A Visit to Crab Mountains
In April of 1991, we dove to the depths of Chiniak Bay, in Kodiak, Alaska, using a two-person submersible called the Delta.
Our goal was simple. We had observed Tanner crabs mating in the shallow waters of nearby Women's Bay, and captured many of them for research. Every female we caught was a first-time spawner, known as primiparous. We know that Tanner crabs can live for a number of years after reaching maturity, but we did not find any older female crabs, which we call multiparous. Similarly, all of the males found mating with females in shallow water were much smaller than the large males captured by the fishery, and were probably first time spawners themselves. Where were the older male and female crabs? and where were they mating? Our findings were published as
Morphometry and maturity of male Tanner crabs, Chionoecetes bairdi, grasping pubescent and multiparous females in shallow and deepwater environments. Can. J. Fisheries and Aquat. Sciences 50:1504-1516.
Conversations with fishermen led us to believe that the older multiparous females might be found in the deeper portions of Chiniak Bay, below 400 feet. But how could we find or catch them? A submersible seemed like the best solution. With funding from the NOAA Undersea Research Program at the University of Alaska Fairbanks, we dove to depths of 600 feet in Chiniak Bay in April, 1991. As we cruised around the muddy bottom, we say very few crabs. After several days we began seeing groups of females buried in the mud in a small area of the bay. Here you can see them almost completely buried, with only their eyestalks sticking up above the surface layer of the sediment.
For several days the water was extremely silty, and our visibility was only a foot or two. Looking out through the porthole near the bottom of the sub, I could barely see the bottom. By the end of the week, the tidal current began to increase, and helped to sweep the silty waters away. Soon we began seeing larger groups of crabs, spread out across the bottom, but closely in contact with each other, almost like parquet flooring. I began to call this a "mat" of crabs.
Cruising along the bottom, we suddenly ran into a wall of crabs. The sub pilot stopped and we both looked in awe. We were surrounded by small mountains of crabs, standing over a meter in height, and several meters in diameter. Nobody had ever seen Tanner crabs (or their relatives snow crabs) form aggregations of this sort. It was an amazing discovery but we did not know what they were doing.
Over the next two weeks, we surveyed the area with the Delta to photograph and map out all the crab mounds we could find. We concluded that there were about 200 mounds, and each mound contained an average of about 500 crabs. Over 100,000 crabs were clustered into an area about the size of a football field. Using a remotely operated vehicle (ROV), we were able to capture some crabs from the tops of the mounds. All the crabs in the mounds were multiparous female Tanner crabs, carrying eyed eggs that were about to hatch. This discovery was published as
Stevens, B. G., J. A. Haaga, and W. E. Donaldson. 1994. Aggregative mating of Tanner crabs Chionoecetes bairdi. Can. J. Fish. Aquat. Sci. 51:1273-1280.
Stevens, B. G., W.E. Donaldson, J. A. Haaga, and J. E. Munk. 1993.
Over the next five years, we revisited the site in spring, but did not observe such mounds again. After analyzing 5 years of data, I realized that the timing of the event was connected with the tidal regime. By aggregating into mounds, female crabs could facilitate hatching their larvae up into the water column away from the silty benthic boundary layer. I called this phenomenon "Larval launch pads".
Stevens, B. G., J. Haaga, and W. E. Donaldson. 2000. Mound formation by Tanner crabs (Chionoecetes bairdi): Tidal phasing of larval launch pads? Crustacean Issues 12:445-456.
In 2000 and 2001 we revisited the site using a camera sled and ROV, beginning our search several weeks ahead of the lowest spring tide. The location of the aggregation changed by a few hundred m every year, so the camera sled allowed us to search a large area quickly and cheaply. Once we located the crabs, we used the ROV to take a closer look. Over the next few weeks, we observed mats of female crabs form into mounds. We captured some female crabs with the ROV and brought them into the lab, where they began releasing their larvae on the next high spring tide. We also placed a current meter on the bottom of Chiniak Bay, so we could measure the water currents near the crab aggregation.
We concluded that mass hatching by so many crabs in a brief period helps to overcome predators, represented by the euphausids (krill) that we saw swarming in abundance around the crab mounds. By waiting for the greatest tide of the spring, larvae were released at a time when the tidal currents would carry them into the shallows where they could find suitable habitats to settle and grow. The answer to the mystery of crab mountains was finally published 12 years after we first saw it.
Stevens, B. G. 2003. Timing of aggregation and larval release by Tanner crabs, Chionoecetes bairdi, in relation to tidal current patterns. Fisheries Research. 65:201-216.
