Turtle recovery could take many decades.

Under current protective laws, it would take at least 70 years to achieve a 10-fold increase in populations of the loggerhead sea turtle of f the coast of the southeastern United States, according to a new computer forecast. Wider use of turtle excluder devices (TEDs) - trapdoor-like mechanisms that allow sea turtles to escape shrimp-trawling nets - would reduce this recovery time by only 30 to 40 years, the forecast indicates.

Biologists consider a 10-fold population increase crucial to saving this threatened species.

The new computer model is the first to take into account the differing effects of TEDs on the survival of loggerhead turtles of various ages and sizes, says Selina Heppell, a biologist at North Carolina State University in Raleigh. Heppell described the model last week at the joint annual meeting of the American Institute of Biological Sciences and the Ecological Society of America, held in Honolulu.

Because TEDs are less effective in saving smaller, younger turtles, she says, the model suggests that fewer than anticipated numbers of loggerheads will survive to reproductive age.

Most TEDs consist of a panel of metal bars inserted into a trawling net at an angle leading up to a hole in the top of the net. The bars are designed to allow shrimp to pass through and accumulate in the sack-like end of the net, while diverting larger marine animals - such as sea turtles - up and out of the net through the hole.

The US. National Marine Fisheries Service (NMFS) estimates that TEDs have slashed by 67 percent the annual mortality rate of sea turtles caught in trawling nets in U.S. coastal waters since the 1988 enactment of a federal law requiring shrimp trawlers to use the devices during selected times of the year in most offshore areas. Despite the use of TEDs, however, the NMFS projects that at least 4,360 sea turtles inhabiting U.S. coastal waters will drown in trawling nets this year.

Heppell's forecast rests on the assumption that TEDs reduce the mortality rate of net-trapped juvenile loggerheads by only 34 percent each year, because these smaller turtles sometimes get swept between the bars of the devices and become caught in the nets. This potentially fatal generation gap skews the demographics of the loggerhead population toward older turtles with fewer remaining reproductive years, Heppell says. This, in turn, slows the recovery rate of the species as a whole, she asserts.

"TEDs have had a very positive effect on increasing the population [of loggerhead turtles]," Heppell says, "but 70 years is a longer time to see a 10-fold population recovery than we'd expected." She adds that although the number of nesting loggerheads has increased within the past three years, indicating the overall benefits of TEDs, "we have to be a little cautious in saying we've found the answer to saving the [loggerhead) turtle population.... It's going to take a long time."

Last April, the NMFS proposed new regulations that would expand the requirements for TED use by mandating trawlers to use TEDs year-round at all U.S. inshore and offshore locations. This would achieve a 97 percent reduction in trawler-related turtle mortality, according to agency estimates.

However, the proposal does not call for new TED designs less likely to trap juvenile loggerheads. Nor does it address what Heppell describes as a potential threat to all sea turtles: the harvesting of sargassum, a type of seaweed that grows in floating mats.

Commercial harvesting of these vast mats for use in pharmaceuticals or livestock feed could destroy an important habitat for "small juvenile" sea turtles, Heppell says. According to he model, this category of juveniles constitutes the second most important age group for the recovery of the loggerhead population. increasing the mortality rate of small juveniles through sargassum harvesting could delay a 10-fold increase in loggerheads to 140 years, Heppell projects.