seaturtle.org : MTN : ARCHIVES : Sign In

Mass nesting at these beaches often occurs within a few kilometers of the river mouth, with only sporadic nesting along the rest of the coastline. Even along large stretches of coast where only sporadic nesting occurs, nesting densities are often higher near river mouths. Tripathy et al. (2003) found that the nesting density of olive ridleys on the east coast of India was several times higher within 5 – 10 km of major river mouths than the rest of the coast. On Great Nicobar Island (India) in the Bay of Bengal, olive ridleys share nesting beaches with leatherback turtles, at the mouths of the Rivers Galathea, Alexandria and Dagmar (Andrews et al. 2006). This pattern of higher densities of nests near river mouths has also been observed in South America and Africa, including Sergipe in Brazil (J.C. de Castilhos pers. comm. 2019) and in Gabon and the Republic of Congo (K. Metcalfe & B. Godley pers. comm. 2019). At La Flor in Nicaragua, the highest concentrations of nesting occur in front of small estuaries that do not break open during the nesting season (S. Honarvar, pers. comm. 2020).

There is evidence of dynamic shifts in topography at many of these nesting beaches, including at all the mass nesting beaches in Odisha, India. In Gahirmatha to the North, nesting occurred on a several kilometer-long spit, which was part of the mainland coast at the mouth of the River Maipura from the early 1970s onwards when it was first documented (Bustard 1976) till the late 1980s (Pandav et al. 1998). In 1989, this spit broke away during a cyclone and formed a separate island where nesting subsequently occurred. This island was then further divided into smaller sand bars during cyclones that occurred in the late 1990s. These sand bars undergo a significant amount of erosion and accretion, which has also resulted in the spatial movement of these islands. In Rushikulya, a monitoring program over the last decade has shown substantial shifts in the nesting beach, caused by erosion and accretion at the river mouth (Chandarana et al. 2017).

A similar pattern of beach erosion and accretion has been observed at the mass nesting beaches in Ostional (Valverde et al. 2012) and other locations in Costa Rica (R. Arauz pers. comm. 2019). In Suriname, there have been significant changes in the topography of Eilanti beach caused by the Marowijne river, resulting in shifts in nesting (Hoekert et al. 1996; Goverse 2003). Biologists working at many other olive ridley nesting sites have observed that beaches near lagoons and estuaries undergo frequent erosion and accretion (J.C. de Castilhos, Brazil; M. Girondot and V. Plot, French Guiana; K. Metcalfe, Gabon, pers. comm. 2019; K.S., Orissa and Andamans, unpubl. data).

In island systems such as the Andaman and Nicobar Islands, olive ridleys rarely nest on beaches with reefs and rocks in the offshore waters, like green and hawksbill turtles do (Andrews et al. 2006). This may be partly due to the fact that, as small animals, they are averse to getting knocked about against hard substrates and injured. Hence, the safest offshore approaches on islands may be on beaches and sand bars created by river mouths. But why do they choose locations near river mouths even on mainland coasts that have vast stretches of sandy beaches?

There are multiple hypotheses for the affinity of olive ridley turtles for beaches near river mouths. I argue that it is most likely related to their mass nesting behavior. Every year, at any mass nesting beach, these turtles lay millions of eggs in a small area, often less than one hundred meters wide, and 2 to 3 km long. A significant proportion of these eggs are destroyed, and rot, leading to an accumulation of soil microbes and fauna. After a few years of nesting at more or less the same site, the potential for infection and infestation could increase dramatically. There is evidence that nest density can affect hatching success at mass nesting beaches (Fonseca et al. 2009; Ocana et al. 2012) through either increased microbial load (Bezy et al. 2015) or its effect on oxygen and carbon dioxide concentrations (Honarvar et al. 2008).

