The Bering Sea tanner crab biomass remains below the threshold necessary to allow a fishery. The Alaska Board of Fisheries harvest strategy for Bering Sea tanner crab specifies a mature female biomass threshold of 21 million pounds and a minimum GHL of 4.0 million pounds in the waters east of 168 degrees longitude. Both thresholds must be met to allow a harvest. The 2004 survey estimate of mature female biomass is 13.2 million pounds, a drop of 37 percent from the 2003 estimate of 20.8 million pounds. Since the mature female biomass was not met, the fishery will not open in 2004.
National Marine Fisheries Service fisheries biologist Braxton Dew gave a presentation in Kodiak last month sponsored by the Alaska Marine Conservation Council (AMCC) that focused on the question of whether bottom trawling in Bristol Bay's red king crab brood stock refuge contributed to the collapse of the fishery. According to a story in the Kodiak Daily Mirror, Dew lined up some theories as to why trawling might have contributed to the collapse of the red king crab fishery. Dew pointed out that in 1959, the Japanese implemented the Bristol Bay no-trawl zone known as the Pot Sanctuary. In 1976, the Magnuson-Stevens Act effectively eliminated this. The boundaries of this refuge closely matched the well-defined distribution of the red king crab population's mature female brood stock, Dew said. In 1980, the point at which the commercial harvest of Bristol Bay legal male red king crab had reached an all-time high after a decade-long increase, domestic bottom trawling in the brood stock sanctuary began with the advent of a U.S.-Soviet, joint venture, yellowfin sole fishery, he said.
"Red king crab disappeared from areas of high trawl density after 1980. By 1983, the Bristol Bay red king crab population dropped to zero," Dew said.
The Mirror reported that there are at least two popular theories that attempt to explain why this happened. The dominant theory says the fishery's collapse in the early 1980s was due to natural ecological circumstances, and the other theory, which Dew supports, says crab may have been adversely affected by fishing. Due to the well-defined diurnal schedule of the crabs and the fact they aggregate during the day, their podding behavior may have led to the sudden collapse in areas that were trawled, Dew said, noting there can be 500 crab per square meter in a pile. "By doing some night diving, I discovered the typical dome-shaped pod observed resting during the daytime becomes an actively foraging aggregation at night. Each morning at dawn these crabs stop foraging and spend an hour or so rebuilding the daytime resting pod," Dew said. "It's not difficult to imagine the eventual demise of a population that reassembles into a single aggregation each time it is divided by trawling. It is predictable that trawling would be disastrous to the population," he added.
The Mirror reported that not everyone agrees with Dew, however. Former Alaska Department of Fish and Game employee Jim Blackburn, who attended the presentation, said, "I don't think (Dew's data) is the most plausible explanation for the decline. His explanation is basically not accepted. It is clear that there were environmental changes. Cod abundance in the Bering Sea increased tenfold at that time and cod are a major predator of crab. From work Guy Powell did here, cod tend to eat female king crab when they are soft-shelled, which is right at reproduction," Blackburn said. He noted there were declines in crab all over the place, even in areas that were not fished and trawling did not occur. "It is hard to make a real clear case that cod fish caused the decline of king crab, but they certainly contribute, and there are a number of other predators out there on king crab," Blackburn said.
Dew disagreed. "The existing data from NMFS shows that cod don't eat anywhere near enough red king crab to have caused that collapse," Dew said, adding that the most cod do is pick off the legs of recently molted females. "Although there is a correlation between the upsurge of cod and the precipitous decline of the red king crab, correlation doesn't say anything about causation. It's just one of those deals where you have two events happening independently and you can put them together and say they must have caused each other. But data suggests that is not the case," Dew said.
