Chapter 11 the Biology and Ecology of the Oceanic Whitetip Shark, Carcharhinus Longimanus

Chapter 11 The Biology and Ecology of the Oceanic Whitetip Shark, Carcharhinus longimanus Ramón Bonfi l, Shelley Clarke and Hideki Nakano Abstract The oceanic whitetip shark (Carcharhinus longimanus) is a common circumtropical preda- tor and is taken as bycatch in many oceanic fi sheries. This summary of its life history, distribution and abundance, and fi shery-related information is supplemented with unpublished data taken during Japanese tuna research operations in the Pacifi c Ocean. Oceanic whitetips are moderately slow-growing sharks that do not appear to have differential growth rates by sex, and individuals in the Atlantic and Pacifi c Oceans seem to grow at similar rates. They reach sexual maturity at approximately 170–200 cm total length (TL), or 4–7 years of age, and have a 9- to 12-month embryonic development period. Pupping and nursery areas are thought to exist in the central Pacifi c, between 0ºN and 15ºN. According to two demographic metrics, the resilience of C. longimanus to fi shery exploitation is similar to that of blue and shortfi n mako sharks. Nevertheless, reported oceanic whitetip shark catches in several major longline fi sheries represent only a small fraction of total shark catches, and studies in the Northwest Atlantic and Gulf of Mexico suggest that this species has suffered signifi cant declines in abundance. Stock assessment has been severely hampered by the lack of species-specifi c catch data in most fi sheries, but recent implementation of species-based reporting by the International Commission for the Conservation of Atlantic Tunas (ICCAT) and some of its member countries will provide better data for quantitative assessment. On the basis of its life-history characteristics, this species is presently considered by the World Conservation Union to be vulnerable to, but not currently threatened by, pelagic fi sheries. Key words: oceanic whitetip shark, Carcharhinus longimanus, Carcharhinidae, age and growth, reproduction, distribution, abundance, fi sheries. Introduction The oceanic whitetip shark (Carcharhinus longimanus, Carcharhinidae) is one of the most common top predators in open waters of all tropical oceans of the world, and is the only truly oceanic shark of its genus. This species and the blue shark (Prionace glauca, Carcharhinidae) are the most abundant oceanic sharks, and they seem to have evolved an Sharks of the Open Ocean: Biology, Fisheries and Conservation. Edited by M. D. Camhi, E. K. Pikitch and E. A. Babcock © 2008 Blackwell Publishing Ltd. ISBN: 978-0632-05995-9 Biology and Ecology of the Oceanic Whitetip Shark 129 effi cient partitioning of the oceanic environment, with blue sharks dominating the temper- ate seas and oceanic whitetips prevailing in tropical areas (Nakano, 1996; Matsunaga and Nakano, 1999). Despite its worldwide distribution and frequent appearance in most high-seas fi shery catches in tropical areas, little attention has been paid to oceanic whitetip shark biology and ecology. Since Bigelow and Schroeder (1948) pointed out that “astonishingly little is known of the habits of longimanus, considering that it is one of the members of its genus that has been recognized the longest,” only a handful of papers have focused on this shark. Studies by Backus et al. (1956) in the western North Atlantic and Strasburg (1958) in the eastern Pacifi c Ocean were among the fi rst to describe the distribution, abundance, size structure, diet, behavior, sex segregation, and reproduction of the oceanic whitetip. However, nearly 30 years passed before Saika and Yoshimura (1985) further reported on the natural history of this species, and theirs was a limited analysis of the ecology and biology of populations in the western Pacifi c Ocean. More recently, a paper by Savel’ev and Chernikov (1994) assessed the presumed ability of oceanic whitetip sharks to search for food from smells in the air, and Seki et al. (1998) and Lessa et al. (1999) studied the age, growth, and reproduction of populations in the Pacifi c Ocean and equatorial West Atlantic, respectively. The latest contributions to our knowledge of this species come from a pair of papers, Baum et al. (2003) and Baum and Myers (2004), describing declines in shark populations in the Northwest Atlantic and Gulf of Mexico. We also present data from fi sheries that catch oceanic whitetip sharks, as well as unpublished data on the bycatch from Japanese tuna research and training cruises (including a total of 209,530 longline sets and 444 million hooks). Distribution and movements The oceanic whitetip shark is a tropical, epipelagic species occurring from the surface to at least 152 m depth. It has a clear preference for open ocean waters and its abundance increases away from continental and insular shelves (Backus et al., 1956; Strasburg, 1958; Compagno, 1984). Although it can be found in waters between 15ºC and 28ºC, it