The Future of Sea Turtle Migration

By Jake Arakawa

Animal migration is a common behavior observed where species will move between two locations during their lifetime for a reason. This movement is undistracted and persistent as animals travel to their destination without stopping for resources (Bauer and Klaassen. 2013). Migration transverses different species and through different environments. The green sea turtle (Chelonia mydas),one of the five species of turtle that inhabits Hawaiian waters, engage in this migratory behavior moving between feeding and breeding sites throughout the course of their lifetime.

Green sea turtles start their migration as soon as they are born. If infant turtles, or hatchlings, can survive the gauntlet of predators on the sand-looking for an easy meal and reach the waterline, they begin a “frenzy period” or a 24 to 48-hour period of continuous swimming. The goal of this period is to quickly reach offshore developmental habitats where the turtles can mature (Pankaew and Milton 2018). To orient themselves towards the ocean, it is hypothesized that turtles use the direction of the waves to navigate out to sea. Since waves move from the open ocean towards the beach, by swimming in the opposite direction of the waves, the turtles can move further out to sea (K. Lohmann and C. Lohmann 1992). After they complete the initial frenzy period, infant turtles switch to a more passive strategy opting to swim only during the daytime (Okuyama et al. 2009).  It is estimated that only 1 in 1000 hatchlings will reach adulthood, meaning that many hatchlings are lost in this initial phase of their lives.

Traveling far from the birth beach can be beneficial for sea turtles for several reasons. Coastal environments have more predators and are therefore more dangerous for smaller turtles. Also, migrating to a site can lead to less competition over resources as it reduces population density (Cox 1966).  Once the turtles reach their new habitats, their main goal is to grow and the best way to do that is to eat. At this stage in their life, green sea turtles are omnivores, meaning they consume a combination of plants and small animals (Arthur et al. 2008). The juveniles will stay in this location until they reach adulthood at which point they will undergo another migration and move to coastal environments that are closer to their nesting areas. Adult green sea turtles can often be spotted by snorkelers in Hawaii on coral reefs along the coast. At this point in their life, the turtles are large enough to avoid predators and they become herbivores (Arthur et al. 2008).

Sea turtles can cover thousands of miles during their lifetimes. This begs the question: how do turtles know how to return to their unique birth beach? This phenomenon is called natal homing and describes when a species migrates away from its geographic birthplace only to return to the same beach to reproduce. Interestingly enough, Hawaiian monk seals are known to do the same. Sea turtles can sense magnetic waves and use these waves to navigate the waters. Magnetic fields surround the earth and different geographic locations will have different magnetic signatures (K. Lohmann and C. Lohmann 2019). The turtles’ natal beaches are marked locations because of the unique magnetic signatures coming from these beaches. This system is like an internal GPS for the turtle! Using these signatures, turtles can return to their birth beaches to nest, year after year.

As incredible as it is that turtles can memorize the locations of their natal beaches, this ability is useless if there is no beach to return to. Human activity near the coast threatens turtle nesting. The problem with coastal development is the conflict between landowners and natural processes. Overtime, erosion caused by rising sea levels pushes beaches further inland. However, with construction of structures like seawalls and other protective devices, the shoreline cannot continue to move upwards. As erosion continues, the beach can disappear if the shoreline isn’t allowed to push forward (Fox 2017).  If nesting beaches were to disappear, the turtles won’t have a place to lay eggs which could threaten the future of the species. This has been observed in Hawaii, where destruction of islands of the French Frigate Shoals in the northwestern Hawaiian islands (NWHI) forced turtles to find new nesting beaches elsewhere. With 90% of nesting occurring in the NWHI, there has been a shift with more green sea turtles forced to nest in the main Hawaiian islands (Tiwari et al. 2010). Nesting on Oahu has become more noticeable in recent years.

