Cold-Stunned Sea Turtles on the Texas Coast: The Record Event of February 2021

By Donna J. Shaver and J. Shelby Walker, Padre Island National Seashore. First published in Park Science magazine, Volume 38, Number 1, Summer 2024.

Volunteers at Padre Island National Seashore rescue a cold-stunned sea turtle on the Laguna Madre shore, on the park's west side. Credit: National Park Service.

When another bout of extreme cold weather arrives, staff and volunteers of Padre Island National Seashore's sea turtle programme bundle up, pick up long-handled nets and walk the shoreline. All sea turtles in U.S. waters are protected under the Endangered Species Act, but a sudden drop in water temperature can trap them in shallow bays and stun them, and the wind pushes them ashore. Sluggish from the cold, they are easily struck by boats or killed by predators. Although climate change is warming the world on average, it is also linked to more extreme winter cold snaps — and park research shows green turtle numbers fall after such events.

Why the cold is so dangerous

Sea turtles are ectothermic: they cannot control their body temperature through metabolism or activity, and depend on outside heat such as warm water and sunshine. Sudden temperature changes hit them hardest in shallow water like the Laguna Madre, the salty lagoon along the seashore — one of the most important habitats for juvenile green turtles in the western Gulf of Mexico. Averaging only about four feet deep, it can cool very quickly.

February 2021: a record

During a severe cold front in February 2021, partners in NOAA's Sea Turtle Stranding and Salvage Network documented 13,326 cold-stunned green turtles in Texas.

• It is hard to count a mobile species, but the seashore's turtle team estimates the event likely affected every green turtle in Texas inshore waters.
• By park data it was more than triple the previous largest Texas event, in winter 2017–2018, and nearly three times the previous largest in the whole United States, in Florida.
• It was the largest sea turtle stranding ever recorded worldwide.

A volunteer lifts a cold-stunned green turtle from the Laguna Madre. Credit: National Park Service.

Park scientists now consider cold stunning the greatest cause of green turtle strandings and deaths in Texas. Some stunned turtles at first have milder injuries, such as non-lethal inhalation of water or weakened immunity, but without rescue and rehabilitation they may fall to predators or fail to survive the months of recovery. A recent park analysis found green turtle numbers drop in the years after large, lethal cold-stunning events — a sign of population-level effects.

Racing the cold

Every year park staff, volunteers and partners spend hundreds to thousands of hours monitoring, rescuing and releasing cold-stunned green turtles. Patrols go out when air and water temperatures drop, to find turtles while they are still alive — usually successfully: in a relatively small event in 2022, 97 percent of the 436 turtles found were alive. But in 2021 there were so many that responders found only 32 percent alive; the seashore and its partners lacked the people, time and boats to reach them all before exposure or predators did.

Donna Shaver scans a cold-stunned green turtle for a passive integrated transponder tag, used to identify and follow individual turtles over time. Credit: National Park Service.

Scientists have linked these cold events to Arctic warming, which pushes polar vortices south into mid-latitude North America. As climate change makes weather more extreme, more such disasters are likely.

Why save them

Green turtles might not be endangered at all but for people. In south Texas they were caught and harvested nearly to extinction from the mid-1800s to 1900, and extreme cold in 1894 and 1899 then killed many of the few left. Their numbers in Texas have rebounded in recent years, but worldwide they remain far fewer than they once were.

Six sea turtle species in the United States — green, hawksbill, Kemp's ridley, leatherback, loggerhead and olive ridley — are listed as endangered or threatened. Phillippe Tissot, a co-principal investigator at the National Science Foundation's AI Institute for Research on Trustworthy AI in Weather, Climate, and Coastal Oceanography, calls sea turtles sentinel species, notes that the impacts of climate change are only beginning, and expects climate-ready programmes like Padre Island's to be critical to their recovery.

A one-two punch from climate change

Sea turtles have been around since the dinosaurs, but disease and habitat loss to development threaten them, and climate change adds more:

• Delayed migration. As shallow waters stay warm longer each year, juveniles put off their winter move to deeper, warmer water — and are caught in deceptively warm shallows when a severe cold front cools them fast.
• Hot beaches. A hatchling's sex depends on the warmth of the sand around the egg: warmer nests produce more females, sometimes no males at all, and sand that is too hot kills the embryos.

Turtles can be trapped when severe cold fronts rapidly cool normally warm shallow water. Credit: National Park Service.

Holding the line

Turtles found and rehabilitated quickly recover fairly fast, but those hurt by predators, boats or partial drowning need long-term care. In Texas many are rescued with community support and help from agencies and non-profits, and public releases of rehabilitated turtles build support for the work.

Sources

• Donna J. Shaver (chief, Division of Sea Turtle Science and Recovery, Padre Island National Seashore; Texas coordinator of the Sea Turtle Stranding and Salvage Network) and Jennifer Shelby Walker (biological technician, same division), "Significant Sea Turtle Cold-Stunning Event Tied to Climate Change Impacts on the Texas Coast," Park Science 38(1), Summer 2024. https://www.nps.gov/articles/000/significant-sea-turtle-cold-stunning-event-tied-to-climate-change-impacts-on-the-texas-coast.htm
• The authors credit the Sea Turtle Stranding and Salvage Network, the public, NPS staff and volunteers, and partner agencies, universities and non-profits.
• Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.