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available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/biocon

Pelagic longlines: A threat to sea turtles in the Equatorial Eastern Atlantic Alvar Carranzaa, Andres Domingob,*, Andres Estradesc a

Investigacio´n y Desarrollo, Facultad de Ciencias, Universidad de la Repu´blica, Igua 4225, Montevideo, Uruguay Direccio´n Nacional de Recursos Acua´ticos, Recursos Pela´gicos, Constituyente 1497, C.P.11.200, Montevideo, Uruguay c CID-KARUMBE´, Juan Paullier 1198/101, 11200 Montevideo, Uruguay b



Article history:

Pelagic longlines are widely known to interact with several species of sea turtles, and there

Received 21 June 2005

is an increasing concern about the by-catch of turtles in commercial fisheries and its

Received in revised form

impact on their populations. However, information on sea turtle by-catch in the South

29 January 2006

Atlantic Ocean is scarce, and there are no quantitative by-catch data available on olive rid-

Accepted 9 February 2006

leys for the Equatorial Eastern Atlantic. In this paper we analyze data collected by observers

Available online 17 April 2006

on board an Uruguayan long-liner targeting swordfish in two areas in the Equatorial Eastern Atlantic: off the Gulf of Guinea and north of Saint Helenaa Island. Specimens of Lepid-


ochelys olivacea and Dermochelys coriacea were hooked or entangled in 26 longline sets. All

Sea turtle

registered interactions with olive ridleys took place off the The Gulf of Guinea, with cap-


tures ranging from 1 to 3 specimens in a single set. The captured specimens, though not


measured directly, appeared to be juveniles. In addition, the examination of the stomach

Lepidochelys olivacea

contents of one female mako shark showed dermal scutes, vertebrae and the complete

Dermochelys coriacea

head of a sea turtle identified as L. olivacea, allowing us to estimate its curved carapace length. In contrast, adult specimens of D. coriacea were caught in the two fishing areas. The capture of 10 individuals in a single set was recorded. Due to the high rate of sea turtle by-catch observed off the The Gulf of Guinea (1.02 ind/1000 hooks) conservation programs in the area should take into consideration the possible existence of a developmental and feeding area in this zone. Accordingly, longline fisheries in this area should be monitored and mitigation measures put in place to avoid or minimize damage to the pelagic phase of African populations of sea turtles. Ó 2006 Elsevier Ltd. All rights reserved.



There is increasing concern about the by-catch of sea turtles by commercial longliners and its impact on their populations, since incidental capture in various types of fishing gears is one of the main causes of injury and mortality of juvenile and adult sea turtles (NRC, 1990; Lutcavage et al., 1997; Oravetz, 1999).

Almost all known sea turtle species occur as by-catch in longline fisheries, in spite of the relative selectivity of the fishing gear (Witzell, 1984; Hall, 1996; Lewison et al., 2004). This interaction has been reported in several studies, mainly in the North Atlantic, and Pacific Oceans and the Mediterranean Sea (Nishemura and Nakahigashi, 1990; Bolten et al., 1996; Williams et al., 1996; Witzell, 1996). For the South Atlantic Ocean some reports focused on loggerheads (Caretta caretta)

* Corresponding author: Tel./fax: +598 2 4004689. E-mail addresses: [email protected] (A. Carranza), [email protected] (A. Domingo), [email protected] (A. Estrades). 0006-3207/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2006.02.003