What better way to deal with this situation than have nature replace the beach periodically. In Odisha, for example, with the northeast monsoon and seasonal cyclones, the mass nesting beaches experience significant erosion every few years and are sometimes completely destroyed (Chandarana et al. 2018). As we have observed, river mouths are dynamic; heavy rains or storms often result in changes in the courses of rivers and in particular, the locations of river mouths. Thus, stretches of beach keep getting washed away on a periodic basis. At the same time, new beaches and sand bars are created that were not present before with the accretion of sand. In effect, the patch of sand with millions of putrefying eggs and/or their microbial communities is washed away and a new beach with clean sand is prepared for the olive ridleys. This could significantly increase hatching success and productivity and contribute substantially to population recruitment. In one instance, hatching success at Nancite increased significantly after a flooding event (R. Valverde pers. comm. 2020).

There are other hypotheses that may explain this pattern of nest site selection. Sand bars of the type that are found near river mouths may harbor fewer predators. Since olive ridleys lay shallow nests that are susceptible to predation, this could explain the affinity for river mouths in both solitary and mass nesting populations of the species. While there can be on-shore factors influencing nest-site selection, there could be in-water or oceanographic factors as well. Mesoscale oceanographic features such as currents and eddies may also influence the location of mass nesting beaches (Coria-Monter et al. 2019) due to their effect on productivity and adult or hatchling energy expenditure. Being near a river mouth may, in particular, aid in hatchling transport away from the beach, which could increase their fitness (see Putman et al. 2010, 2012).

Since arribadas may have played a significant role in ridley evolution, through predator satiation, (Eckrich & Owens 1995; Bernardo & Plotkin 2007), it seems compelling that they would select sites where hatching success may be maximized. While I provide a possible explanation of the evolutionary cause for olive ridley preference for river mouths, there are likely to be proximate cues as well such as salinity, sea surface temperature, moisture, bathymetry or other physiographic features. Future studies should explore both ultimate and proximate causes of nest site selection in olive ridley turtles.

Acknowledgements. Since there was relatively little published information about olive ridley nest site preference, I relied on the experience of several sea turtle biologists and conservationists, prominently Matthew Godfrey, for general advice and M. Muralidharan for discussions over many years. I thank the following colleagues for their prompt responses and valuable inputs on nesting beaches in different countries/regions: Shaya Honarvar (Nicaragua), Randall Arauz and Roldan Valverde (Costa Rica), Colum Muccio (Guatemala), F. Abreu-Grobois (Mexico), Thane Wibbels (Kemp’s ridleys in Mexico), Marc Girondot, Virginie Plot and Benoit de Thoisy (French Guiana), Jacqueline C. de Castilhos (Brazil) and Brendan Godley and Kristian Metcalfe (West Africa). And finally, I thank two anonymous reviewers for their detailed comments on the manuscript.

Figure 1. Tracks from nesting olive ridley sea turtles during an arribada on Rushikulya nesting beach in Odisha, India (photo by Kalyan Varma).

ANDREWS, H.V., S. KRISHNAN & P. BISWAS. 2006. Distribution and status of marine turtles in the Andaman and Nicobar Islands. In: Shanker, K. & B.C. Choudhury (Eds.). Marine Turtles of India. Universities Press, Hyderabad, India. pp 33-57.

BERNARDO, J. & P.T. PLOTKIN. 2007. An evolutionary perspective on the arribada phenomenon and reproductive behavioral polymorphism of olive ridley sea turtles (Lepidochelys olivacea). In: P. Plotkin (Ed.). Biology and conservation of ridley sea turtles. The Johns Hopkins University Press, Baltimore. pp 59-87.

BÉZY, V.S., R.A. VALVERDE & C.J. PLANTE. 2015. Olive ridley sea turtle hatching success as a function of the microbial abundance in nest sand at Ostional, Costa Rica. PLoS ONE 10(2): e0118579.

BUSTARD, H.R. 1976. World’s largest sea turtle rookery? Tigerpaper 3: 25.

CHANDARANA, R., M. MANOHARAKRISHNAN & K. SHANKER. 2017. Long-term Monitoring and Community-based Conservation of Olive Ridley Turtles in Odisha. CMPA Technical Series No. 7. Indo-German Biodiversity Programme, GIZ-India, New Delhi. 43pp.

CORIA-MONTER, E., S. DE LEÓN, A. MONREAL-GÓMEZ & E. DURÁN-CAMPOS. 2019. Are hydrographic mesoscale structures associated with the mass nesting behavior of the olive ridley sea turtle? International Journal of Oceans and Oceanography 13: 199-209.