is most commonly found in waters with temperatures above 20ºC. It is one of the most abundant oceanic sharks, and although it generally does not school, it can form aggre- gations around food sources. Catch rates for this species have been shown to decrease with increasing depth between 80 and 280 m, suggesting that oceanic whitetips are found in shallow surface waters more commonly than other pelagic sharks, such as the bigeye thresher (Alopias superciliosus, Alopiidae; Nakano et al., 1997). Preliminary data from Japanese research and training tuna longliners (H. Nakano, unpublished data) indicate that, in the Pacifi c Ocean, oceanic whitetips are most abundant in a belt between 10ºN and 10ºS, are common between 20ºN and 20ºS, and can occur up to about 30ºN in the northwestern Pacifi c (Fig. 11.1). These data also show that pregnant oceanic whitetips occur mainly in a wide area of the North Pacifi c between 140ºW and 150ºE, with higher concentrations in the central part of this distribution just above 10ºN (Fig. 11.2). Newborn sharks occur between the equator and 20ºN, but mainly in a narrow strip just above 10ºN in the central Pacifi c, coincident with the higher concentrations of 130 Sharks of the Open Ocean 100 40ЊN 10 1 20ЊN 0Њ 20ЊS 40ЊS 120ЊE 140ЊE 160ЊE 180Њ 160ЊW 140ЊW 120ЊW 100ЊW80ЊW Fig. 11.1 Catch rate of oceanic whitetip sharks by Japanese tuna research and training vessels for each sampled 1° quadrant in the Pacifi c Ocean. Bubble size is proportional to the number of sharks per thousand hooks (aggregated data from 209,530 longline sets with a total of 444 million hooks for the period 1992–1998, from the National Research Institute of Far Seas Fisheries). 40ЊN 30ЊN 20ЊN 10ЊN 0Њ 10ЊS Pregnant females 20ЊS Newborns 30ЊS 120ЊE 140ЊE 160ЊE 180Њ 160ЊW 140ЊW 120ЊW 100ЊW Fig. 11.2 Distribution of pregnant females and newborns of oceanic whitetip shark in the tropical Pacifi c Ocean (data from Japanese tuna research and training vessels for the period 1992–1998, from the National Research Institute of Far Seas Fisheries). pregnant females. This suggests that the area between 150ºW and 180ºW and just above 10ºN might be a pupping ground for oceanic whitetip sharks. However, incomplete sam- pling limits a better defi nition of this area. Little is known of the migrations and movements of this species; Backus et al. (1956) reported that oceanic whitetip sharks move out of the Gulf of Mexico during the winter Biology and Ecology of the Oceanic Whitetip Shark 131 and may move southward from the waters north of Cape Hatteras when temperatures drop. They hypothesize that the avoidance of shallow-water habitats may be a mechanism to avoid competition for food with faster-swimming coastal sharks. In the Cooperative Shark Tagging Program of the US National Marine Fisheries Service, 542 oceanic whitetips were tagged in the Atlantic Ocean between 1962 and 1993, but only 6 were recaptured. Maximum time at liberty was 3.3 years, maximum distance traveled was 2,270 km, and maximum estimated speed was 32 km/day (Kohler et al., 1998). These data indicate movements from the northeastern Gulf of Mexico to the Atlantic Coast of Florida, from the Mid-Atlantic Bight to southern Cuba, from the Lesser Antilles west into the central Caribbean Sea, from east to west along the equatorial Atlantic, and from off southern Brazil in a northeasterly direction. Biology and ecology Diet Oceanic whitetip sharks are one of the main apex predators in tropical open waters, and feed mostly on oceanic teleosts and cephalopods (Backus et al., 1956). Their diet consists of lancetfi sh, oarfi sh, threadfi ns, barracuda, jacks, dolphinfi sh, tuna, skipjack and other scombrids, white marlin, and squid, and occasionally stingrays, seabirds, turtles, marine gastropods, crustaceans, carrion from marine mammals, and garbage (Compagno, 1984). Reproduction Similar to other carcharhinid species, the oceanic whitetip shark is viviparous with pla- cental embryonic development. There are few reproductive studies for this species, and most of the information comes from the Pacifi c Ocean populations considered in Seki et al. (1998). In the North Pacifi c, mating takes place during June and July, and parturition occurs from February to July. These fi ndings, based on a sample size of 52 embryos, suggest a 9- to 12-month embryonic development period. From a more limited sample size for the South Pacifi c (n 16), parturition appears to occur in November. Size at maturity ranged from 168 to 196 cm total length (TL) for males and from 175 to 189 cm TL for females, although a 137-cm-TL pregnant female was also recorded but not fully investi- gated (lengths converted from precaudal (PL) to total length using Seki et al.’s formula of TLϭ1.397 PL). Size at birth was between 55 and 75 cm TL, and the number of embryos in a litter ranged from 1 to 14, with a mode of 5 and an average of 6.2 (Seki et al., 1998).

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