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Chapter 11 The and of the Oceanic Whitetip , longimanus

Ramón Bonfi l, Shelley Clarke and Hideki Nakano

Abstract

The (Carcharhinus longimanus) is a common circumtropical preda- tor and is taken as bycatch in many oceanic fi sheries. This summary of its life history, dis- tribution and abundance, and fi shery-related information is supplemented with unpublished data taken during Japanese research operations in the Pacifi c . Oceanic whitetips are moderately slow-growing that do not appear to have differential growth rates by sex, and individuals in the Atlantic and Pacifi c 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 suggest that this 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 (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 . This species and the (Prionace glauca, Carcharhinidae) are the most abundant oceanic sharks, and they seem to have evolved an

Sharks of the Open Ocean: Biology, 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 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 repro- duction 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 . 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 (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 40N 10 1 20N

0

20S

40S

120E 140E 160E 180 160W 140W 120W 100W80W

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).

40N

30N

20N

10N

0

10S

Pregnant females 20S Newborns

30S 120E 140E 160E 180 160W 140W 120W 100W

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 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 , from east to west along the equatorial Atlantic, and from off southern 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 and (Backus et al., 1956). Their diet consists of lancetfi sh, oarfi sh, threadfi ns, , jacks, dolphinfi sh, tuna, skipjack and other scombrids, white , and , and occasionally stingrays, seabirds, turtles, marine gastropods, , 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, sug- gest 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 TL1.397 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). There was a weak positive correlation between female size and litter size. Ancillary observations (Bigelow and Schroeder, 1948; Backus et al., 1956; Bass et al., 1973; Lessa et al., 1999) on the reproduction of oceanic whitetip sharks in other areas of the world have usually been based on smaller sample sizes than in Seki et al. (1998). In South African waters, males were found to attain sexual maturity around 194 cm TL and females around 170–180 cm TL; in the northwestern Atlantic, females mature between 189 and 198 cm TL; both sexes attain maturity between 180 and 190 cm TL in the equato- rial western Atlantic. Size at birth is around 65–75 cm TL in the northwestern Atlantic and 132 Sharks of the Open Ocean

60–65 cm TL off South Africa. A positive correlation between female size and number of pups per litter was found in the northwestern Atlantic, where litter size averages 6. In this area, the gestation period is about 12 months, and mating and parturition have a defi ned period from late spring to summer. In South African waters, parturition also occurs in late spring and summer. There may be a pupping ground in the equatorial western Atlantic off Brazil.

Age and growth Until recently, little was known of the age and growth of this species. The fi rst studies were conducted by Saika and Yoshimura (1985), who provided initial estimates for the growth coeffi cient (k) of 0.04–0.09 for both sexes in the western Pacifi c based on vertebrae of 13 specimens (Table 11.1). Also working in the Pacifi c, Seki et al. (1998) subsequently ana- lyzed thin sections of vertebral centra from 111 males and 114 females and estimated a higher k of 0.103. No difference in growth between the sexes was observed. The authors assumed that one growth band is laid per year, and estimated through marginal increment analysis that annuli formation takes place in spring. According to their results, both sexes mature at 4–5 years of age, and the maximum age in their sample was 11 years. In the western equatorial Atlantic, Lessa et al. (1999) analyzed unstained thin sections of vertebral centra from 110 individuals (44 male, 60 female, and 6 of undetermined sex) and also found no evidence of differential growth between the sexes. Growth coeffi cients varied from 0.075 for back-calculated lengths to 0.099 using the observed-length-at-age method (Table 11.1). However, the authors reported that back-calculated lengths did not match well with those from direct readings, thus raising some doubts about their back- calculated growth model parameters. This study assumed that one translucent ring and one opaque ring were deposited each year and estimated that annuli are completely formed by July. The age at sexual maturity for both sexes using the observed-length-at- age method was estimated at 6–7 years. The maximum age found in the female samples was 13 years, although using their von Bertalanffy growth equation they estimated a max- imum age of 17 years for a larger female that was not aged through vertebral analysis. Prior to the age and growth studies by Seki et al. (1998) and Lessa et al. (1999), Branstetter (1990) used growth parameters to classify a number of carcharhinoid and lamnoid sharks by life-history strategy. Using the earlier estimates of k from Saika and Yoshimura (1985), and an estimate of the asymptotic average maximum body size (L)

Table 11.1 Von Bertalanffy growth parameters for oceanic whitetip sharks.

