Yatsu, 1995), and the 

 longline fishery for southern bluefin tuna (Thunnus 

 maccoyii) and bigeye tuna (Thunnus obesus) in the 

 southern Indian and Pacific Oceans (Stevens et al., 

 1983; Francis et al., 1999). In the New Zealand long- 

 line fishery, porbeagles are the second most commonly 

 caught shark after the blue shark (Prionace glauca) 

 (Francis et al., 1999). 



The collapse of the northwest Atlantic fishery in the 

 1960s provides ample justification for a cautious ap- 

 proach to managing porbeagles. In view of recent in- 

 creased landings in the North Atlantic (O'Boyle et al.. 

 1996), and the size and scope of the tuna longline fish- 

 ery in the southern oceans, there is an urgent need for 

 improved information on reproduction, growth, and 

 stock productivity as a basis for effective management. 

 Much of the Southern Hemisphere longline fishei-y oc- 

 curs in international waters, making monitoring and 

 management difficult. Recently, the New Zealand and 

 Australian governments implemented scientific ob- 

 server programs to monitor catches of foreign and do- 

 mestic longline vessels in their respective Exclusive 



Economic Zones (EEZs). These programs provided an 

 opportunity to collect information on the reproduction 

 and growth of porbeagles. In this paper, we describe 

 the geographical distribution and length composition 

 of sharks taken by longline vessels in the southwest 

 Pacific, estimate the growth rate of embryos and juve- 

 niles, and describe embryonic development and ooph- 

 agy. We also estimate the length of the gestation pe- 

 riod, the timing of parturition, and the size at birth, 

 and compare these with estimates for North Atlantic 

 porbeagles. 



Materials and methods 



Data sources 



Most of our data and specimens were collected by sci- 

 entific observers aboard Japanese and domestic tuna 

 longline vessels operating in the New Zealand and 

 Australian EEZs. In New Zealand, fishing and ob- 

 server effort was concentrated in two regions: 1 ) north- 

 east New Zealand (north and east coasts of North Is- 

 land and the Kermadec Islands), and 2) southwest 

 New Zealand (east and west coasts of South Island) 

 (Fig. 1). In Australia, most effort was around Tasma- 

 nia (Fig. 2). New Zealand obsei'vers began recording 

 the quantity of bycatch in 1987, measuring and sexing 

 porbeagles in 1990, and examining females for em- 

 bryos in 1992. In Australia, the respective years were 

 1988, 1990 and 1991. The primary task of observers 

 was to monitor the target tuna species (mainly south- 

 ern bluefin and bigeye tuna). Porbeagles were counted 

 on most longline sets but were not always measured 

 or examined for embryos. Therefore our data repre- 

 sent a subsample of the catch taken by observed long- 

 liners. The opportunistic nature of this collection pro- 

 cess, and the low catch rate of pregnant females, ne- 

 cessitated the accumulation of specimens and data 

 over a lengthy period. 



Embryos collected by observers were supplemented 

 by specimens and data from other sources, and the lit- 

 erature, including three litters from Heard and Ker- 

 guelen Islands in the southern Indian Ocean (Table 1). 

 The four embryos from the Kerguelen female were de- 

 posited in the Museum National d'Histoire Naturelle 

 (MNHN 1981-1432-1981-1435) (Duhamel and Ozouf- 

 Costaz, 1982), and were photogi-aphed and remea- 

 sured for us by Duhamel.' A 185-cm-fork length (FL) 

 female from Macquarie Island (Fig. 1) was the only 

 intact pregnant female we examined. 



' Duhamel. G. 1997. Museum National d'Histoire Naturelle 

 ( MNHN), 75231 Paris cede.x 05, France. Personal commun. 



