White abalone, the endangered shellfish that once numbered in the millions off the Southern California coast, have declined precipitously over the last decade and are on the brink of extinction, a study has found.

In research published this week, scientists for the National Oceanic and Atmospheric Administration reported “a dramatic and continued decline” in the population of hard-shelled sea snails, a trend that has only worsened since they were protected from overfishing in the 1990s.

Underwater surveys found a 78% drop in the number of white abalone lodged between rocks off the coast of San Diego since 2002, with most of those remaining either so old or isolated from one another they can no longer reproduce. Researchers warned that, without the ability to spawn a new generation, the aging sea creatures, which can live up to 35 years, will not be able to recover on their own.

“At this point, without human intervention, the species could go extinct within our lifetimes,” said co-author Melissa Neuman, white abalone recovery coordinator for NOAA’s National Marine Fisheries Service.

The report, published in the journal Biological Conservation, urges “immediate, proactive conservation” by breeding white abalone in captivity and releasing them in the wild, saying it may be the only way to save them.

“The study highlights a new sense of urgency about the importance of captive breeding,” said Kevin Stierhoff, lead author of the study and research fisheries biologist at NOAA’s Southwest Fisheries Science Center in La Jolla, which operates an aquarium facility designed to culture young white abalone to boost wild populations. The University of California, Aquarium of the Pacific and Cabrillo Marine Aquarium are working on similar captive breeding programs, he said.

White abalone were abundant in kelp forests and rocky reefs from Point Conception to Baja California until the 1970s, when commercial divers plucked some 350,000 of them from the ocean for food. The overharvesting caused landings to plunge near zero and the fishery was shut down in 1997. White abalone was listed as a federally endangered species in 2001.

Currently, only a few thousand are left; A 2000 government report predicted white abalone would disappear entirely by 2010.

White abalone is one of the seven abalone species that live in California waters and were historically considered to be immune from extinction. Black abalone, a relative, was listed as endangered in 2009.

“The case of the white abalone, " the report notes, “illustrates that marine invertebrates, and particularly species with high commercial value, are unarguably vulnerable.”

tony.barboza@latimes.com