How Deep Do Sea Turtles Dive?

 

By George Schillinger

Researchers watch as a satellite-tagged leatherback hatchling dives into the ocean. Tags like these have helped researchers learn about sea turtles’ diving abilities. © Lazaro Ruda

Sea turtles spend most of their lives underwater. Hard-shelled species generally stay close to shore in shallow waters, but leatherbacks mostly stay in the open sea, diving deep and often while migrating over vast distances. Some hard-shelled turtle species can dive to beyond 300 meters (980 feet), but leatherbacks are the true record holders. They can descend to below 1,000 meters (3,280 feet), deeper than any other reptile, where pressures are intense (more than 100 atmospheres) and temperatures can drop to less than 5 degrees Celsius (41 degrees Fahrenheit). Among air breathers, leatherbacks share those depths with sperm whales and elephant seals, which are capable of reaching depths of more than 2,000 meters (6,560 feet)! 

All sea turtles have special adaptations that allow them to hold their breath underwater for extended periods, as described in the article on the opposite page. And even among sea turtles, leatherbacks are especially adapted to store oxygen in their blood and tissues to help them dive. Beyond their breath-holding abilities, leatherbacks have also evolved unique physiological adaptations that allow them to brave the extreme conditions of the deep sea. Those adaptations include: 

  • Compressible carapace: The thick, leathery shell of a leatherback can compress during descent and expand during ascent, unlike the shells of hard-shelled turtles.

  • Nitrogen absorption: Leatherbacks can absorb nitrogen through their carapace, thereby avoiding decompression sickness while diving.

  • Thermal inertia: Their giant size means that leatherbacks cool slowly. Their black color also helps them recuperate heat more rapidly when basking on the surface after a dive. 

  • Countercurrent heat exchange: A leatherback’s arteries, which carry warm blood, lie close to the veins returning colder blood to the core, so that heat is conserved.

There is now an array of sensor-equipped satellite tags, animal-borne cameras, and multisensor recorders that help scientists understand those enigmatic underwater behaviors. While using early satellite tag technology in the 2000s, I was astounded to see leatherbacks diving to depths that maxed out our sensors at 1,210 meters (3,970 feet)! But now we know that leatherbacks routinely dive into the bathypelagic zone (1,000–4,000 meters’ depth, or 3,280–13,120 feet). But why?

During 2004 to 2008, our team equipped 35 adult eastern Pacific leatherback turtles with satellite relay data loggers integrated with thermistors and pressure sensors, with the goal of better understanding the whys behind leatherbacks’ deep-diving behavior. From 42,234 recorded dives, we observed that during the day the leatherbacks dove deeper and shorter than at night, when their dives were shallower and longer. Those observations pointed to a few possible explanations. One is that the turtles may be searching for prey during the daytime and feeding at night, when their prey also migrate closer to the surface. Another is that the turtles may be staying near the surface during daytime to warm up via solar radiation. We also observed that the turtles dove deeper when warmer water allowed them to, suggesting that temperature regulation is an important factor influencing their diving behaviors.

Not only are adult leatherbacks deep and frequent divers, but juveniles are too! A team of scientists from Upwell and Florida Atlantic University observed that among dozens of individuals studied, several dove over 40 meters (130 feet), including one to over 100 meters (330 feet) shortly after being released. As a group, those young animals spent more than 70 percent of their time underwater and made routine dives to beyond 70 meters (230 feet). 

From a conservation perspective, understanding diving behavior in sea turtles is important because the threats they face occur throughout the water column. Impacts can come from deep-set longlines (30–100 meters deep, or 100–330 feet), to ghost nets at any depth, to boat strikes on the surface. By integrating turtle movement datasets (both horizontally and vertically) with those on anthropogenic threats, such as datasets on fisheries, vessel traffic, or offshore development activities, we can develop new tools that identify areas of highest risk to inform targeted conservation strategies.

Author Affiliation

George Shillinger, Upwell Turtles, California, U.S.A.

This article originally appeared in SWOT Report, vol. 19 (2024). Click here to download the entire article as a PDF.

 
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