(Delmarva Now) -- Blue crab numbers dropped by nearly one-fifth this year in the Chesapeake Bay, according to a new scientific survey that presents a mixed outlook for the key crustacean.

“The big picture is the stock overall is in pretty good shape,’’ said John Bull, head of the Virginia Marine Resources Commission. “This year’s harvest should be substantial and consumers should find plenty of tasty crabs for the dinner table."

But he added that obstacles remain, suggesting that regulators should remain conservative with the catch limits they set for the commercial crabbing industry.

The Maryland Department of Natural Resources and Virginia Institute of Marine Science have partnered since 1990 on annual winter surveys of the blue crab in Chesapeake Bay. In all, they estimated this year's population at 455 million, down 18 percent from 2016.

Watermen should have no trouble filling their crab pots during the first half of this year's harvest because female crabs abound, the survey showed. The female stock increased 31 percent to a record high of 254 million crabs.

But troubles await during the second half — from August onward. At that point, this year's juvenile crabs are expected to be large enough for keeping, but their numbers slipped 54 percent to the fourth-lowest total on record, the survey found.

Scientists attribute the decrease to natural fluctuations — unfavorable winds, currents, temperature and crab-on-crab cannibalism.