Neeman, Noga, Nathan J. Robinson, Frank V. Paladino, James R. Spotila, and Michael P. O’Connor. “Phenology shifts in leatherback turtles (Dermochelys coriacea) due to changes in sea surface temperature.” Journal of Experimental Marine Biology and Ecology, 462 (2015): 113-120. DOI:10.1016/j.jembe.2014.10.019
The Migration of Leatherback Turtles
How will climate change affect organisms in the ocean? This is a huge question with many different components, unknowns, and complications. One commonly studied aspect of climate change is how it changes phenology. Phenology refers to the timing of a seasonal event (such as when trees flower in the spring) and these events often use temperature as a “start” cue. Many species adapt to climate change by changing their own event timings. For example, some trees have begun to flower earlier. (Interested in this topic?)
Enter our species of interest: the adorable leatherback turtle (Dermochelys coriacea). Leatherback turtles spend much of their time in cold waters, looking for tasty gelatinous zooplankton. Every 2-7 years, leatherback turtles migrate to warmer, subtropical beaches to mate and lay their eggs. The timing of these migrations between foraging and nesting grounds is very important. For example, baby turtles are dispersed by ocean currents when these hatchlings enter the ocean for the first time. Any change in when nesting occurs could change hatchling dispersion, meaning that the fate and survival of young turtles could change!
So why is this topic so complicated? Firstly, it is unclear which temperature cue causes leatherback turtles to migrate. The leading hypothesis is that warming waters in foraging grounds trigger migration to the nesting area. However, some researchers believe that leatherback turtles migrate early to nesting grounds and wait for a temperature cue to begin egg laying. Secondly, changes in leatherback turtles’ migration to nesting areas have been observed, but they do not appear to be correlated to any climate indices (for example, no connection to warming).
Previous research has demonstrated some of the negative effects warming surface water temperatures have caused for leatherback turtles. For example, increasing temperatures have decreased nesting abundance, increased egg mortality, and even caused an increase in the female to male sex ratio. This study by Neeman et al. aimed to determine: 1) if leatherback turtles use temperature as their cue to migrate to a nesting area and 2) how, or whether, climate change will affect this migration.
The approach: Turtles, Temperature, and Timing
Three nesting areas were investigated in this study: Playa Grande (Costa Rica), Tortuguero (Costa Rica), and Sandy Point (St. Croix). (You can track the turtles nesting at Tortuguero here). Nesting events, or when leatherback turtles crawled up on the beach to dig a nest and lay eggs, were counted by looking for tracks during surveys every day and night (except for Tortuguero, which occurred every 3 days). These nesting track surveys were conducted from October-March for 19 seasons at Playa Grande, March-June for 9 seasons at Toruguero, and March-August for 27 seasons at Sandy Point. The start date of nesting was designated to be when 5 or 10% of the total nests made that year were established. This percentage approach was a precaution in case “early” nesters arrived before the surveys began.
Both maximum and minimum sea surface temperatures were determined using a NOAA data set in order to determine if any temperature changes acted as cues for migration. Additionally, net primary productivity was estimated using the Ocean Productivity website and was used as a proxy for food availability at the turtles’ foraging grounds. Seasonality was assessed by looking at deviations in temperature at the leatherback turtles’ foraging grounds and then correlating these deviations with when those first nests were established.
What did they find?
There was no correlation between the start of nesting and the local sea surface temperature but there was at least one relation at each site between nesting start date and the foraging region’s sea surface temperature. This finding supports the hypothesis that leatherback turtles use a temperature cue at their foraging region to begin migrating to their nesting grounds.
The most apparent trend was that increased temperatures at foraging grounds were correlated to later nesting dates. This actually contradicted other observations from different studies! Additionally, although the data was not as complete as the scientists would have liked, there was no relationship found between net primary productivity and sea surface temperature at the foraging grounds. This could imply that phenology changes were not likely caused by food shortages.
Migratory marine critters are difficult to study. Turtles at the nesting sites could have come from many different foraging areas, all of which could experience temperature changes differently. In the future, it would be helpful to determine whether or not those late nesters all came from the same foraging region. For example, the Gulf of Mexico foraging region had very different trends observed than the other foraging areas in this study (Figure 2). Neeman et al. hypothesized that the warmer temperatures could have caused more food to be available in the Gulf of Mexico region, which could be linked to river discharge instead of their net primary productivity proxy. (So these turtles may have been late to the nesting party because the food was just so good!). This also implies that factors other than temperature could be as play. For example, increased discharge from the Mississippi River means more nutrients, which is usually associated with more productivity (and thus jellyfish!).
Phenological observations (timing of migration) are complex and variable by site, but the overall trend observed a connection between increasing water temperatures and later nesting. This trend supports the hypothesis that leatherback turtles use a temperature cue at their foraging grounds to start their migration to their nesting regions. However, we need more research to better understand the factors other than temperature that affect turtle phenology, such as hypoxia and streamflow, to better enact policies and management plans for these critters.
To conclude the paper, Neeman et al. asked the question: “Will delayed seasons help mitigate climate change effects on [leatherback turtles] or exacerbate them?”
What do you think?