THRESHER SHARK OFF SUFFOLK
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THE OCCURRENCE OF THRESHER SHARK OFF THE SUFFOLK COAST J. R. ELLIS Common thresher shark Alopias vulpinus (Bonnaterre, 1788) (Lamniformes, Alopiidae) is one of three species of thresher shark, and the only species known to occur in the coastal waters of the British Isles (Quéro, 1984). Big eye thresher Alopias superciliosus (Lowe, 1839) occur in offshore areas in the North Atlantic and may be landed occasionally (Thorpe, 1997). It has also been suggested that a fourth species may exist in the Pacific (Eitner, 1995), but specimens to confirm this are lacking. Common threshers have a circumglobal distribution in tropical and temperate waters, and are found in both oceanic and coastal ecosystems (Gubanov, 1972, 1976; Kabasakal, 1998; Compagno, 2001), with the young typically found in shallower waters (Compagno, 2001). It has been suggested that nursery grounds occur in parts of the North-east Atlantic and Mediterranean, including off the Iberian Peninsula (Moreno et al., 1989), although their distribution off North-west Europe is less well known. A. vulpinus predate on a variety of pelagic fishes (e.g. herring, horse mackerel and mackerel) and cephalopods (Pascoe, 1986; Preti et al., 2001). Thresher sharks give birth to live young, with the developing young nourished by a yolk sac and subsequently by feeding on ovulated eggs (oophagy). Fecundity is low, with usually 2–7 pups (Hixon, 1979; Compagno, 2001) born at a length of 114–160 cm (Compagno, 2001). Little is known about the age and growth of thresher sharks (Cailliet et al., 1983), though they are reported to mature at a length of 3·1–3 5 m (males), 3·7–4 m (females), and maximum lengths reported are in the range of 5·6–6·1 m (Quéro, 1984; Compagno, 2001). Thresher sharks have long been known to occur off the coast of Suffolk, with many records resulting from them becoming caught in nets set for herring and mackerel (e.g. Patterson, 1910; Collings, 1933). There are several earlier records of thresher sharks in the North Sea, and these are listed in Table 1. From this it is apparent that both small individuals (<2 m) and larger fish are most often reported in the summer and autumn (June–November). Furthermore, most of the reported captures are from the southern North Sea, with comparatively few captures north of 55ºN (Figure 1). Reasons for this apparent seasonal occurrence of thresher sharks in the southern North Sea is unclear, but could possibly be that the area acts as a nursery ground, or that it is associated with the seasonal abundance of small pelagic fishes, or simply that it reflects seasonal patterns in fishing practices. Three of the more recent captures of A. vulpinus (a 150·5 cm female caught in June 2001, a 146·5 cm male caught in June 2004 and a 132 cm female caught in July 2004) have been examined and morphometric data collected (Table 2). The male specimen was whole and examined fresh. Total weight (WT) was approximately 7·17 kg, of which the liver weighed 652 g (9·09% WT), the alimentary tract weighed 320 g and the gutted weight about 6·2 kg. Stomach contents were composed of unidentifiable digested fish remains. Interestingly, two similarly sized thresher sharks were caught off the Suffolk coast on 17 June 2004, one landed at Aldeburgh (East Anglian Daily
Trans. Suffolk Nat. Soc. 40 (2004)