Anyone else have an overwhelming sense of hopelessness when thinking about all of the problems facing our oceans? Then let’s talk about a success story for a change: sea turtles. Back when we were undergraduates (a long time ago), pretty much every population of sea turtle in the world was crashing. Extinction seemed inevitable for all seven sea turtle species and the point of no return was approaching quickly. It was pretty depressing stuff. The story of how sea turtles were snatched from the jaws of extinction is a great lesson in how science, government, industry, and individual citizens can come together to solve problems.
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| Green turtle on a Hawaiian reef (photo by David Burdick). |
Sea turtles have complex life cycles, and each life stage requires a different habitat: adult females, eggs, and hatchlings need nesting beaches; early juveniles drift around the great oceanic gyres; and late-stage juveniles and adults live in coastal or neritic waters. The threats differ for each life stage. Eggs and hatchlings are vulnerable to threats on land, some of which aren't obvious: tire tracks in the sand left by offroad vehicles can be a pitfall trap to tiny hatchlings and lights from waterfront hotels and distant cities can disorient hatchlings, causing them to march inland toward a certain death. This is on top of natural nest predators, such as raccoons and ghost crabs, and introduced predators, such as feral pigs and fire ants. At sea, late-stage juvenile and adult turtles are susceptible to collisions with fast-moving boats and drowning in fishing gear such as gillnets, longlines, pound nets, and, especially, shrimp trawls. (The unintentional capture of non-target species in fishing gear is called "bycatch.")
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| At Nannygoat Beach, on Sapelo Island, Georgia. Red light is OK because the turtles can't see it. |
With so many sources of human-caused mortality, it wasn't clear what strategies to pursue and early conservation efforts in the 1980’s were haphazard. At the time, nobody knew how effective any of them were. Two of these conservation strategies were head-starting and using Turtle Excluder Devices in shrimp trawls. Head-starting involves collecting eggs from the nesting beaches, incubating them, and raising the hatchlings in captivity for several months before releasing them into the ocean. Head-starting is labor-intensive and hugely expensive. Being a nanny to a bunch of sea turtle hatchlings is a 24-7 job and it requires specialized aquarium facilities. A Turtle Excluder Device, or TED, is a rigid grate that is sewn into a shrimp trawl net. It prevents large objects, such as turtles, from passing through the "throat" of the trawl net, while retaining smaller objects, such as shrimp. The goal in developing TEDs was to reduce the rate of lethal bycatch in the shrimp fishery. The idea and original design of TEDs came from shrimp fishermen themselves, who later teamed up with engineers and fishing gear technologists working for the U.S. National Marine Fisheries Service (commonly called "NOAA Fisheries") to improve on the original design. The real impetus for TEDs was to save the shrimp fishery. The turtles being caught by the fishery were formally designated as endangered and it seemed very likely that provisions of the Endangered Species Act would require NOAA Fisheries to ban shrimp fishing in large areas of the Atlantic and Gulf of Mexico, unless something could be done to reverse the precipitous declines of sea turtle populations.
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| Shrimp fishing vessel with its trawl nets hanging in the rigging. The oval-shaped grates in the upper parts of the nets are Turtle Excluder Devices (Photo by William Folsom). |
Early on, it wasn't clear which, if any, of these conservation measures would contribute to the recovery of sea turtle populations. Based on preliminary data on the performance of TEDs, NOAA Fisheries began requiring shrimpers to use TEDs in certain areas in 1987. We knew that head-starting improved survivorship of baby turtles in their first few months of life and that TEDs reduced the number of turtles drowning in shrimp nets, but would either strategy grow the turtle populations? Scientists had to evaluate each of these conservation measures and compare their effectiveness so we could get the biggest bang for our buck. Sounds easy, right? Well, not really. It's a bit more complicated than simply counting the number of turtles saved by each measure. Because not all turtles in a population are equal, it was also important to consider the demographic group (age and sex) of turtles that these conservation efforts targeted and the reproductive values of the individuals in those groups. And because they mature slowly and live a long time, it would take many decades of counting turtles or turtle nests to tell if these measures were working. We didn't have the luxury of taking that long to find out what worked and what didn't. We needed an answer quickly, before these populations reached the point of no return. If neither head-starting nor TEDs were the solution, we needed to know soon so alternative strategies could be developed.
