redOrbit Staff & Wire Reports – Your Universe Online
For loggerhead sea turtles, the number of returning nesting females in the Northwest Atlantic combined with favorable climate conditions in the preceding years are strongly related to the number of nests produced in a given year, according to a study published this month in the journal PLOS ONE.
Furthermore, in what may be good news for loggerheads, which are considered threatened under the US Endangered Species Act, nesting increases since 2008 may be a recovery response in this population, the researchers said.
Sea turtles are a long-lived, late maturing species that live throughout the world’s oceans. But estimated survival rates for loggerhead sea turtles in the Northwest Atlantic are very low, with less than 0.2 percent of turtles born in a given year surviving to the age of 30.
Because the reproductive value is highest for sea turtles that have reached or are approaching maturity, management strategies protecting these life stages from human threats are critical.
In the current study, Vincent Saba, a research fishery biologist at NOAA’s Northeast Fisheries Science Center (NEFSC), and colleagues used annual nest counts from Florida and a time-series of climate data in turtle-nesting population models to assess observed changes in nest counts, and to project future nesting trends in the Northwest Atlantic loggerhead sea turtle population – the largest in the world.
“Our study suggests that the cumulative survival from hatchling to maturity, which may take 30 years, combined with present-day climate effects on mature females, has a greater influence on annual nesting population size than does the exclusive impact of survival during the first year of life as hatchlings,” Saba said. “The first year of life represents only 3 percent of the time elapsed through age 31.”
The study suggests that protection for older juveniles and sub-adults to ensure they reach maturity and breed multiple times is at least as important as protecting hatchlings, findings that have significant implications for management strategies aimed at the recovery of Northwest Atlantic loggerheads and the projected recovery rates of this threatened population.
Nearly 90 percent of all loggerheads nesting in the Northwest Atlantic occurs in Florida. Saba and his team derived nest count data between 1989 and 2012 from 15 index beaches located on the east coast of Florida between Canaveral National Seashore and Boca Raton. The researchers used these nest counts to model the importance of first-time nesters (age 31) to future nesting numbers. To accomplish this, they projected the number of nesting females up to 2020 and 2043 based on the estimated number of hatchlings produced between 1989 and 2012.
The decline in the annual nest counts observed between 1998 and 2007 was not projected to occur during 2029-2038, suggesting that annual nesting variability and trends are influenced more by returning nesters, older females, than by first-timers. If first-timers contributed more to nesting variability and trends, a decline in the years 2029-2038 would be expected.
Saba and colleagues also found that adult female sea turtles are influenced by environmental conditions – referred to as “climate forcing” – when it comes to annual reproductive activity.
“This study offers a different perspective than earlier work that suggested most of the annual variability in loggerhead sea turtle nest counts in Florida between 1989 and 2010 could be explained by climate forcing on hatchling survival,” said Saba, a member of the NEFSC’s Ecosystem Assessment Program.
“We reached a much different conclusion. The annual variability and trends in loggerhead nesting numbers in Florida are associated with long-term survival at sea from hatchling to maturity, combined with climate-driven changes in mature female foraging areas within a year or two before nesting,” he added.
Not all female sea turtles in the population nest each year, a phenomenon attributed by some researchers to climate-driven changes in energy consumption at foraging areas in the years prior to nesting.
Limited tagging data from nesting beaches suggest that the reproductive longevity of loggerheads may exceed 25 years, the researchers said.
Saba and colleagues used population models to evaluate the relative influence of climate indices including the Atlantic Multidecadal Oscillation, the North Atlantic Oscillation, and the El Niño Southern Oscillation. They then tested for differences in model fits using annual nest count data to see how the model performed with respect to environmental factors. They also assessed the relative contribution of first-time nesters to projected annual nest counts. Finally, they created a survival matrix to compare the relative importance of survival in the first year of life relative to the next 30 years before maturity was reached.
With passage of the US Endangered Species Act (ESA) in 1973, protection measures have been implemented for all loggerhead life stages. Since the lowest annual loggerhead nest count data from Florida’s index beaches occurred thirty years after the ESA was enacted, the researchers suggest that the nesting decline that occurred during 1998-2007 represented a lagged response to historical human-induced impacts on juveniles and adults.
The subsequent increase in nest counts since 2008 reflects a potential recovery response, the researchers concluded.
Saba, who has conducted modeling studies on the impacts of climate change on endangered leatherback sea turtles in the eastern Pacific Ocean, says the Northwest Atlantic loggerhead study offers a new approach in understanding how climate variability affects sea turtle populations. The study should also help management efforts aimed at protecting these populations because it suggests that the protection of large juveniles and adults should be a priority, he said.