Atlantic Loggerheads: Why Isn’t the Best Understood Sea Turtle Recovering?
By BLAIR WITHERINGTON, ALAN B. BOLTEN, KATE L. MANSFIELD, LUCIANO SOARES, SIMONA A. CERIANI, and NATHAN F. PUTMAN
The Atlantic Ocean has served as a laboratory for pioneering work to save sea turtles. It is where Professor Archie Carr—and many whom he inspired— first addressed some profound mysteries that had stymied the conservation of such enigmatic marine animals. Loggerhead sea turtles became an exemplar of this work, revealing critical concepts such as the oceanic dispersal, the nature of the “lost years,” the migratory connections, and the relative importance of different life stages to population growth. These puzzle pieces have guided strategic sea turtle conservation for decades.
In addition to being a cradle for sea turtle conservation science, the Atlantic also happens to contain the largest population of loggerhead sea turtles on Earth (consequences of recent misfortune, rather than achievement, see SWOT Report, vol. XIV article on Oman). If the adage “Where much is granted, much is expected” applies, this statement makes the stewards of the Atlantic disproportionately responsible for our future with sea turtles. So how are we doing? Is our understanding of Atlantic loggerheads benefiting them? Well, let’s just say, it’s never too late to show responsibility.
Humans have had an extensive presence in the Atlantic Ocean and its coastlines. Since before recorded history, native people on both shores drew upon resources from Atlantic coastal waters. And then, just 10 or so loggerhead generations ago, Europeans began ocean crossings that broadly spread both human appetites and the industrial capacity to satisfy them. Especially in the past century, people have consumed from the Atlantic and changed for the worse the waters where loggerheads live.
Loggerheads aren’t picky, which has led some to suggest that the sea turtles might be resilient to all this human presence and might comfortably persist with us in the Sea of Atlas. They are globally distributed marine animals that forage widely. Their nesting range is almost as broad and includes islands, barrier strands, and continental beaches. Loggerheads are also generalists in where they live and what they eat—from estuaries, coastal shallows, and oceanic waters, with associated habitats spanning seagrass pastures, hard bottom, coral reefs, and the open sea. Their diet is satis ed by prey that vary over many phyla, from jellies to heavily armored crabs, clams, and large marine snails.
Perhaps because of their liberal and accommodating conduct, loggerheads and their kin have a track record of persistence. Their direct forebears have endured for more than 100 million years, with generalist species like the loggerhead surviving the cataclysmic events that snuffed out the dinosaurs. They prevailed through multiple ice ages and warming trends that shuffled habitats and drove sea-level changes of more than 200 meters. So we might imagine that loggerheads, with a capacity to fill such varied niches and to survive global tumult, would be able to avoid adverse effects from a single, albeit tough, competitor. But we’d be wrong.
Conventional wisdom accepts that the majority of the world’s sea turtle populations are depleted owing to human actions. Many of these actions involve direct harvesting of eggs and turtles. But aren’t we past that era? Shouldn’t loggerheads be on the rebound? The recent IUCN Red List assessment (2013), drawing from decades of extensive monitoring on nesting beaches all over the world, concluded that the species is still vulnerable. But the assessment is complex (see SWOT Report, vol. XII, “The Conservation Status of Loggerhead Populations Worldwide”). Ten loggerhead subpopulations make up the global species, with three in the Atlantic (excluding the Mediterranean). Of those three, the Northeastern Atlantic subpopulation is considered endangered because of its small size and restricted distribution, whereas the two Western Atlantic subpopulations (north and south) are listed as being of least concern. In this context, least concern does not mean recovered; they remain depleted but are holding their own for now. Why aren’t these Atlantic populations recovering?
Overall, we’ve shown considerable conservation progress within the range of Atlantic loggerheads—we value them, seek to understand them, and attempt to manage our detrimental actions. Loggerheads in the Atlantic enjoy life on and off the shores of wealthy nations that show high conservation awareness. The Bahamas, Brazil, European countries, Mexico, the United States, and others are testament to this concern in their rule of law at sea—all of those nations have banned direct harvest of sea turtles.