Reference Growth coeffi cient, k Asymptotic body size, L Hypothetical age at length zero, t0

Bass et al. (1973) – 270–300 TL – Saika and Yoshimura (1985) 0.04–0.09 – – Seki et al. (1998) 0.103 244.58 PL 2.698 Lessa et al. (1999) Observed length-at-age 0.099 284.9 TL 3.391 Back-calculated 0.075 325.4 TL 3.342 Biology and Ecology of the Oceanic Whitetip Shark 133 from Bass et al. (1973) (Table 11.1), the oceanic whitetip shark was characterized as a slow-growing species on the basis of k less than 0.10, growth in the fi rst year less than 30% of birth length, and length at birth greater than 20% of maximum length. Other pelagic sharks, such as the blue shark (k 0.11–0.25), shortfi n mako ( oxyrinchus, ; k 0.20–0.27), and (Carcharhinus falciformis, Carcharhinidae; k 0.10–0.15), were classifi ed as fast-growing species (Branstetter, 1990). The more recent estimates of the growth coeffi cient for the oceanic whitetip by Seki et al. (1998) and Lessa et al. (1999) suggested that its growth characteristics are intermediate to the archetypal slow- and fast-growth species described by Branstetter (1990).

Demographic analyses Demographic techniques have been used to characterize the vulnerability of various species to exploitation based on survival, maturation, and fecundity estimates. Although demographic metrics are sensitive to various assumptions and input parameters, such studies provide a framework for relative comparison of oceanic whitetips to other pelagic and coastal shark species. Using a method that incorporates density dependence, Smith et al. (1998) compared the productivities of 26 sharks representing fi ve orders and nine families. In this method, the maximum sustainable yield (MSY) population size is approximated by assuming that total mortality is equal to double the adult instantaneous natural mortality (M ) for each species. Two different assumptions are applied regarding species-specifi c fecundity (b), resulting in two different values of r2M (the intrinsic rate of increase when total mortality is 2M, or the intrinsic rebound potential) for each shark. In this study, the oceanic whitetip was ranked as the sixth most productive shark of the 26 species considered, with a mid- point for the two r2M estimates that was higher than that for the blue shark but slightly lower than that for the (Alopias vulpinus; Table 11.2). Cortés (2002) also used demographic analysis, and an assumption that vital rates are independent of density over time, to estimate values of the population growth rate (λ, where r log λ) for 38 species of sharks representing four orders and nine families. In this study, the value of λ for the oceanic whitetip from the western Pacifi c was eleventh highest among 41 shark populations (Table 11.2). This value indicates that oceanic whitetip productivity is in the range of other pelagic sharks, including Prionace glauca and Alopias vulpinus.

Table 11.2 Demographic metrics for three common pelagic sharks: The intrinsic rate of increase and the population growth rate provide relative measures of species resilience to exploitation.

Common name Scientifi c name Intrinsic rate of increase, Population growth r2M (midpoint of 1.00b rate, λ (Cortés, 2002) and 1.25b estimates) (Smith et al., 1998)

Common thresher Alopias vulpinus 0.084 1.125 Oceanic whitetip Carcharhinus longimanus 0.081 1.117 Blue Prionace glauca 0.074 1.401 134 Sharks of the Open Ocean

