More than 50 people attended a presentation sponsored by the Alaska Marine Conservation Council (AMCC) by National Marine Fisheries Service fisheries biologist Dr. Braxton Dew last week. The discussion focused on the question of whether bottom trawling in Bristol Bay’s red king crab broodstock refuge contributed to the collapse of the fishery.

Dew worked in the Kodiak NMFS facility for 13 years and gleaned a lot of his data by scuba diving and photographing the behavior of the crabs in their habitat.

AMCC representative Theresa Peterson said, “He was one of the first ones to see that crab alternate between daytime resting and nighttime foraging. He photographically documented the podding behavior of adult red king crab which are remarkable similar to juvenile crab.

“Braxton’s research has very significant implications for future fisheries management and the need to safeguard the long-term viability of the marine eco-systems,” she said.

Dew presented one hypothesis offering an explanation why trawling might be a contributing factor to the collapse of the red king crab fishery.

In 1959, the Japanese implemented the Bristol Bay no-trawl zone known as the Pot Sanctuary. In 1976, the Magnusen-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 broodstock, 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 broodstock 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.

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 — the pod most people have seen in shallow waters around Kodiak.

“It didn’t take long to realize the behavior of these crabs was very organized and their intention was to stay together,” Dew said.

“Whatever happened to the crab happened quickly. There was a steep decline. Podding increases crab vulnerability to trawling by increasing the probability of extinction,” he 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.

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.

“In the 1960s, here, the only place you could find cod was off the continental shelf edge, and by the late 1970s they had moved into the bays. Their abundance increased and the ones that were here moved into the areas that were the prime habitat for crab. Previously, the cod were not in crab habitat.

“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.

He said crab also declined in Kodiak and Adak and in Southeast Alaska and all along the Alaska Peninsula where there had never been any trawling.

However, he admitted that it is not easy to pinpoint the cause of the decline of the crab population.

In light of this rationale, Dew remains adament that fishing — specifically trawling — had everything to do with the red king crab decline.

“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.