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To Catch a Crab: Undersea with the Delta Sub in Chiniak Bay, Alaska (1991)

[female Tanner crab pile, thcbpod.jpg=7KB] [grasping Tanner crab pair, male and female; th0020.jpg=5KB]
[Ed Wyman's crab catching basket and Delta sub, th0012.jpg=9KB] [crab catching claw, thclaw.jpg=5KB]

In April 1991, Dr. Bradley Stevens was exploring the Chiniak Bay  in the Gulf of Alaska with the DSV Delta researching fish waste when he inadvertently discovered an aggregation of Tanner crab (Chionoecetes bairdi). This was the first discovery and description of complex, high-density mating aggregations of Tanner crab observed at continental shelf depths (greater than 150m). Aggregation behavior is common to many species of crustaceans including spiny lobsters (Panulirus interruptus), American lobsters (Homarus americanus), and juvenile and adult red king crabs (Paralithodes camtschaticus). Descriptions of aggregative behavior in majid crabs, of which family Tanner crabs belong, is anecdotal and limited.

Piles of Tanner crab, such as the one shown at left, were found at about a 150 meters depth in Chiniak Bay, Kodiak Island, Alaska (57.72 N, 152.32 W). This pile, and others like it, consisted primarily of oldshell, multiparous female crabs and measured approximately 1-2 meters in diameter, 0.5 - 1.0 meters high, and were spaced at intervals of 1 - 2 meters apart. Each mound contained hundreds of crabs; the entire aggregation covered an area of about 2.2 ha and included approximately 100,000 crabs.

Male Tanner Crabs were found mating with females, as shown at right, at the periphery of the large aggregation, with sex ratios varying from 1:10 to 1:100. After the initial discovery in 1991, these aggregations were seen in the same vicinity in the following two years. The use of the DSV Delta submersible allowed observation of crab behaviors not previously documented at these depths.

The crab catching basket and grasping arm shown in the pictures below were designed by Ed Wyman of Neptune Marine Products and used by the DSV Delta to collect Tanner crab from the ocean floor. When crab were observed, the sub could position alongside them and the grasping arm, controlled from the inside, could be used to gently grab the crab and place them into the basket. Collected crab were brought to the surface at the end of the dive for various measurements and study.

(Text excerpted from Dr. Bradley Stevens et al. article, Aggregative mating of Tanner crabs, Chionoecetes bairdi, Can. J. Fish. Aquat. Sci., Vol 51, 1994. Photographs by Dr. Bradley Stevens)

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