Fisheries

Catch and catch-rate data Although oceanic whitetip sharks are seldom explicitly targeted, they are one of the most common bycatch species in tuna fi sheries in offshore tropical waters. They are frequently caught in small- multispecies shark fi sheries, for example, in the Gulf of Aden (R. Bonfi l, unpublished data) and along the Pacifi c coast of Central America (Bonfi l and Abdallah, 2004). Despite their abundance, quantifi cation of catch numbers or biomass for this species is hindered by the lack of complete and accurate logbook data. Using an estimate of hooks deployed north of 20ºN in the Pacifi c by Japanese and South Korean longline fl eets in 1988, and hooking rates of 0.07 oceanic whitetips per 1,000 hooks from Strasburg (1958), Bonfi l (1994) estimated that 7,253 oceanic whitetips, or about 145 metric tons (t), were taken annually as incidental catch in the North Pacifi c. Applying Strasburg’s (1958) hooking rate for the eastern equatorial Pacifi c of 5.46 whitetips per 1,000 hooks, and an estimate of total hooks fi shed by Japanese, South Korean, Taiwanese, and Australian longliners in that area in 1989, Bonfi l (1994) estimated that another 539,946 individuals (about 10,799 t) were taken annually in the central and South Pacifi c. Similar extrapolations were not provided for the Atlantic and Indian Oceans owing to high expected variation in catch rates by area and season. Other fi shery-specifi c studies provide anecdotal information on catches and catch rates for oceanic whitetips in the Atlantic, Indian, and Pacifi c Oceans. In Brazilian waters, Lessa et al. (1999) found they were the second most abundant shark species caught by longline in equatorial regions between 1992 and 1997. In contrast, observer data collected from the Japanese longline fl eet in the Atlantic in the period 1995–2003 indicated that oceanic whitetips composed less than 1% of all shark bycatch (Senba and Nakano, 2004). Such differences in catch rates are likely due to potential differences in fi shing gear and mode of operation between the two fl eets, and notably the localized activity of Brazilian longliners compared to the wider coverage of the Japanese fl eet. Very low catches of oceanic whitetip sharks relative to other sharks were also found in the US Atlantic pelagic longline fi shery logbooks, where the average number of indi- viduals taken per year between 1990 and 2000 was 165, compared to an average annual take of 17,380 blue sharks and 11,953 shortfi n makos (Cortés, 2001). Data from Spanish surface longline fi sheries for 1999 indicated that of the 32.7 t of sharks landed from the Atlantic that year, only 0.2 t (0.6%) were oceanic whitetips (Mejuto et al., 2001). They were present in 4.72% of the sets of French and Spanish purse-seine tuna fl eets in the tropical eastern Atlantic (Santana et al., 1997). Using available data from the longline fl eets based in Uruguay and Brazil, Domingo (2004) reported that this species appears to be rather rare (only 0.006 sharks per 1,000 hooks) in the southern Atlantic; data from the Uruguayan fl eet in fi shing grounds off western equatorial Africa showed a catch rate of 0.09 sharks per 1,000 hooks. In the , data from research longline vessels in the late 1960s showed that C. longimanus composed 3.4% of the shark catch, compared to 22.5% in the western Pacifi c and 21.3% in the eastern Pacifi c (Taniuchi, 1990). Oceanic whitetips were present in 16% of the tuna purse-seine sets of Spanish and French fl eets in the western Indian Biology and Ecology of the Oceanic Whitetip Shark 135

Ocean (Santana et al., 1997). Recent surveys in the and Gulf of Aden have shown this species to be a common catch in medium- and small-scale directed multispe- cies shark fi sheries using gill nets (Bonfi l, 2003). Analyses of data from Japanese research longliners in the Pacifi c offer a historical com- parison between the late 1960s and the early 1990s for the pelagic environment stretch- ing from Papua New Guinea to Hawaii (Matsunaga and Nakano, 1999; Nakano, 1999). Oceanic whitetip catch per unit effort (CPUE) was characterized for 1967–1970 (n 912 sets; 1,777,000 hooks) and 1992–1995 (n 5,700 sets; 12,293,000 hooks) and standardized for differences in hook depth as fi shing operations changed through the years. Signifi cant changes in CPUE between the two periods were only observed in the eastern half of the study area (east of 180º latitude). Immediately north of the equator (0–10ºN), C. longimanus CPUE increased by 40–80%, whereas farther north (10–20ºN) catch rates decreased by 30–50%. The authors concluded that further standardization would be necessary to more clearly interpret abundance trends. Two studies analyzing catch-rate indices for oceanic whitetips in the Northwest Atlantic and Gulf of Mexico suggested a strong trend of declining populations. In the Northwest Atlantic, US commercial pelagic longline catch rates for C. longimanus from 1992 to 2000 showed declines of 70%, although the authors caution that such trends are more diffi cult to interpret for oceanic shark species because their habitats extend beyond the fi shing grounds (Baum et al., 2003). In the Gulf of Mexico, research longline surveys in 1954 and 1957 (n 170 sets; 82,972 hooks) were used to establish baseline catch rates that were then compared to observer data from US commercial longlines between 1995 and 1999 (n 275 sets; 219,461 hooks; Baum and Myers, 2004). In the earlier period, the oceanic whitetip was the most common shark caught, accounting for 61% of all hooked sharks and present in 64% of all sets. By the latter period, however, catch rates for this species had declined by 99%. Despite differences in gear and operational deployment between the two surveys, the authors concluded that this species is in danger of extirpation at this particular edge of its distribution.

