New low-cost solutions could save sea turtles from a climate change-induced sex crisis | TheHill – The Hill
Because of global warming, most newborn sea turtles are female, which could put their long-term survival at risk. New research reveals that in addition to more widely used techniques such as shading and irrigation, the splitting of the turtle nests, which has not been tried before, could even out the sex ratio of turtle hatchlings.
Scientists also argue that to increase the resilience of sea turtles, turtle conservation must address threats beyond climate change, including ocean pollution, unsustainable fishing and illegal wildlife trade.
Understanding turtle reproduction
While the sex of mammal offspring is determined genetically, certain reptile groups such as crocodilians, lizards and turtles rely on temperature-dependent sex determination, or TSD, meaning that the gender of the hatchling would depend on the egg’s incubation temperature.
According to the National Oceanic and Atmospheric Administration (NOAA), sea turtle eggs which incubate below 81.86 Fahrenheit – around 28 Celsius degrees – will typically hatch into a male baby, while incubation temperatures above 87.8 Fahrenheit – 31 Celsius degrees – would tend to produce female hatchlings.
Temperatures that fluctuate between the two would result in a mix of female and male baby turtles, while extreme temperatures beyond this range could lead to embryo malformation and higher mortality rates. The exact range might also vary slightly based on the type of sea turtle or the location of the nesting beach.
Feminization of sea turtles threatens their future
Global warming is skewing the sex ratio of sea turtles, and research undertaken in Australia, Brazil, the Caribbean, Florida and the Mediterranean revealed an increasingly female-biased population in each of these locations.
Although male turtles are able to mate with multiple females, so more females could actually increase the population’s reproductive potential, questions arise over how much female bias is too much.
In the rapidly warming northern part of Australia’s Great Barrier Reef which is home to one of the largest populations of green turtles, more than 99 percent of juvenile turtles are female, as are over 86 percent of adults. Scientists warn that the potential complete feminization of the sea turtle population would endanger their long-term viability.
Shading and splitting of nests could even out sex ratio
Metabolic heat, which is produced by the eggs as they develop, has been shown to further increase nest temperatures, shares Leo Clarke, lead author of a study which explored low-cost solutions to reducing the impact of climate change on turtle reproduction.
Clarke and his team aimed to reduce metabolic heating and overall nest temperatures, by halving the number of eggs, or “splitting” the clutch, a technique that has not been explored before, and through the shading of the nests, which has been used in previous studies.
The experiments were conducted on the island of Boa Vista, Cabo Verde off the coast of West Africa, the third largest nesting aggregation of the loggerhead sea turtle.
“We found that both techniques result in cooler nests temperatures compared to natural nests. The proportions of females produced in natural nests was 69 percent on average, compared to 45 percent in split nests and just 1.5 percent in shaded nests,” shares Clarke. “Splitting was therefore effective, but to a lesser extent than shading, although importantly, splitting the clutch requires less cost and manpower, which are important for turtle rookeries that are often in remote locations.”
Cooling methods can have unintended consequences
Interventions that promote favorable nest conditions for turtle reproduction have been adopted by many conservation projects around the world, but they require a lot of effort to implement and monitor, as conditions vary from location to location, and from species to species, cautions wildlife ecologist Bryan Wallace.
Projects should work towards sex ratios that will ensure long-term population growth, and not simply the production of more hatchlings, advises Wallace, who is the authority coordinator of the IUCN SSC Red List for the Marine Turtle Specialist Group. Too much cooling might lead to more balanced sex ratios and more hatchlings in the short term, but could reduce overall population numbers on the long haul if too few females are born, he says.
The ideal sex ratio between male and female turtles for a healthy, genetically diverse population is not currently known, shares Roderic Mast, president and CEO of the non-profit Oceanic Society and co-chair of the IUCN SCC Marine Turtle Specialist Group.
Both Wallace and Mast recommend that conservation efforts take a more holistic approach and focus on reducing other — and mostly human-induced — threats to turtles, which will increase their resilience to the effects of climate change.
Additional threats: bycatches, illegal poaching and pollution
Sea turtles lay hundreds of eggs in one nesting season, but few make it to adulthood, with estimates ranging from one in 1,000 to one in 10,000. Hatchlings are devoured by vultures, racoons, coyotes, crabs and other opportunistic predators as they journey from nest to ocean, and even if they manage to reach the open sea, many fall prey to fish, dolphins, seabirds and other marine life.
While adult turtles face fewer natural threats, aside from apex predators such as tiger sharks, jaguars or orcas, they grapple with anthropogenic hazards, including the illegal harvesting and trade of their eggs, meat, skin and shells, habitat loss, pollution and unsustainable fishing.
Mast rates fishing bycatches and ocean pollution as the two most significant risks faced by sea turtles. Turtles regularly mistake plastic trash for food or get trapped in discarded fishing gear, causing them to choke or drown. Many are poisoned by oil spills or get caught in longline hooks, trawler or gill nets intended for other animals.
Already at risk because of rising sea levels, nesting beaches are also disrupted by infrastructure development, while feeding grounds are dwindling as sedimentation from the clearing of land or toxic run-off from agricultural activities destroy coral reefs and seagrass beds.
Innovation in fishing practices and regulation
Mast advocates for the temporary closure of fisheries during nesting season and highlights the need for government regulation which mandates more sustainable fishing practices.
As an example, in the late 1980s, the US made the installation of turtle excluder grid (TED) devices mandatory on US shrimp trawler vessels which would allow larger animals such as turtles or sharks to escape the net, and introduced legislation that made shrimp imports contingent on turtle conservation. The EU, the world’s largest seafood market, doesn’t currently have a similar law in place.
“One of the most significant threats to sea turtle conservation is bycatch in fisheries, but innovations in the design of fishing gear such as excluder grids in trawls, or use of circle hooks and different bait in longline fisheries that reduce the chance of turtles swallowing the hooks, have helped reduce turtle bycatch,” adds Clarke.
“It all boils down to human behavior – what people put in the ocean and what we take out of it. To protect sea turtles and maintain healthy oceans, we must change human practices first,” concludes Mast.