Scientists Track Endangered Sea Turtles In San Diego Bay
Jeff Seminoff’s small boat eased down the Chula Vista Marina boat ramp until the vessel was off its trailer and floating freely in the water.
The marine ecologist’s thick rubber boots were propped on the side of the craft as he started the boat’s engine. There’s a small puff of smoke and a familiar hum as the engine caught and power was fed to the rotors.
Seminoff works at the Southwest Fisheries Science Center in La Jolla, and he has spent years tracking the sea turtles in the bay.
“There are a lot more dead turtles showing up in San Diego Bay than we’d like,” Seminoff said.
He finds that concerning.
Boat strikes cause some deaths. Age and disease, others. But individual carcasses lack the context that researchers need to assess what is really happening to the local sea turtle population.
Green sea turtles are born on Mexican beaches and spend much of their young lives drifting in the nearshore ocean currents. The young turtles begin to look for coves and bays when they get to be about the size of a dinner plate, according to Seminoff.
If there is plenty of food, like in San Diego Bay, the creatures can grow to nearly 600 pounds.
Seminoff estimated there could be as many as 100 sea turtles living in the San Diego Bay. But he said the population could be closer 60 or even smaller. He just doesn’t have an exact figure.
Sea turtles are not small animals
The endangered creatures are large — an adult can reach four feet long — but they still have a talent for stealth.
“They’re big animals and the fact that they’re here is probably one of the best-kept secrets for wildlife in Southern California,” Seminoff said. “Few people realize that we have this thriving green turtle population in San Diego Bay.”
The bay’s sediment-rich water feeds eel grass and invertebrates, both favorite foods of the underwater swimmers. The turtles commonly look for food and rest in shallow murky waters off the south bay area in the evenings and that is where researchers hope to catch them.
The team used a long net held up by several buoys. The net’s webbing is stitched together in squares that are more than a foot wide and tall. That creates enough space for everything but a turtle to swim through.
“Those nets are out there,” Seminoff said, pointing at the shallow water just south of the Chula Vista Marina. “The animals get entangled in those nets. It’s a special net that doesn’t harm the turtle. So once we see that it’s been captured in the net we go and pull it out and we’ll bring it to shore.”
About an hour after the first net was put in place, the team snagged a young turtle. There was a struggle, but the team finally pulled it into their craft and took it to shore.
A research team led by Tomo Eguchi already set up a field station on a spit of land on the old South Bay Power Plant property. The location was close enough to the feeding grounds to catch and examine the animals and then release them back into the bay.
Like Seminoff, Eguchi works at the Southwest Fisheries Science Center and he has been studying the bay’s sea turtles since 2004.
The old South Bay Power Plant transformed the bay
The dismantled power plant was a long-time draw for the animals, according to Eguchi, who said the plant created a sort of turtle hot tub as it pumped heated, bubbling water into the bay.
“When the power plant was operating and the warm water was coming out, their growth rate was much faster than what you would expect at this latitude,” Eguchi said.
Local sea turtles were growing just like their cousins in much warmer climes in Florida or the Bahamas. But the power plant was torn down in 2013 and the property has long since sat empty. No more turtle Jacuzzi.
“Now that the power plant is gone, we expect them to grow much slower and that’s why we measure them every time we catch them,” Eguchi said.
Turtles are harder to catch these days because scientists can’t just camp out near the power plant. The turtles have adjusted and so too have researchers tracking the creatures.
The turtle captured on this spring evening is not new to the team. It had a tag on its right front flipper. Eguchi scanned the left flipper for an embedded chip.
A beep on the scanner confirmed the chip was there, and he rattled off the identifying number to Robin LeRoux. She is the deputy director of the marine mammal and turtle division at the Southwest Fisheries Science Center.
LeRoux checked the number against the team’s records and found this turtle was last caught in 2014. And that was not the first time.
“The first time we caught this turtle was in August of 2013,” Laroux said as she thumbed through a stack of records.
The animal weighed 27 kilograms, or 60 pounds, just four years ago. The weight on this day was 65 kilograms or 143 pounds.
And that 143 pounds gave the turtle some weight to throw around. It saw an opening in the makeshift wooden pen and scrambled toward it.
The turtle census helps scientists learn
LeRoux and Eguchi scrambled to close the open side of the box. The escape route was cut off and the team draped a washcloth over the turtle’s head to keep it calm.
Volunteer Sabrina Mashburn reached in and offered a gentle caress. Sea turtles are her passion and she has set up a website to record and track local sightings.
Mashburn recently found that many commercial boaters know about the turtles, but recreational boaters do not.
“One benefit of educating recreational boaters about turtles is that they’ll be looking for them and hopefully we can decrease the number of boat strikes that we have on our local turtles,” Mashburn said.
The turtle calmed down and researchers moved to attach a satellite transmitter. It is an autonomous device that sends a signal every time the sensor breaks the water’s surface long enough for it to register air.
“I’m using alcohol to rub off any algae or anything else on the carapace so that the glue can stick well,” Eguchi said.
A thick layer of epoxy was put on the turtle’s back with a caulking gun. Once the transmitter was in place, volunteers used a hairdryer to heat the glue until it cured and became hard.
The transmitter will work anywhere from a few weeks to as long as three months.
Local sea turtles have been tracked for hundreds of miles as they return to their breeding beaches in Mexico. Once their reproductive job is done, the local turtles typically return to San Diego Bay.
The new tracking information will be added to a growing database that keeps tabs on where the local population goes.
With the transmitter firmly attached, Eguchi readied a syringe. He injected a dose of tetracycline. The antibiotic is absorbed by the turtle’s bones and that makes it easier to figure out how old the sea turtle is once its carcass is discovered. Scientist age turtles by counting the rings in their large leg bones, just like they would age a tree by counting its rings.
If a turtle was captured and injected, the ring from that year would stand out. A count from that ring to the outside of the bone can be compared to capture records. That increases the accuracy of efforts to age a turtle.
Two hours after capture, this young turtle finally had a chance to return to the now dark waters of San Diego Bay. It did not hesitate. The turtle slipped into the nighttime waters in less than a minute.
But the creature left behind critical information and its new transmitter will gather even more. That data will help researchers better understand the elusive turtles and the habitat they live in.