We’ve also studied and monitored Atlantic loggerheads for decades, leading to those populations serving as the discovery point for comprehending sea turtle life histories, population biology, and ecology. If we draw on an index of numbers of nests made on Atlantic beaches, then we know much about how many adult loggerheads there are. The trend is disappointing—not dire, but certainly no recovery. Why? Perhaps we’re impatient. The eggs we protected from poaching in the 1980s produced turtles that are only now coming home to nest. But the greatest toll we’ve taken on loggerhead populations has come from effects on life stages that are the most valuable to the population—older juveniles and adults with the highest probability of breeding. After only a couple of decades of protecting these animals in the water, we should now be seeing effects on recovery. Why don’t we? Ostensibly, an answer lies not in our success toward ceasing the harm being done to them on purpose but in our inability to address the harm that occurs by accident.
CONNECTIONS—LIFE HISTORY, ECOLOGY, AND HAZARDOUS INTERSECTIONS
The loggerhead conservation puzzle finds clues in their connections to other organisms, ecological systems, human enterprise, and geopolitical states. Like all sea turtles, loggerheads are connected to the beaches where they lay their eggs. More so than with other sea turtles, loggerhead nesting covers many latitudes, with nests recorded as far north as New Jersey, on the U.S. coast, and as far south as the southern state of Paraná, Brazil. That range spans more than 3,700 nautical miles north to south. The southeastern coast of Florida is the center of nesting abundance, but considerable nesting also occurs in the remaining southeastern United States, on Mexico’s Yucatan Peninsula, across Sergipe and northern Bahia states in Brazil, and in the Cabo Verde archipelago of western Africa.
Almost everywhere throughout the loggerhead’s Atlantic nesting range, eggs are protected from poaching. But threats to nests and emerging hatchlings are common. Sources of mortality that are most severe stem from the incidental effects of coastal development. Human population centers close to beaches bring heavy-handed defense against erosion (sand pumping and seawalls), which leads to nesting habitat loss. However, the most injurious effects from development come from artificial lighting, which draws nocturnally emerging hatchlings away from the sea and causes high mortality on many beaches.
Hatchlings that survive the beach make a frenzied swim into the offshore Atlantic. This two-day sprint limits exposure to intense coastal fish predation and ends with little loggerheads settling in to the numerous surface features produced by converging ocean currents. The convergence zones collect the pelagic algae Sargassum, along with a host of small, slow-moving invertebrates that provide food for young sea turtles. Sargassum drift habitat is unique to the Atlantic, forming a massive oceanic habitat, with patches large enough to be viewed from space.
The currents that carry this drifting material manage to transport small loggerheads much more widely than they could disperse under their own swimming power. In the North Atlantic, the Gulf Stream and connected currents at the western edge of a clockwise, ocean- spanning North Atlantic Gyre carry neonate loggerheads across higher latitudes and into the eastern Atlantic. Hatchlings from Brazilian beaches may be transported even more widely, being either swept into the North Atlantic or circulated deeper into the South Atlantic and eastward toward Africa.
Until recently, a loggerhead’s life in the open sea was almost completely unknown. How do they find their way? Answers came from Ken Lohmann and students at the University of North Carolina, who demonstrated that hatchlings use GPS-like cues from Earth’s magnetic field to derive their location from the bi-coordinate grid of magnetic field strength and the angle of that field relative to the Earth’s surface. From Atlantic loggerheads we now know that the relatedness of female loggerheads on their nesting beaches can be explained by the similarity of magnetic field values where the turtles nest. So it seems, at least in part, that magnetic fields guide loggerheads home. In the Atlantic, there is also considerable support for the hypothesis that young loggerheads use their magnetic sense to swim toward favorable open-sea habitat. And in the case of southeastern U.S. loggerheads, this directed swimming increases the odds of reaching their next life-history milestone—continuing to mature in the foraging habitats in the eastern Atlantic and Sargasso Sea.
