Paper: Shimada, T., C. Limpus, R. Jones, J. Hazel, R. Groom, and M. Hamann. 2016. Sea turtles return home after intentional displacement from coastal foraging areas. Marine Biology 163:8. DOI: 10.1007/s00227-015-2771-0.
Writing this post prompted more introspection than I had expected. I often feel I am displaced, like a sea turtle captured and released far from home. I spent the first 19 years of my life in St. Louis, MO. My family is there as are many of my friends. I also feel comfortable there: I recognize the schools and parks, know where the hospitals are, and most importantly, I know where to find the good pizza (Imo’s!). My choices pulled me away for college, a few marine conservation internships in southwest FL, grad school in RI… Recently, when I was looking for the next step, I wanted to be somewhere familiar (but needed an ocean nearby) so back to southwest FL I came! It’s great most of the time, but it’s still not home.
Sea turtles that get displaced overwhelmingly head for home, so maybe I will follow their lead one day…
Sea Turtle Displacement
Why do sea turtles get moved away from their homes in the first place? Well, they don’t make the decision on their own, unlike myself. In certain situations, people remove them from their normal habitat and release them somewhere else. For example, it may be logistically difficult to return rehabilitated or formerly captive animals where they were found or it may seem beneficial to release them where better conditions exist. In some cases, animals are moved to establish new populations or increase genetic diversity. Finally, in response to catastrophes, such as oil spills, vulnerable animals may be moved out of the impacted region. But, the effectiveness of moving them often depends on them staying where released.
Sea turtles are known to exhibit site-fidelity, meaning they return to the same breeding sites and foraging grounds. While some studies have investigated turtles returning to breeding grounds, there is little information about the ability of turtles to return to their other homes—their feeding (foraging) grounds. The researchers involved in this study wanted to know whether sea turtles remain at their new locations after displacement and hypothesized that they would return home. They also asked what factors influence whether they stay where released, including distance they were moved and how long they lived in captivity And finally, they asked what environmental factors influenced their speed of return and routes taken.
So, how do scientists study something like this? Due to ethical, logistical and funding considerations (satellite tags are expensive!), displacing a bunch of sea turtles was not a good option. Therefore, the authors used satellite tracking data that had been gathered for other purposes in work with sea turtles in coastal foraging grounds.
They examined 113 tracks of sea turtles from foraging grounds in north-eastern Australia from 1996-2014. About half were released back to their home area while the other half were displaced after being captured for research, as bycatch, or rescued and rehabilitated. Their tracks were statistically analyzed, but breeding migrations were eliminated.
Displaced turtles returned home! 88% (52/59) returned to their original areas and another 4 individuals moved toward home but settled between 1.8 and 14.1 km of capture location. Two had transmission cease during travel. Only one did not move towards its capture location during a 120 day tracking period. All non-displaced turtles remained in their home foraging zones.
For homing turtles, sea surface temperature (SST) best predicts traveling speed and straightness of route with cooler SST indicating faster travel speeds and straighter paths. These trends are contrary to what would be expected based on sea turtle physiology (they are ectotherms so tend to have greater activity in warmer waters). Sea turtles also traveled faster in the daytime than at night.
So what does that all mean?
Sea turtles do not stay where humans move them. There is strong evidence that they have strong homing abilities. Even those displaced by a great distance (up to 117.4 km or 72 mi.) or held in captivity for a long time (up to 514 days) returned to their homes. Site fidelity is indeed strong in these animals. The benefit of this behavior is as yet unknown, but may relate to site familiarity (something hard to measure). It makes sense though; long residence in a particular area suggests the sea turtles learn where to find food, shelter, where predators hang out and places to avoid them (just like I know where to find my parents’ and friends’ houses, restaurants/grocery stores, etc. in St. Louis). Local knowledge would benefit the turtles greatly so they could maximize foraging while minimizing predation risk. Therefore, there are great costs in being displaced: energy to travel back, certainly, but also higher risk and less efficient feeding in the unfamiliar areas. They are clearly capable of finding their way back. For conservation, that means that moving them is only a short-term fix and may be useful during an event like an oil spill. Since we will not succeed in moving sea turtles to good habitat zones, we must instead protect the habitats that they already use.