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Olive Ridley

Lepidochelys olivacea

 

Olive ridleys are the second smallest marine turtle species, and also the most abundant (1). The species is found throughout tropical and subtropical waters worldwide, and nests along the beaches of nearly sixty different countries (2). In some locations olive ridleys display an unusual synchronous nesting strategy called an arribada, in which up to hundreds of thousands of females lay their eggs in the span of several days on the same stretch of beach (3). The largest arribadas occur at on the Pacific coasts of Central America and Mexico, and at multiple locations in the Indian state of Orissa (4). Arribada nesting can leave olive ridleys particularly vulnerable to direct harvest and egg collection (5), practices which are still common in many parts of the world even where sea turtles are protected by law (2,6,7,8). Direct harvest for leather and consumption was responsible for the species’ decline in abundance, particularly in Mexico, in the twentieth century (7). A conservative estimate of the species’ global decline, based on observations of nesting females, is 31-36% (2).

Olive ridleys are listed under Appendix I of the Convention on International Trade inEndangered Species of Flora and Fauna and classified as Vulnerable by the International Union for Conservation of Nature (IUCN) (2). In the United States they are listed as Threatened under the U.S. Endangered Species Act (populations breeding off the Pacific coast of Mexico are listed separately as Endangered) (2,5). Sea turtles are protected in many countries at the national level as well.

Olive ridleys interact with pelagic longline fisheries targeting mahi mahi, swordfish, billfish and tuna throughout much of their range (9,10). Mortality can occur when turtles bite baited longline hooks and drown. A study of post-release mortality indicates that olive ridleys survive interactions with shallow-set longline gear if they are lightly hooked and subsequently released; however, mortality rates can be affected by many variables including hook size and shape, the depth at which hooks are set, and how deeply the turtle is hooked (10,11). Gillnet, purse seine, and trawl fisheries are also responsible for high levels of bycatch mortality in the western Atlantic and along the coasts of Central America and India (2). An estimated 90,000 turtles were killed in less than a decade by gillnet and trawl fisheries off the Orissa coast of India (4). Although large-scale industrial fisheries are often the focus of bycatch mitigation, small artisanal fisheries can function as bycatch “hotspots”(7).

General strategies for preventing sea turtle bycatch can include regulatory controls like time and area closures; for example, seasonal restrictions on fishing effort near nesting beaches (8). Other spatially based strategies focus on reducing fishing effort in areas where bycatch is most likely to occur. Real-time fleet communication can help fishermen identify and avoid bycatch hotspots in the short term (12), while remote sensing and tracking data can be used to identify important migration and foraging habitats (1).

Longline mortality can be reduced with modifications to fishing gear and fishing techniques, including the use of circle hooks (11) and the elimination of shallow sets, which account for most longline mortalities (13).  Turtle excluder devices (TEDs) are required in trawl fisheries in many countries, including the United States and Australia. TEDs can be very effective, as in Australia’s northern prawn fishery (14), but noncompliance, inadequate design, and lack enforcement may limit their effectiveness even where they are required by law (4,15,16).

Another strategy for reducing trawl mortality is to restrict tow times [the amount of time the gear is submerged]. Bycatch prevention strategies are more likely to succeed when they are developed on a fishery-specific basis (9), involve collaboration among stakeholders, incorporate monitoring, and maintain high levels of compliance (15). In artisanal and subsistence fisheries, bycatch mitigation may require the development of economic alternatives to replace or supplement fishing effort (8)

 

1.  Polovina, J.J., G.H. Balazs, E.A. Howell, D.M. Parker, M.P. Seki, and P.H. Dutton. 2004. Forage and migration habitat of loggerhead (Caretta caretta) and olive ridley (Lepidochelys olivacea) sea turtles in the central North Pacific Ocean. Fisheries Oceanography 13 (1): 36-51

2.  Abreu-Grobois, A. and Plotkin, P. 2008. Lepidochelys olivacea. In: IUCN 2008. 2008 IUCN Red List of Threatened Species. <www.iucnredlist.org>. Downloaded on 15 October 2008.

3.  Ekrich, C.E., and D.W. Owens. 1995. Solitary versus arribada nesting in the olive ridley sea turtles (Lepidochelys olivacea): a test of the predator-satiation hypothesis. Herpetologica 51(3): 349-354

4.  Shanker, K., B. Pandav, and B.C. Choudhury. 2003. An assessment of the olive ridley turtle (Lepidochelys olivacea) nesting population in Orissa, India. Biological Conservation 115: 149-160

5.  NOAA. NMFS. Office of Protected Resources. Olive Ridley Turtle (Lepidochelys olivacea). http://www.nmfs.noaa.gov/pr/species/turtles/oliveridley.htm Downloaded 15 October 2008.

6.  Chan, S. K., I.J. Cheng, T. Zhou, H.J. Wang, H.X.Gu, and X.J. Song. 2007. A comprehensive overview of the population and conservation status of sea turtles in China. Chelonian Conservation and Biology 2007 6(2): 185-198

7.  Koch, V., W.J. Nichols, H. Peckham, and V. de la Toba. 2006. Estimates of sea turtle mortality from poaching and bycatch in Bahía Magdalena, Baja California Sur, Mexico. Biological Conservation 128: 327-334

8.  Weir, C.R., T. Ron, M. Morais, and A.D.C. Duarte. 2007. Nesting and atsea distribution of marine turtles in Angola, West Africa, 2000-2006: occurrence, threats and conservation implications. Oryx 41(2): 224-231

9.  Gilman, E., E. Zollett, S. Beverly, H. Nakano, K. Davis, D. Shiode, P. Dalzell and I. Kinan. 2006. Reducing sea turtle by-catch in pelagic longline fisheries. Fish and Fisheries 7:2-23

10.  Swimmer, Y., R. Arauz, M. McCracken, L. McNaughton, J. Ballestero, M. Musyl, K. Bigelow, and R. Brill. 2006. Diving behavior and delayed mortality of olive ridley sea turtles Lepidochelys olivacea after their release from longline fishing gear. Marine Ecology Progress Series 323: 253-261

11.  Read, A.J. 2003. Do circle hooks reduce the mortality of sea turtles in pelagic longlines? A review of recent experiments. Biological Conservation 135: 155169

12. Gilman, E.L., P. Dalzell, and S. Martin. 2006. Fleet communication to abate fisheries bycatch. Marine Policy 30: 360-366

13.  Polovina, J.J., E. Howell, D.M. Parker, and G.H. Balazs. Divedepth distribution of loggerhead (Caretta caretta) and olive ridley (Lepidochelys olivacea) sea turtles in the central North Pacific: Might deep longline sets catch fewer turtles? Fishery Bulletin 101(1): 189-193

14.  Brewer, D., D. Heales, D. Milton, Q. Dell, G. Fry, B. Venables, and P. Jones. 2006. The impact of turtle excluder devices and bycatch reduction devices on diverse tropical marine communities in Australia’s northern prawn trawl fishery. Fisheries Research 81: 176-188

15.  Cox, T.M., R.L. Lewison, R. Zydelis, L.B. Crowder, C. Safina, and A.J. Read. 2007. Conservation Biology 21(5): 1155-1164

16.  Lewison, R.L., L.B. Crowder, and D.J. Shaver. The impact of turtle excluder devices and fisheries closures on loggerhead and Kemp’s Ridley strandings in the western Gulf of Mexico. Conservation Biology 17(4): 1089-1097