To graduate to larger oceanic juveniles, neonate loggerheads must survive threats inherent to being small. Fish predation is believed to be high in this stage, but for eons, loggerhead reproduction (having lots of young) and behavior (rapid dispersal past risky coastal waters) have minimized this mortality. However, recent threats are developing that are too abrupt for accommodation. In a single loggerhead generation (roughly 45 years), an extraordinarily pernicious, human- generated ingestion hazard has spread into every crevice of the Atlantic—plastic litter. And it is in those crevices where little loggerheads find their food. We don’t know the lethal magnitude of this incidental human threat, but we do know it is pervasive. Nearly all of the neonate loggerheads swept dead onto Atlantic beaches during severe storms had ingested shards of degraded plastics, with compromised nutrition and gut blockage being a likely cause of death.
One of the greatest mysteries of a loggerhead’s life at sea was how long they spent there. We first came to understand from Atlantic loggerheads that their enigmatic “lost year” in the open sea was more like a lost decade. In 1982, Helen Martins (University of the Azores) began tagging platter-size loggerheads found near the Azores archipelago in the eastern Atlantic. She forwarded the turtles’ size data to Archie Carr, who used the information to lay out the case for a connection between loggerheads nesting in the western Atlantic and those swimming in eastern-Atlantic Azorean waters.
In what has now been a 35-year collaboration, the Archie Carr Center for Sea Turtle Research, the University of Florida (principally, Alan Bolten and Karen Bjorndal), and the University of the Azores have made advances in understanding loggerhead conservation challenges. Revelations include how fast loggerheads grow and compensate for periods of low growth, their extent of migratory movements, their ecological connections, and their odds of survival. is work also revealed important new findings on threats from plastic pollution, nutrient dilution, and—most important— bycatch in oceanic longline fisheries. e latter hazard is compounded by the decade that loggerheads endure as oceanic juveniles. Bycatch mortality is an onerous consequence of biology intersecting with economic enterprise; it occurs in both the northwestern and southwestern Atlantic loggerhead populations, whose open-ocean life stage makes up a third of the animal’s maturation.
Following many years of growth in the open sea, most loggerheads return to the vicinity of their natal coast, a profound habitat shift accompanied by changes in behavior and diet. The turtle’s mouth has grown along with its body size, making it capable of crunching a wide variety of shelled sea-bottom invertebrates. is shift is not always permanent. Some Atlantic loggerheads settle permanently into coastal waters, others move between coastal and offshore waters, and some remain largely oceanic while moving into shallow seas only during breeding migrations to their nesting beaches. Yet as most loggerheads grow closer to a mature size, they become more likely to occupy waters where they dive to the bottom for their food.
Coastal loggerheads are faithful to specific foraging grounds, but their fidelity is punctuated by seasonal migrations. Water temperature changes drive juveniles and adults into warming northern waters in the spring and south again in the fall as waters cool. Many North Atlantic coastal loggerheads move north and south of Cape Hatteras during such seasonal migrations, as they spend the winter over deep water along the western edge of the warm Gulf Stream current. Those turtles follow three-dimensional thermoclines within their habitats, remaining mostly at the surface during colder months, when ocean temperatures are more stratified, and feeding on the bottom once those deeper waters mix in the spring and summer.
Again, an unfortunate proximity of biology and human enterprise burdens larger loggerheads with exposure to a variety of coastal hazards. Perilous fisheries include trawling for shrimp and finfish, dredge fishing for scallops, and gill netting for finfish. Those fisheries are regulated throughout much of the loggerhead’s Atlantic range, and fishers have made efforts to modify their gear and methods, yet incidental drowning in fishing gear remains a critical source of loggerhead mortality. Boat traffic in general constitutes another severe coastal threat to loggerheads, with boat strikes being the most common identifiable cause of death for sea turtles stranded on U.S. Atlantic shores.
Loggerheads that survive to adulthood are exposed to most of the same threats felt by younger coastal turtles. One additional array of threats experienced by adults may result from their breeding migrations. In the North Atlantic, breeding movements may be similar to coastal north-and-south travel seen seasonally in younger loggerheads. But some breeding migrations are more extensive, like the common route females take between Chesapeake Bay foraging grounds and Florida nesting beaches, and the route between the northern Gulf of Mexico and eastern Florida. These periodic coastal movements multiply the risk of lethal interactions with an array of coastal hazards.