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| Kemp's Ridley sea turtle (Photo by Steven Wilkie). |
Then along came some of the heroes of this story, including our friends, Selina Heppell and Larry Crowder. They created mathematical models of loggerhead and Kemp's Ridley sea turtle populations and used these models to evaluate the likely effectiveness of head-starting and TEDs as ways of reversing the population declines in these species. Their work showed that TEDs could be extremely effective, whereas head-starting likely had very little benefit for the population. The key point is that because shrimp (and shrimp trawls) are found in the same areas inhabited by late-stage juvenile and adult turtles, the TEDs reduced mortality in these two age classes for both loggerheads and Kemp's Ridleys. In terms of their potential future reproductive contributions to the population, late-stage juveniles and adults are much more valuable than are eggs, hatchlings, or early-stage juveniles (these younger demographic stages are the ones targeted by head-starting). Sea turtles have evolved a life history strategy in which they produce a large number of offspring, but the mortality rates of early life stages are very high. Their lifestyle hedges against increases in mortality of the early life stages. Once a turtle grows to a certain size, there are very few predators that can kill it (besides shrimp trawls and some other commercial fishing gears). A loggerhead turtle has to survive for about 25 years before reproducing for its first time. A female loggerhead turtle that has managed to survive to adulthood will lay about 100 eggs in each nest, she'll nest multiple times in a season, and she'll be reproductively active for decades. The probability that an individual embryo in an egg will survive to adulthood and reproduce is well below 1%. For hatchlings, the probability of reaching adulthood is slightly higher, but not by much. However, if a loggerhead turtle manages to live for a couple of decades, its probability of surviving and successfully reproducing in the future is close to 100%. To put it another way, saving one adult or sub-adult loggerhead turtle is equal to saving many hundreds or thousands of hatchlings. Because of their life history, sea turtle populations can handle increases in mortality of early life stages, but even small increases in mortality of older turtles spell trouble.
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| Turtle being ejected from a shrimp trawl by a Turtle Excluder Device or TED. This photo was taken in 1985, during early testing of TEDs (Photo credit: NOAA NMFS SEFSC Panama City Beach Laboratory). |
Thanks to this body of research, fisheries management agencies around the world had clear guidance on how to mitigate the conflict between sea turtle conservation and shrimp fisheries. The take-home message was to put your resources into adopting the use of TEDs and don't bother with head-starting (at least not for the purpose of restoring populations). The decision by NOAA Fisheries to begin requiring TEDs back in 1987 was very likely making a difference for sea turtles, and TEDs were soon required in all of the bottom trawl shrimp fisheries in the Mid-Atlantic and Southern U.S. Following the publication of Selina & Larry's papers, many other countries also began requiring TEDs in their shrimp fisheries. Now that TEDs have been required for more than 25 years in many areas, we believe we are seeing real-world evidence of growth in some populations. In a study using long-term datasets in nesting activities for 299 sea turtle populations around the world, the number of populations in which annual nest counts were growing outnumbered those that were shrinking by a margin of 3 to 1. Here in the U.S., loggerhead nesting has been relatively stable in Florida but increasing steadily in Georgia, North Carolina, and South Carolina; leatherback nesting has been increasing slightly in Florida; the number of green turtle nests has been increasing explosively in Florida, and nesting of Kemp's Ridley turtles has been increasing steadily in Texas (Sea turtle nesting statistics by U.S. state and various locations around the world). The increase isn’t just seen in nests. Underwater count surveys of green and hawksbill turtles at 53 coral reef sites throughout the Pacific showed increasing trends in abundance from 2002 - 2015 (the last year of the published study).
The results of these recent studies are in stark contrast to the situation in the late 1980s, presumably thanks, in large part, to TEDs. But not only is this a good news story for sea turtles; it's also good news for the shrimp fisheries. Thanks to this simple technological fix, these fisheries have been able to continue operating. The industry incurs costs from the turtle conservation regulations (purchase, installation, maintenance of the TEDs, as well as a slight reduction in shrimp catch). But the shrimp fisheries have been able to continue operating.
Sea turtles are not out of the woods quite yet. There are still many sea turtle populations that are endangered and there are other human-caused threats besides shrimp trawls (e.g., leatherback turtles caught in commercial longlines set for swordfish and tuna, and the combination of coastal development and a rising sea level, resulting in decreased nesting habitat). We can’t let our guard down but it's important to celebrate the successes we've had, understand why these particular efforts worked, and figure out what lessons we can apply to other similar problems. We can make a difference, if we set our minds to it. Sea turtles are just one example among many.
Sorry for getting so serious. We'll try to get back to doing more posts like the one on farting manatees. Life is all about balance, after all.
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| Tracks left on the beach at Sapelo Island by a female loggerhead turtle that had come ashore to nest during the previous night. |
