Grand Turtles of Grand Riviere
Grand Riviere, on Trinidad’s northeast coast, is a narrow strip of shoreline backed by mountainous emerald jungle that tumbles to the broad sea. Almost as soon as the sun goes down, dark shapes dot the gleaming surf. And you realize those shapes are big, and they’re moving up the beach.
Grand Riviere has so many nesting leatherback turtles that it’s probably the world’s densest nesting beach for this world’s largest sea turtle. (They average about 800 pounds and can weigh a ton.)
We’re here to shoot an episode for our upcoming PBS series Saving the Ocean, with Carl Safina. The crew consists of director Dave Huntley, cameraman Dan Lyons, sound capturist Tim Wessel, and underwater filmer Valentina Cucciara, with associate producer Mike Olcott.
We came specifically because there are plenty of turtles. You’d think that more turtles is better. But only up to a point. They are so dense here that by the middle of the nesting season, nest-digging females often accidentally dig up eggs laid by other turtles. (The nesting season runs from about April to August, we’re here in early June because we can see both laying and hatching.)
Another reason they’re so dense is that the beach is too small for the roughly 3,000 females who nest here each season. Each lays about 6 clutches per season, at intervals of several weeks. That’s just too many nests for a beach only half a mile long, and so narrow that on the full-moon tides we’re having the highest waves are drenching most of the beach. Waves of the high-running surf are reaching almost all the way to the jungle. (Long-term, sea level rise can’t be helping.) Further, exceptional rainfall has rivers swollen, and where they meet the beach they’re carrying out big wedges of sand.
And so—. The beach is a jumble of churned up sand and churned up eggs. Plus, nests are coming out of eroded sand banks.
All this is good news to the dozens of vultures who incessantly poke around the beach, pecking holes in eggs. They also kill hatchlings lucky enough to have survived the whole incubation period and unlucky enough to emerge before sunset or after dawn (they usually emerge at night).
And that’s only on the beach. In the ocean, many fish and various seabirds are only too happy to cross paths with a hatchling turtle. That’s why leatherbacks lay something like 500 eggs per nesting season.
After they mature at about age thirty, Caribbean leatherbacks nest every 2 to 3 years. (Leatherback maturation age was recently worked out by Larisa Avens at the National Marine Fisheries Service in Beaufort, North Carolina. Avens and colleagues’ study is published in Endangered Species Research, Volume 8, pp 165-177, 2009.) Males never come ashore. Only females do, and only to lay eggs. Between nesting years, she’ll wander the ocean, eating jellies and bulking on weight.
Those lucky enough to live to maturity can live to be 80 years old. Many don’t make it. Adult mortality is about two percent annually. So if a female has, say, a 20-year reproductive life, laying every other year, she’ll lay about 5,000 eggs. Think about the odds; to oversimplify, keeping the population stable means that just two of those 5,000 or so eggs result in one adult female and one adult male. (Some females are astonishingly successful at staying alive and become super-breeders. Dr. Scott Eckert, who has studied leatherbacks for years, tells me that a turtle in St. Croix in the US Virgin Islands (tagged #D-603) was still nesting recently in 2012.)
On top of natural dangers, there’s also fishing gear. As I saw first-hand.