Bolstered by a huge spawn of young crabs, the number of blue crabs in the Chesapeake rose dramatically this winter after several years of declines or no improvement, an annual Baywide survey shows.

This year's "index" - a relative measure of abundance - was 15.5. That was up dramatically from the 8.5 level recorded in last year's Baywide winter dredge survey, and the 9.58 index reported in 1994.

But officials cautioned that the numbers primarily reflected the highly successful reproduction of the blue crabs - 71 percent of those caught were juveniles less than 1 year old. When the tremendous amount of young were factored out, the number of adults in the Bay rose only slightly.

"I think one of the most reassuring things about this year's data is that there was a lot of concern last year about the size of the spawning stock - the number of females," said Bess Gillelan, director of the National Oceanic and Atmospheric Administration's Chesapeake Bay Office. "And with that relatively low, or perceived low level of females, we were still able to get an abundant year class."

The survey results likely will have little impact on this year's harvest, which is already off to a slow start because of abnormally cold conditions this spring. But Gillelan said the large crop of young crabs could be good news for 1997, when most of this year's juveniles will have reached market size. She cautioned, though, that many factors affect the survival of juvenile crabs.

The index is an estimate of the number of blue crabs found per thousand square meters. It is based on samples taken at about 1,200 sites in Virginia and Maryland through the winter. NOAA funds the annual winter dredge survey, which is conducted by the Maryland Department of Natural Resources, the University of Maryland's Chesapeake Biological Laboratory and the Virginia Institute of Marine Science. It is coordinated by the Chesapeake Bay Stock Assessment Committee, which represents state and federal agencies.

The survey began in 1988, but scientists and fisheries managers disregard the first two years of data because sampling techniques changed. Until this year, the survey had indicated a decline in abundance from the relatively high numbers seen in 1990 and 1991 - with indexes of 12.4 and 13.91 respectively.

That apparent decline, coupled with several years of smaller than normal catches, spurred concern that increasing harvest pressure could be threatening the overall crab population. In particular, many scientists and managers expressed concern that too many females were being harvested before they had a chance to reproduce.

As a result, both Maryland and Virginia have taken actions to curb harvests of the Bay's most economically valuable species. Many of the added regulations were aimed at allowing more females to survive and reproduce.

But the picture grew murkier when a scientific review sponsored by the Chesapeake Bay Stock Assessment Committee concluded earlier this year that there was no long-term decline in blue crab abundance. By patching together varying pieces of data, the review team concluded that crab numbers in recent years had merely returned to normal levels after being abnormally high in the 1980s. Their conclusions will undergo a scientific peer review this summer.

While fishing pressure on blue crabs is a concern, the primary factor affecting blue crab reproduction is environmental conditions along the coast. After they are spawned, blue crab larvae leave the Bay to swim along the coastal waters for several weeks before returning.

It has been hypothesized, for example, that the large number of hurricanes last year may have helped push a greater than normal number of juveniles back into the Chesapeake and other coastal bays.

"We know that recruitment of the larvae back into the Bay is dependent on wind," Gillelan said. "There were extremely high numbers of young crab larvae coming up into the Bay last fall."