CORNELIUS, S.E. & D.C. ROBINSON. 1985. Abundance, distribution and movements of olive ridley sea turtles in Costa Rica V. Rept. USFWS, Albuquerque, NM. USA. 143pp.

ECKRICH, C.E. & 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: 349-354.

FONSECA, L.G., G.A. MURILLO, L. GUADAMÚZ, R.M. SPÍNOLA & R.A. VALVERDE. 2009. Downward but stable trend in the abundance of arribada olive ridley (Lepidochelys olivacea) sea turtles at Nancite Beach, Costa Rica for the period 1971−2007. Chelonian Conservation & Biology 8: 19−27.

GOVERSE, E. 2003. Aerial survey of the coastline of eastern Suriname and nesting beach characteristics. The sea turtles of Suriname 2002 Project. Guianas Forests and Environmental Conservation Project (CFECP). Technical Report, World Wildlife Fund Guianas/Biotopic Foundation Amsterdam, the Netherlands. 21pp.

HOEKERT, W.E.J, A.D. SCHOUTEN, L.H.G. VAN TIENEN & M. WEIJERMAN. 1996. Is the Surinam olive ridley on the eve of extinction? First census data for olive ridleys, green turtles and leatherbacks since 1989. Marine Turtle Newsletter 75:1-4.

HONARVAR S., M.P. O’ CONNOR & J.R. SPOTILA. 2008. Density-dependent effects on hatching success of the olive ridley turtle, Lepidochelys olivacea. Oecologia 157: 221-230.

KAR, C.S. 1982. Discovery of second mass nesting ground of the pacific olive ridley sea turtles Lepidochelys olivacea in Orissa, India. Tiger Paper 9: 6-7.

OCANA, M., M. HARFUSH-MELENDEZ & S.S. HEPPELL. 2012. Mass nesting of olive ridley sea turtles Lepidochelys olivacea at La Escobilla, Mexico: linking nest density and rates of destruction. Endangered Species Research 16: 45-54.

PANDAV, B., B.C. CHOUDHURY & C.S. KAR. 1994. Discovery of a new sea turtle rookery in Orissa, India. Marine Turtle Newsletter 67:15-16.

PANDAV, B., B.C. CHOUDHURY & K. SHANKER. 1998. The Olive Ridley sea turtle (Lepidochelys olivacea) in Orissa: an urgent call for an intensive and integrated conservation programme. Current Science 75: 1323-1328.

PLOT, V., B. DE THOISY, S. BLANC, L. KELLE, A. LAVERGNE, H. ROGER-BÉRUBET, Y.

TREMBLAY, S. FOSSETTE & J.Y. GEORGES. 2012. Reproductive synchrony in a recovering bottlenecked sea turtle population. Journal of Animal Ecology 81: 341-351.

PUTMAN, N.F., R. SCOTT, P. VERLEY, R. MARSH & G.C. HAYS. 2012. Natal site and offshore swimming influence fitness and long-distance ocean transport in young sea turtles. Marine Biology 159: 2117–2126.

PUTMAN, N.F., T.J. SHAY & K.J. LOHMANN. 2010. Is the geographic distribution of nesting in the Kemp’s ridley turtle shaped by the migratory needs of offspring? Integrative and Comparative Biology 50: 305-314.

SHANKER, K., B. PANDAV & B.C. CHOUDHURY. 2003. An assessment of the olive ridley turtles (Lepidochelys olivacea) nesting population in Orissa, India. Biological Conservation 115: 149-160.

TRIPATHY, B., K. SHANKER & B.C. CHOUDHURY. 2003. Important nesting habitats of olive ridley turtles Lepidochelys olivacea along the Andhra Pradesh coast of eastern India. Oryx 37: 454-463.

VALVERDE, R., C. ORREGO, M. TORDOIR, F. GÓMEZ, D. SOLÍS, R. HERNÁNDEZ, G. GÓMEZ, L. BRENES, J. BALTODANO, L. FONSECA & J.R. SPOTILA. 2012. Olive ridley mass nesting ecology and egg harvest at Ostional Beach, Costa Rica. Chelonian Conservation & Biology 11: 1-11.