With more nesting occurring in the main Hawaiian islands, another threat that has arisen is artificial coastal lighting. Sea turtles rely on the light of the moon and stars to navigate to the ocean, meaning that artificial light can disorient the hatchlings away from the shoreline, causing them to die from exhaustion (Thums et al. 2016).  Furthermore, a study conducted on green sea turtle hatchlings showed that artificial light also affected their ability to navigate in the water. If the turtles cannot reach deeper waters, this can increase their risk of predation, reducing their likelihood of survival (Thums et al. 2016).

While these issues can be challenging, there is evidence of nesting at beaches where they were never seen before. On June 11th, 2020, the Honolulu Star Advertiser reported that for the first time in documented history, green sea turtles nested at Bellows beach in Waimanalo (Wu 2017). This is a promising development for the future of green sea turtle nesting in Hawaii and hopefully will rally momentum and conservation actions regarding the threats facing this species. A great way to get involved is to minimize coastal light if living on the beachside, volunteering and donating to sea turtle conservation groups, or properly reporting any sea turtle nests, hatchlings, and emergencies immediately.

References:

  • Arthur, Karen E., et al. “Ontogenetic Changes in Diet and Habitat Use in Green Sea Turtle (Chelonia Mydas) Life History.” Marine Ecology Progress Series, vol. 362, 2008, pp. 303–311. JSTOR, www.jstor.org/stable/24872585. Accessed 18 Aug. 2020.
  • Bauer, Silke, and Marcel Klaassen. “Mechanistic Models of Animal Migration Behaviour – Their Diversity, Structure and Use.” Journal of Animal Ecology, vol. 82, no. 3, 2013, pp. 498–508., www.jstor.org/stable/24034618. Accessed 18 Aug. 2020.
  • Cox, George W. âThe Role of Competition in the Evolution of Migration.â Evolution, vol. 22, no. 1, 1968, pp. 180â192. JSTOR, www.jstor.org/stable/2406662. Accessed 22 Aug. 2020.
  • Fox, Alex. “Threatened Sea Turtles in Hawaii Losing Ground to Rising Oceans.” Eos, 14 Dec. 2017, doi:10.1029/2017eo089065.
  • Lohmann, Kenneth J, and Katherine M Fittinghoff Lohmann. “Orientation to Oceanic Waves by Green Turtle Hatchlings.” Journal of Experimental Biology, ser. 171, 6 May 1992, pp. 1–13. 171.
    Lohmann, Kenneth J., and Catherine M. F. Lohmann. “There and Back Again: Natal Homing by Magnetic Navigation in Sea Turtles and Salmon.” The Journal of Experimental Biology, vol. 222, no. Suppl 1, 2019, doi:10.1242/jeb.184077.
    Okuyama, Junichi, et al. “Ontogeny of the Dispersal Migration of Green Turtle (Chelonia Mydas) Hatchlings.” Journal of Experimental Marine Biology and Ecology, vol. 379, no. 1-2, 2009, pp. 43–50., doi:10.1016/j.jembe.2009.08.008.
  • Pankaew, Karen, and Sarah L. Milton. “The Effects of Extended Crawling on the Physiology and Swim Performance of Loggerhead and Green Sea Turtle Hatchlings.” The Journal of Experimental Biology, vol. 221, no. 1, 2017, doi:10.1242/jeb.165225.
  • Thums, Michele, et al. “Artificial Light on Water Attracts Turtle Hatchlings during Their near Shore Transit.” Royal Society Open Science, vol. 3, no. 5, 2016, p. 160142., doi:10.1098/rsos.160142.
  • Tiwari, M, et al. “Estimating Carrying Capacity at the Green Turtle Nesting Beach of East Island, French Frigate Shoals.” Marine Ecology Progress Series, vol. 419, 2010, pp. 289–294., doi:10.3354/meps08833.
  • Wu, Nina. “Unprecedented’ Turtle Nesting at Bellows Beach Closes Campground until Sept. 4.” Honolulu Star Advertiser, 11 June 2020.