While recent surveys have shown a drop in the blue crab population over the last several years, a leading crab researcher is now suggesting that stocks of the Chesapeake's most valuable species have actually been in a "low phase" for more than two decades.
Rom Lipcius, a scientist at the Virginia Institute of Marine Science who also advises the Virginia Marine Resources Commission, and Rochelle Seitz, a doctoral candidate at VIMS, theorize that the crab's decline dates to Tropical Storm Agnes in 1972.
The population dropped after the storm, Lipcius said, and never recovered to normal levels. The recent crab decline, he said, is actually a further decline from an already-low population.
"We have been in a relatively low phase, apparently, for 20 odd years," Lipcius said. "That's what brings us to more concern. If we're in the trough of a low phase, then we had better be cautious in terms of management."
Lipcius and Seitz are finalizing a paper on the hypothesis, which they expect to submit for publication in a peer-reviewed journal in November.
Agnes was the most severe storm to hit the Bay and its watershed in the past century or more. It dumped between 5 and 18 inches throughout the watershed over a three-day period, washing huge amounts of sediments and nutrients in to the Bay.
Lipcius said that fisheries data and trawl surveys indicate that crab abundance was "substantially" higher in the two decades before Agnes. Population surveys and harvest reports all show sharp declines in the years after the storm, he noted.
"Beyond drops in the crab population, beds of submerged aquatic vegetation an important habitat for juvenile crabs declined dramatically after Agnes." Also, Seitz noted, soft-shelled clams and other major blue crab food source s were nearly Òwiped out."
"We believe that year they took at least a double hit; both their nursery grounds and food were devastated," Lipcius said. "We think that had a long-term impact, basically bringing [the crab population] to a lower phase."
Earlier, Lipcius and others attributed the decline of the blue crabs to overfishing. But now, Lipcius suggests it was a natural event that dropped the population. However, he also believes that gradually increasing fishing pressure on blue crabs has kept the population from bouncing back as grass beds and food populations recovered.
Surveys have found increasing numbers of juveniles in grass beds, Lipcius said, but that has not translated into a larger population. "There is so much fishing pressure that all those extra crabs that are coming back are being harvested," he said. "So they are not becoming effective spawners, which then keeps the population at a relatively lower level."
"When marine populations are reduced to low levels, Lipcius said, it often takes an awful lot, a big effort, to get the population back up to a hig her phase."