and leatherbacks (Dermochelys coriacea) are available (Acha´val et al., 2000; Domingo et al., 2002; Kotas et al., 2003; Pinedo and Polacheck, 2004). With regard to Lepidochelys olivacea Escholtz (1829), Kleiber (1998), Arau´z et al. (2000), and Polovina et al. (2003) reported by-catch in pelagic longline fisheries off the Pacific coast of Costa Rica and Hawaii. Data on by-catch from the North Atlantic Ocean, the Caribbean Sea and the Gulf of Mexico are also available. However, the information available on this species in the South Atlantic Ocean is scarce. A ‘‘possible’’ single specimen was captured off the Brazilian shelf (Pinedo and Polacheck, 2004) and another was caught off Rio Grande do Sul, Brazil (Serafini et al., 2002). The olive ridley turtle is a pantropical species, limited to waters at or above the 20 °C isotherm. Olive ridleys live in coastal habitats, but some captures far offshore indicate that there may be a pelagic stage in their life cycle (Marcovaldi, 2001). These turtles converge on beaches in isolated areas for nesting (Ma´rquez, 1990). Reliable nesting data for the Western African coast are available for Sa˜o Tome´ Island (290 nesting females in 1998–1999 season), Bioko Island (141 nests in 1997–1998 season), and Ghana (78 nests in the 2001 season). Minor nesting beaches are found in Cote d’Ivoire, Cameroon, Equatorial Guinea, Gabon, Congo and Angola (Formia, 2003). This turtle is currently listed as ‘‘Globally Endangered’’ due to ‘‘direct observation of population reduction of at least 50% over the last 10 years and ‘‘actual or potential levels of exploitation’’ (criteria EN A1 a b d, in the IUCN Red List of threatened species, Hilton-Taylor, 2000). However, far more dramatic is the situation faced by leatherback turtles, as documented in a growing number of publications (e.g., Spotila et al., 2000; Bell et al., 2003; Casale et al., 2003; Hays et al., 2003). Dermochelys coriacea (Vandelli, 1761) is the largest and most widely distributed sea turtle, occurring worldwide in tropical and temperate waters of the Atlantic, Pacific, and Indian Oceans. The adult leatherback preys mainly upon jellyfish, but it is also known to feed on squid, crustaceans, tunicates, fish, blue-green algae, and floating seaweed. Dramatic declines in the number of nesting leatherbacks have occurred over the last two decades along the Pacific coasts of Costa Rica and Mexico, the latter population being now less than one percent of its 1980 estimated size (Spotila et al., 1996). Similarly, estimates of global population sizes for leatherbacks suggest a reduction of over 70% for the global population of adult females in less than one generation (Pritchard, 1989). Data on longline by-catch for this species are especially relevant for their conservation, since the decline of leatherback populations in the Pacific is thought to have been caused by high mortality of adults in oceanic longline and gillnet fisheries (Eckert and Sarti, 1997; Crowder, 2000; Spotila et al., 2000). In Equatorial Africa, the most important nesting population of leatherbacks is located in Pongara, Gabon, with approximately 300–500 nesting females per season (Sounguet et al., 2004; Billes and Fretey, 2004). Data for nesting leatherbacks in Angola are also available (Carr and Carr, 1991). In this context, the assessment and reduction of incidental capture and mortality of sea turtles are of extreme importance and a major conservation issue (IUCN/Species Survival Commission, 1995). Considering the critical status of the populations of these species and the few data available on their

1 3 1 ( 2 0 0 6 ) 5 2 –5 7


interaction with the longline fishing gear, all information that can be gathered is very valuable. In this paper we analyze the data on sea turtle by-catch obtained in a fishing trip on board a commercial longliner in the Equatorial Eastern Atlantic Ocean, and provide new data on the distribution area of L. olivacea and D. coriacea. We also report for the first time a record of predation on olive ridley by a mako shark.


Materials and methods

Data were collected by a scientific observer of the PNOFA (Programa Nacional de Observadores de la Flota Atunera), of the DINARA (Direccio´n Nacional de Recursos Acua´ticos, Uruguay) in a single three month trip. Observations were carried out from a 36 m long commercial fishing vessel targeting swordfish (Xiphias gladius), blue shark (Prionace glauca) and tunas (Thunnus albacares, Thunnus obesus and Thunnus alalunga). The fishing gear consisted in a standard US style monofilament polyamide longline, approximately 80 kms long, with an average of 1300 hooks baited with frozen squid (Illex argentinus) or mackerel (Scomber scombrus) per set, at a depth of 40– 60 m. (see Domingo et al., 2001 for a detailed description). The date, time, geographical position, number of hooks, depth for the fishing gear and the sea surface temperature were recorded for each set in seventy-nine sets carried out from May to September 2003. The fishing operations took place in two areas: The Gulf of Guinea, delimited by 3°35 0 N and 1°04 0 S and 0°37 0 E and 17°33 0 W and north of Saint Helena Island, delimited by 8°55 0 S and 13°25 0 S and 8°23 0 W and 3°32 0 W (Fig. 1). Results were grouped by geographic area for fishing operations, but this spatial difference is related also to a temporal difference. The Gulf of Guinea area was

Fig. 1 – Portion of the Equatorial Eastern Atlantic Ocean showing sets location with the reported interactions. Note that some interactions involved more than one turtle. Fishing effort for each area is also indicated. Symbols: +, D. coriacea; , L. olivacea.



in general fished earlier during the period than the St. Helena area, but the vessel switched between areas frequently, thus interspersing both factors. The total capture was classified as catch, discard (by-catch and release, following Hall, 1996) and lost catch (Domingo, 2002). Capture Per Unit of Effort (CPUE), as number of specimens per 1000 hooks, was calculated for both species. Histograms showing frequency of by-catch, as number of individuals per set and in each one-degree sea surface temperature category were also constructed. Differences in sea surface temperature between areas and species were examined by means of a Student t-test for independent samples. Relations between sea surface temperature and number of specimens caught were explored by means of linear regression analysis. The potential misidentification between olive ridleys and loggerheads (see Frazier, 1985; Ma´rquez, 1990) is not likely to affect these observations due to the good training of the scientific observers of the PNOFA. Particularly, the identification was based on the number of lateral scutes. The size of olive ridleys was not recorded due to the fact that all turtles were released by cutting-off the fishing line (discarded). Specimens of leatherbacks were also discarded in a similar way, though these sea turtles were mostly entangled in the fishing gear. However, the head of a L. olivacea found in the stomach of a mako shark was measured in length and width. The turtles’ carapace curved length was estimated according to morphometric data reported by Frazier (1984).



