A commercial fisher hoists aboard a crab trap with a meager catch. Although catches are highly variable, most fisheries managers believe the Dungeness crab population is healthy.
In another dramatic decline, catches of the commercially harvested Dunge-ness crab near San Francisco collapsed from 12 million pounds to less than a million pounds in the late 1950s, just after the major El Niño of 1957–1958. Several explanations have been proposed, among them a permanent shift in ocean conditions. The cause of the decline is still not known with certainty.
Further north, in Northern California, Oregon and Washington, Dungeness crab catches go through regular cycles of about 10 years. Since the late 1970s, research has focused on the causes of the cycles, so that they might be managed to avoid these boom-and-bust cycles. Groups at UC Davis and Humboldt State University used mathematical models combined with field data to show how the negative influences of adult crabs on recruitment of juveniles (including cannibalism of adults on newly settled juveniles and density-dependent fecundity) could contribute to these cycles.
Research since the mid-1980s has indicated that annual variability in the way currents transport the planktonic larvae of crabs and other species also has a critical effect on their population booms and busts. Data from Russian plankton cruises off the California coast indicate that Dungeness crab larvae disperse several hundred kilometers offshore, but may be transported back toward suitable nearshore habitat by migrating vertically between layers of water flowing in different directions. Modeling studies have shown that the timing of egg hatching and larval release into the plankton relative to the timing of the “spring transition” to the upwelling season also has a critical effect on the number of Dungeness crab larvae that successfully return to near shore. Other modeling studies of the Dungeness crab subpopulations from Northern California to Washington have shown that the patterns of larval dispersal between subpopulations can have a critical effect on whether populations cycle and how synchronous these cycles are along the coast (Botsford et al. 1994).
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