Saving the Green Giants California's Supersized Turtles Raise Questions About Altered Habitats

Even most locals in Chula Vista, California—the second-to-last city you pass through before crossing the Mexican border into Tijuana—are unaware of what lies beneath San Diego Bay. Strange how a colony of 60 to 100 sea turtles has somehow thrived, virtually undetected, in one of the busiest, most developed natural harbors in the world. These particular Eastern Pacific green turtles are supersized, some bettering 550 lbs., an unprecedented enormity, almost doubling the normal size records of the same species in other habitats.

“They’re monsters, I tell ya,” says Jeffrey Seminoff, ecologist and assistant team leader for the Marine Turtle Research Program, part of the National Oceanic and Atmospheric Association’s (NOAA”s) Southwest Fisheries Science Center. He’s wearing orange coveralls and flip-flops, standing at the Boston Whaler’s pedestal helm as we glissade over the dusky water. Twice a month, he and his team use two 17-foot Whalers to make sweeps of an area called South Bay, netting green turtles and hauling them back to shore to be weighed, tagged, sampled and released as part of a new pathbreaking study.

There are four of us crammed in the little boat, including NOAA Fisheries ecologist Tomoharu Eguchi. He’s the man with all the stats—including those showing that some of these enigmatic turtles are growing eight to 10 centimeters a year. “That’s quadruple the rate of green turtles at other sites,” he shouts over the outboard motor. Genetically similar green turtles in the Sea of Cortez only grow about two centimeters a year and rarely top 300 lbs.

South Bay is a unique habitat of intertidal mudflats and salt marshes, granting refuge to over-wintering birds. Hidden in the murky waters, green turtles take sanctuary among the eelgrass beds. The one we’re going after is just ahead. It struggles in the buoyed net, flippers wind-milling and splashing, head breeching for air. It’s a 350-lb. female, so burdensome she makes the boat’s starboard gunwale dip underwater as three men grunt her aboard.

Water warmed by a power plant may play a role.

Back onshore there’s already a 139-lb. juvenile and a whopping 450-lb. female being processed at the makeshift lab camp—not much more than a tent canopy, pitched on a land spit that juts out below the hulking South Bay Power Plant. The steam electric power generating facility, built in 1956, is a primary reason the turtles are so active here. It uses what’s called a “once-through wet-cooling system” that draws seawater from the bay. The cooling water is heated after re-condensing the steam that goes through the turbine generators then flushed back out to sea, warming a discharge channel of South Bay up to 15 degrees above normal temperature.

The turtles are attracted to, and affected by, this warm-water effluent. The higher water temperature decreases the amount of oxygen in the water and, in turn, increases their metabolic rate. Carl Mayhugh, a Prescott College graduate student doing his Master’s thesis on the movements and habitat use of these turtles, theorizes that the animals’ supersized growth may be because the warmer water allows them to continue foraging in winter—a time when green turtles in the Sea of Cortez “bury themselves down in the mud and go into a state of torpor for a few months when the waters get cooler,” Mayhugh says.

Seminoff isn’t ready to corroborate this theory, because there have been studies of sea turtle populations in other power plant-warmed waters with no unusual size differential. But both researchers suspect the turtles’ normal herbivory diet of eelgrass and algae is being supplemented with animal matter. Turtles in the Sea of Cortez have been observed feeding on tube worms, sea hares, jellyfish and other slow-moving, soft-bodied sea creatures.

Under the tent canopy, the 450-lb. female’s plastron rests on an old truck tire so that her flippers cannot touch the ground and lend her escape. A wet rag covers her face to keep her calm. Seminoff squats next to her head and, with a neurosurgeon’s delicacy, takes a dime-sized skin sample from her neck. He will later perform a “stable isotope analysis’ of the sample to determine if the animal is a pure vegetarian or not. The technology could also be used on humans. “If you ate salad for the last week, then shaved, and I analyzed your whiskers, I could tell you your diet was all vegetarian that week,” says Seminoff.

South Bay represents the northernmost site on the continental West Coast where green turtles live year-round. New animals are expanding the turtle colony every year. Some are as small as 15 lbs. Once they come into the habitat they will stay until reaching sexual maturity, a process that takes between 10 and 25 years. Then they are compelled to return to their natal beach to reproduce and lay their eggs.

Larger adult females are fitted with an Argos satellite tag. Their migration patterns have been tracked over 1,000 kilometers south to the Revillagigedos Islands and other remote areas south of the border. Seminoff says, “They’re a transboundary species. They don’t pay attention to any national borders, so we’re doing our best to work with our Mexican counterparts to ensure the turtles have some level of protection that’s ongoing.”

Transboundary protection is crucial. It’s during their migration that the turtles come into serious peril. Poachers still hunt them throughout Mexico, ignoring international protection laws. Every year about 35,000 turtles are captured in Baja waters alone, butchered for consumption locally or trafficked north to buyers in commercial cities like Ensenada and Tijuana. Add to this a sea turtle’s risk of entanglement as incidental bycatch in fishing nets and longline gear.

Meanwhile, the South Bay Power Plant has outlived its usefulness, operating at one-third capacity. It’s slated to be torn down in the next several years. But the resident green turtles will likely remain in South Bay even after their warm water oasis cools. “After they nest, they come back up into San Diego Bay,” says Seminoff. “So they come back home, basically.”