Turtle-paced Recoveries

 

Green Turtle

Published in the Pacifica Tribune July 28, 2020

What’s Natural?

Turtle-paced Recoveries

Many humans are working hard to prevent any further extinctions of our plants and animals and with growing success. Pelicans are increasing and no longer endangered. Humpback whales are increasing at a rate of 12% per year. Mountain lions and bald eagles are increasingly abundant. In 1982 the California Condor had dwindled to just 25 individuals. A captive breeding program began and today there are about 325 individuals and condors are expanding back into their historical range. This summer, condors were seen in Sequoia National Park for the first time in 50 years.

Over-harvesting, loss of habitat and introduced species are the main causes of endangered species. The green sea turtle, prized for its meat and eggs, was seriously over-harvested and endangered. Their numbers continued to fall as beach resorts disturbed their traditional nesting sites. Many others were killed as by-catch in fishing nets. But several new studies report seeing an uptick in green turtles around the world. One of the world’s largest nesting rookeries, Raine Island off northeastern Australia, just experienced the most abundant nesting season as ~64,000 breeding turtles have arrived. What is the reason for these higher numbers?

Green Turtles nesting on Raine Island

There is a degree of certainty conservation efforts have been effective. Many countries have banned harvesting eggs or turtles for meat, although poaching remains a threat. Fishermen have developed Turtle Exclusion Devices that prevent turtles from being captured in their nets.  Still, it is extremely difficult to reliably measure the success of sea turtle conservation.

Green turtles feed primarily on low-calorie sea grass. The good news is sea grass thrives under higher concentrations of CO2. However, that diet limits turtle growth and it takes 10 to 25 years before turtles reach sexual maturity. So, even if today’s conservation efforts are successful, we won’t see today’s benefits for at least another 10 years when hatchlings return to their birth sites as breeding females.

Despite increasing populations, some researchers have been needlessly gripped by a global warming fear. Like several reptiles, a turtle’s gender is determined by the temperature of the incubating egg. Eggs at the top of a nest are warmed the most and become females. Just a 4° F cooler temperature will create males. Furthermore, sex ratio of turtles from nests along the northern Great Barrier Reef have averaged 88% to 99% females, while populations from the cooler southern Great Barrier Reef average about 66% female. So the fear is, if the earth warms only females will be born and the population goes extinct. But gender determination by temperature is not a fragile system.

Turtles evolved during the age of dinosaurs over 250 million years ago when global temperatures were much warmer. Palm trees grew along the coast of Antarctica and crocodiles roamed the Greenland coast 55 million years ago. Since then temperatures cooled and ice ages ensued, yet turtles did not become all males. Nonetheless, cooler temperatures are an immediate threat. Along the USA’s east coast, many turtles travel northward as waters warm with summer heat. But if they do not return south in time, autumn’s cooler temperatures paralyze them, stranding hundreds on east coast beaches. (Again, humans help out by flying rescued turtles back to warmer waters).

During the Holocene Optimum 6000 to 9000 years ago, temperatures were bout 1.8° F warmer than today without causing extinctions. In 1957, the Whittaker brothers petitioned Australia’s Queensland government to commercially harvest female turtles on Raine Island. They cited surveys that found over 99% of the turtles were female. That high percentage of females has remained despite 60 years of climate change and Raine island remains one of the world’s most abundant nesting sites.

There are good scientific reasons why 99% females are beneficial. Only female turtles go on shore to nest. Males remain offshore mating with every available female. One male fertilizes several females. On shore females suffer more mortality. They come ashore in the evening to avoid the deadly effects of the sun. If they do not return to the sea by early morning, they often die of heat exhaustion. Some females get disoriented or fall into pits, becoming stranded in the midday sun. Some flip over on uneven ground and cannot right themselves. On crowded beaches, some females become so exhausted from jostling with other females they return to the ocean without laying their eggs.

Nesting turtles can flip and die

Females may lay 2 to 5 clutches, each with 100 eggs, during the breeding season. This greatly depletes their energy. So, females only breed every 3 to 5 years, allowing time to replenish their body condition. Finally, it is estimated only 1 of every 1000 successfully hatched turtle ever survives to maturity. So the high percentage of females is certainly not a sign of impending global warming doom. It is an ancient breeding system that maximizes egg production and ensures the species’ survival.

Hatchling Ridley Turtles