ANNAPOLIS, SEPT. 20 -- Reproductive success of rockfish in the Chesapeake Bay slid sharply from last year's 20-year high to a seven-year low this summer, according to Maryland's annual young-of-the-year index.

The 1990 index, which represents the average number of rockfish hatchlings caught in 132 seine hauls at sites around the estuary in July, August and September, was 2.1, far below the state's 36-year average of 8.7, Maryland Natural Resources officials said today.

Last year's figure of 25.2, the second-highest ever, precipitated the scheduled reopening of rockfishing in Maryland Oct. 5 after a 5 1/2-year moratorium to protect the beleaguered state fish.

Natural Resources Secretary Torrey Brown said the new numbers will not affect plans for the five-week recreational and charter season, nor for the month-long commercial rockfishing season set to follow it this winter.

"We have some good news to report today and some news that needs interpretation," said Brown, who noted that stocks of adult rockfish remain plentiful around the Bay as a result of the five-year closure.

As for the young-of-the-year decline, Brown said it was not unusual for peak reproductive years to be followed by lows. He cited cold, rainy weather this spring (poor conditions for spawning) and also offered a pet theory -- that abundant juvenile rock from last year's hatch may have gobbled up many of their new kin shortly after they were born.

In any event, Brown said, the decision to reopen fishing this year is based on spawning figures from 1985-88. He defended the upcoming sportfishing season, which features two-fish-a-day creel limits for private anglers and five-a-day for charter customers, as conservative.

"Ten years ago, we were fishing all year for 12-inch fish and keeping all we wanted," said Brown. "This year, we're fishing five weeks for 18- to-36-inch fish and keeping only two or five a day."

He said Maryland anglers, having done without rockfish for half a decade, "deserve a season if it's done responsibly. They've waited a long time."

Maryland will spend hundreds of thousands of dollars in increased enforcement to make sure recreational and commercial fishermen do not exceed the year's overall rockfish quota of 750,000 pounds.

Fisheries chief Pete Jensen said he expected the recreational season to run the full five weeks, but would not be surprised if the charter-boat season and the commercial season are cut short because the limit is prematurely exceeded.