The total effort during the trip was of 102,700 hooks, averaging 1300 hooks per set.The fishing effort in the Gulf of Guinea was 23,400 hooks (18 sets), while the effort in the 61 sets conducted in St. Helena was 79,300 hooks. The sea surface temperature (SST) showed significant differences between areas (Student t-test, t = 5.25; p  0.01). SST for the Gulf of Guinea ranged from 24.7 to 28.1 °C (25.79 ± 1.04 °C), whereas in St. Helena, SST ranged from 21.9 to 28.5 °C (24.14 ± 1.20 °C). Significant differences were also observed when comparing sea surface temperatures between the areas of occurrence of both species (Student t-test,

t =  3.3; p < 0.01). No significant correlation was found between the number of specimens caught and the sea surface temperature for leatherbacks (F = 1.29; p > 0.05) or olive ridleys (F = 0.02; p > 0.05). Sea turtles were hooked or entangled in 27 longline sets, 34.2% of the sets. Five of these events (6.32% of the total sets) involved L. olivacea (nine specimens). The remaining 22 events involved D. coriacea, with 40 specimens caught. Most interactions involved a single turtle, but up to 10 individuals were caught in a single set (Fig. 2A). The total CPUE for both species of sea turtles in the whole region was 0.48. Discriminated by area, sea turtles CPUE was 1.06 in the Gulf of Guinea, and 0.3 in St. Helena (Table 1). All registered interactions with olive ridleys took place in the Gulf of Guinea, with captures ranging from 1 to 3 specimens in a single set (Table 1), in waters with a mean SST of 26.22 ± 1.00 °C (Fig. 2B). The CPUE for olive ridley’s was 0.38 individuals per 1000 hooks in The Gulf of Guinea and 0 in St. Helena. The maximum fishing depth estimated for fishing operations with L. olivacea interactions was 40 m in all but one set, in which the estimated depth was 50 m. All olive ridleys observed were hooked in the mouth. The examination of the stomach contents of one female mako shark (TL = 3.21 m, following Sadowsky, 1968) captured at 0°25 0 S, 01°44 0 W showed dermal scutes, vertebrae and the complete head of a sea turtle identified as L. olivacea. The head measured 10.1 cm in straight length and 7.5 cm in width. This corresponds roughly to a carapace curved length (CCL) of 40 cm. D. coriacea were caught in both areas: 16 specimens were recorded for the Gulf of Guinea and 24 for St. Helena. Capture by set ranged from 0 to 10 individuals, in waters with a mean SST of 24.63 ± 0.95 °C (Fig. 2B). By-catch occurred with the gear set at an estimated operational depth ranging from 40 to 60 m. When the observation was possible, leatherbacks were found either entangled in the lines (8 specimens) or hooked through the flippers or carapace (1 specimen). Direct mortality of leatherbacks was observed in two opportunities, due to heavy entanglement of individuals and posterior suffocation (5% of leatherbacks).


D. coriacea L. olivacea



No Individuals

No of Sets

14 12 10 8 6 4 2 0




4 5 6 7 No of Individuals




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26 24 22 20 18 16 14 12 10 8 6 4 2 0

D. coriacea L. olivacea



23 24 25 26 Sea Surface Temperature (˚C)



Fig. 2 – Histograms showing frequency of by-catch, as number of individuals per set (A) and number of individuals vs. sea surface temperature (B) for L. olivacea and D. coriacea.