Utilization Numerous products are derived from oceanic whitetip sharks: meat and skin for human consumption, hides for leather production, and vitamin A derived from liver oil (Compagno, 1984; McCoy and Ishihara, 1999; Vannuccini, 1999). Fins from this spe- cies are one of the most distinctive and common products in the Asian shark fi n trade and compose at least 2% by weight of shark fi ns auctioned in Hong Kong (Clarke et al., 2005, 2006). Molecular genetic testing of 23 fi n samples collected from nine traders and representing three ocean basins demonstrated a 100% concordance between the trade name “Liu Qiu” (Chinese for “rolling ball,” presumably a reference to the fi n’s rounded edges) and C. longimanus (Clarke et al., 2006). Shark fi n consolidators, working outside of Asia, reportedly include oceanic whitetip fi ns within the category of “brown” sharks (McCoy and Ishihara, 1999), which indicates that some oceanic whitetip fi ns may be sold in unspecifi ed mixed lots. As a result, quantities labeled as “Liu Qiu” in the shark fi n market may underestimate the true quantity of oceanic whitetip fi ns in trade. Despite the availability of “Liu Qiu” or “brown” fi ns in the market, these fi ns are not graded 136 Sharks of the Open Ocean particularly highly by shark fi n traders (Parry-Jones, 1996; McCoy and Ishihara, 1999; Fong and Anderson, 2000). Wholesale prices for oceanic whitetip fi n sets (size and weight not given) originating in the South Pacifi c ranged from US $45 to $85 for the period 1997–2003 (Clarke, 2004). Because of economic and operational differences, the utilization of this species varies from fl eet to fl eet. The Mexican tuna fl eet in the Gulf of Mexico retains 95% of oceanic whitetip sharks caught as bycatch for local consumption or sale, whereas the US fl eet in the same area keeps only 7% of individuals, and releases 69% alive (Gonzalez-Ania et al., 1997). Taiwanese longliners reportedly retain about half of the oceanic whitetip carcasses in their bycatch, whereas Japanese longliners are believed to discard all of the meat from this species (McCoy and Ishihara, 1999). Studies of oceanic whitetips landed by the coastal fi sheries off Japan indicate that the proportion of retained carcass meat to whole weight for this species varies from 53% to 60% (Matsunaga et al., 2003).

Management and conservation

In 1996, ICCAT began requesting that parties submit their shark data using a form that lists eight species of pelagic sharks, including oceanic whitetip shark (Kebe et al., 2001). However, ICCAT recognized that most of its member countries would have diffi culties in immediately fulfi lling this obligation, and this has proven to be the case. In the 2001 posting of the ICCAT shark database (ICCAT, 2001), only fi ve countries – Brazil, Mexico, Spain, St. Lucia, and the United States – had reported oceanic whitetip catches (Table 11.3). More recently, national reports presenting observer data for the French purse-seine fi shery off (Goujon, 2003) and the Japanese longline fl eet in the North and South Atlantic (Senba and Nakano, 2004) have included data for the oceanic whitetip, indicating that other fi sheries do collect species-specifi c catch data for this shark. Since 1997, Japan has required the recording of oceanic whitetip sharks in a separate category in the logbooks of all fi sheries. No stock assessments have been conducted for C. longimanus, in large part because of the lack of historical catch and abundance indices. As more fi sheries, such as those reporting to ICCAT, begin recording pelagic shark catches by species, the prospects for rigorously assessing the impact on oceanic whitetip stocks will improve. Evaluation of the of this species is hampered by the limited avail- ability of standardized catch and abundance data, and the resulting absence of stock assess- ment studies. Oceanic whitetips are considered a highly migratory species under Annex I

Table 11.3 Catches of oceanic whitetip sharks reported to ICCAT’s shark database (ICCAT, 2001).

Country Years Total number of sharks Total weight of sharks (round weight, t)

Brazil 1992–1993 201 – Spain 1997–1998 – 13.466 Mexico 1994–1995 57 5.480 St. Lucia 1995 – 0.076 United States 1983–1999 2,022 62.680 Total 2,280 81.702 Biology and Ecology of the Oceanic Whitetip Shark 137 of the 1982 Convention on the Law of the Sea (FAO, 1994). Castro et al. (1999) classifi ed this species as vulnerable to overfi shing on the basis of its slow growth, limited reproduc- tive potential, and high rate of bycatch in pelagic fi sheries. The World Conservation Union’s (IUCN) Red List considers the oceanic whitetip shark to be a Lower Risk/Near Threatened species (IUCN, 2004). A better understanding of oceanic whitetip biological parameters and improved fi shery statistics are needed to identify management and conservation measures. Until such data are available, precautionary management measures are warranted.

Acknowledgments

R. Bonfi l and S. Clarke thank the Japan Society for the Promotion of Science for the post- doctoral fellowships that supported them in the preparation of this manuscript. All authors acknowledge the assistance of the Ecologically Related Species Team of the National Research Institute of Far Seas Fisheries.

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