Despite what we think we know about how loggerheads breed, some profound mysteries remain. One of those is the presumed threat from hybridization. Although there are sporadic reports of loggerheads worldwide hybridizing with other sea turtle species, only in the southwestern Atlantic do such observations occur at an alarming frequency (see SWOT Report, vol. XI, “Sea Turtles of South America”). In the northeastern Brazilian rookeries of Bahia and Sergipe, hybridization between loggerheads and hawksbills, and between loggerheads and olive ridleys, occur at rates of more than 20 percent. This hybridization is not sex-specific. Both male and female loggerheads mate with other species, and both male and female adult hybrids have been identified. Remarkably, the hybrids do not seem to be at a reproductive disadvantage relative to their parental species in regard to hatchling production, and hybrid hatchlings have similar viability to nonhybrids. The cause and consequences of this blurring of sea turtle species are unknown.
Atlantic loggerheads exemplify the challenges and opportunities characterizing life for sea turtles in a prospering world. Much of our activity in pursuing that prosperity has unintentional consequences for loggerheads. Yet the achievement of economic success over much of the loggerhead’s Atlantic range, as well as the political systems governing that success, allow the people who accidentally harm loggerheads the luxury of purposefully conserving them.
One common thread weaving through the story of loggerheads in the Atlantic is that their lives span their ocean. Each knot in a loggerhead’s life-history thread, which is tied in waters of dozens of different countries or in waters belonging to all, lengthens the strand. But each knot can also break. No single country or entity will save our Atlantic loggerheads.
The international imperative for conserving sea turtles is most obvious when considering ubiquitous global threats. Climate change comes to mind, of course. But what are the implications for loggerheads? In short, climate change is expected to bring about widespread, abrupt, persistent changes in how marine ecosystems function (called ecological regime shifts). Those shifts will lead to altered growth rates, delays in graduation from life stages, and reduced population growth. Evidence for this cascade of effects comes from work on Atlantic sea turtles led by Karen Bjorndal. Her work revealed that somatic growth rates of loggerheads and two other sea turtle species throughout the region began to decline in the late 1990s as the result of an ecological regime shift. The decline continues to the present. Whether this environmental change is natural or anthropogenic matters not to our loggerheads. The result is the same—turtles endure risks for longer periods, delay their breeding, and contribute less to potential population growth. Do conservationists throw up their hands? No, they recognize the exigency and work harder.
Another daunting but crucial area of conservation work for loggerheads is management of global fisheries. Unlike climate-change solutions, resolving threats to fisheries requires sea turtles to play an inspirational role. Already, loggerheads and other sea turtles in the Atlantic are the impetus for global thought, local action, and global action. In the western Atlantic, the Inter-American Convention (IAC) for the Protection and Conservation of Sea Turtles, signed by 15 countries in the Americas and Caribbean, has provided a legal framework for protection as an intergovernmental treaty since 2001. And conservation diplomacy continues.
Recently, a South Atlantic Network was established, within which sea turtle biologists and conservationists in West Africa and South America exchange information and ideas. The latest development from this network is a loggerhead threats analysis conducted by colleagues in Argentina, Brazil, and Uruguay and will soon include West Africa. The work would build on a similar analysis developed by the Northwest Atlantic Loggerhead Turtle Recovery Team. Threat analyses like these will show where conservation action can be directed to provide the greatest benefit to loggerhead populations.
There is justifiable hope. Yes, Atlantic loggerheads su er within the challenging “tragedy of the commons.” The turtles are indeed a shared resource affected by individual users who act in their own self- interest and collectively behave contrary to the common good—in this case, by depleting the oceans’ loggerheads. However, there are solutions to avoid the tragic outcome of such circumstances. They are the underlying goals of conservationists—cooperation and rule of law. To get there, to save our loggerheads, we’ll need measurable progress on all fronts in the Atlantic region. Some will work toward advancing the conservation science. Others will work within the social sciences to understand required sociopolitical relationships. But we will also need guidance on the art of communication, of winning friends, and of generating influence. We are on our way. Go team!
This article originally appeared in SWOT Report, vol. 14 (2019). Click here to download the complete article as a PDF.