Table 1 – Number of individuals of L. olivacea and D. coriacea caught during the fishing operations, fishing effort (as number of sets and number of hooks), number of specimens and CPUE for both species in the different areas. Total CPUE values are also shown Area

CPUE (individuals)

No. sets (No. hooks)

Dermochelys Lepidochelys coriacea olivacea


The Gulf 18 (23.400) of Guinea Saint 61 (79.300) Helena

0.64 (16)

0.38 (9)

1.06 (25)

0.3 (24)

0 (0)

0.3 (24)


0.39 (40)

0.09 (9)

0.48 (49)


79 (102.700)


CPUE values for L. olivacea off the Gulf of Guinea were higher than those mentioned in other sea turtle by-catch reports. For example, the CPUE for olive ridleys during the observation period (0.38) was close to the maximum CPUE reported by Witzell (1996) for all sea turtle species from the North Atlantic (0.0–0.4 ind/1000 hooks). If we consider the total CPUE for both species of sea turtles for the whole region, CPUE values are even higher (0.48). Further, CPUE values for both species increase if only the Gulf of Guinea area is considered (1.06). Though the observation period was rather short (May–September 2003), the effort was considerable (102.700 hooks). The captured olive ridleys, though not measured directly, appeared to be less than adult size. Large series of measurements of breeding L. olivacea from Suriname and Oaxaca, Mexico (Frazier, 1984) indicate that the minimum carapace length of nesting females is 63 cm; notes from onboard observer indicate that all ridleys caught during this study were below this size, and hence not adult. In addition, the estimated size of the L. olivacea found in the mako shark stomach (40 cm) corresponds to a juvenile stage. This supports the idea of the existence of a developmental and feeding area throughout the Gulf of Guinea, related to nearby well known nesting beaches, mainly in Sao Tome´ and Bioko Island, Ghana and Cote d’Ivoire (see Dontaine and Neves, 1999; Fretey, 1999; Fretey, 2001). The fact that all the L. olivacea specimens were captured only in the Gulf of Guinea suggests a preference for warmer waters for this species when compared with leatherbacks. This appears to confirm that olive ridley turtles are not geographically randomly distributed but stay within preferred temperature ranges that are seasonally variable (Coles and Musick, 2000) while leatherbacks are more widely distributed. The presence of L. olivacea in the Gulf of Guinea is not coincident with the reported nesting season, that takes place generally between the months of September and March (Fretey, 2001), suggesting a year-round presence of this species is this zone. However, the distribution area of this species is not well documented for the pelagic South Atlantic region. There is no record of L olivacea in St. Helena Island and adjacent waters (Fretey, 2001). Coincidentally, no L. olivacea was caught or seen in these waters for over 60 days, suggesting the possible ab-

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sence of olive ridleys in this area, at least during the observation period. Shark predation upon sea turtles (C. caretta, Chelonia mydas and Eretmochelys imbricata) has been recently reported, mainly by tiger shark Galeocerdo cuvier and great white shark Carcharodon carcharias (Musick, 2002; Ferguson et al., 2000; Simpfendorfer et al., 2001). However, to our knowledge, there was no record so far of interaction between the mako shark Isurus oxyrinchus and olive ridleys. This shark preys mainly upon fishes and cephalopods (Compagno, 1984; Vaske and Rinco´n, 1998). In contrast with olive ridleys, the leatherback specimens were apparently adults considering their large size. However, carapace length may not be a reliable indicator of maturity or breeding status for sea turtles (Miller, 1997). During this trip leatherback capture, both in absolute terms (total number of individuals) and CPUE was higher at the Gulf of Guinea. This fact, coupled with the observation of 10 individuals caught during a single set in this area may indicate a high concentration of this species during the observation period. Calculated CPUE values for leatherbacks (0.68–0.3 in the Gulf of Guinea and St. Helena, respectively) fall within the range reported by Kotas et al. (2003) for southern Brazil from March to October 1998 (1.59–0.10). Conversely, Acha´val et al. (2000) reported a CPUE of 0.37/1000 hooks for both loggerheads and leatherbacks in approximately the same area. Given the dramatic decline in the olive ridley populations in the Western Atlantic (principally Surinam nesting beaches), all African nesting sites should be considered as priority areas. Particular attention should be paid to beaches in Guinea Bissau, Sierra Leone, Cote d’Ivoire, Ghana, southern Cameroon, Sao Tome´, Bioko Island, Angola and Gabon (Fretey, 2001). Similarly, an exponential decline in leatherback nesting has occurred over the last two decades along the Pacific coasts of Mexico and Costa Rica. In this context, the pelagic longline fishery in the area should be monitored to avoid or minimize damage to the pelagic phase of African olive ridley and leatherback populations.

Acknowledgements This study was made possible by the work of onboard scientific observers of the PNOFA. We also thank the crews of the fishing vessels monitored by the Program. Special thanks to Walter Norbis (DI.NA.R.A., Uruguay) and Angela Formia (Aventures Sans Frontie`res, Gabon) for the critical reading of an earlier version of this manuscript. Thanks to Stella Weng for collaboration with the translation and review.


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Pelagic longlines: A threat to sea turtles in the Equatorial Eastern ...

In contrast, adult specimens of D. coriacea were caught in the two fishing areas. The capture of 10 individuals in a single set was recorded. Due to the high rate ...

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