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New understanding of crab populations may not bring fishermen's delight

University of California - Davis

Populations, whether fish or people, can be even less predictable than the weather, according to an accurate new model created by researchers from the University of California, Davis, and University of Helsinki, Finland.

Population sizes can dive and soar dramatically, sometimes without apparent cause. Trying to figure out what drives the big swings is a question that has taunted researchers for years. In fact, one of the most important and fiercest debates in population biology swirls around whether populations go through boom and bust cycles primarily because of internal ways of regulating their numbers by changing things such as natural life expectancy, egg-laying rates and cannibalism, or because of outside forces, such as predators, weather and food.

It turns out that both sides may be right. A new paper being published in the May 30 issue of Science magazine fingers random external influences that are amplified by nonlinear internal dynamics as guilty parties in the wild swings.

Focusing on Dungeness crabs on the U.S. West Coast, the paper is authored by newly minted UC Davis doctoral recipient Kevin Higgins, UC Davis environmental studies chair Alan Hastings, University of Texas computer science graduate student Jacob Sarvela, and UC Davis wildlife, fish and conservation biology professor Louis Botsford.

The model stands out for several reasons. In particular, the model provides an extraordinary match with 42 years of data gathered from eight fishing regions along the West Coast.

The art of ecological modeling is in figuring out how to simplify complex systems to the few most important components while minimizing distortions. Modelers translate the relationships between those key components into equations and plug them into a computer. A model "works" when the equations spit out answers that resemble the actual data. Then, the modelers believe they have pinpointed some essential dynamics that could help explain how a complicated system works.

In this paper, the researchers charted crab catches from Grays Harbor, Willapa Bay and Columbia River in Wash.; Astoria and Warrenton, Ore.; Tillamook and Gardibaldi, Ore.; Newport and Depoe Bay, Ore.; Coos and Winchester bays, Ore.; Brookings, Gold Beach and Port Orford, Ore.; Eureka and Crescent City, Calif.; and Fort Bragg, Calif.

They matched the catches with a model that accounted for internal crab population dynamics--natural survival rates, egg-laying rates and cannibalism, all of which have been identified by field researchers as effective internal forces that help crabs rebuild populations when the numbers are too low or reduce populations when the numbers are too high.

Then the researchers threw in random external forces, also called "noise," which could be weather, water temperature, ocean circulation or other factors. This works mathematically, they found, but more research is needed to identify those external influences more specifically.

"In the history of population ecology, one of the big-time questions is what regulates populations," Hastings says. "We've got a model showing that the observed dynamics come about because of the interplay between external and internal forces. The complex dynamics are produced by a small amount of noise."

For one of the first few times, a modeled understanding of the process agrees with data from a field population, he says.

Ideally, a model with such a good fit to data could be extended to predict the future of crabs. Yet, like weather, populations seem to defy predictability. Even extended to the year 2950, this precise model seems to confirm the elusiveness of certainty.

"The one thing we can predict with this model is that crab populations will fluctuate drastically," says Higgins, now a postdoctoral researcher at the University of Helsinki. "People have felt populations in nature are in equilibrium. Our model says no, small environmental disturbances are amplified by populations, preventing equilibrium. The natural state of populations is constant change."

The model not only provides a good match for the data, it offers strong theoretical support for field ecologists looking for a more detailed explanation of environmental and other external forces. Co-author Botsford, for example, has been studying the relative influence of environment and internal population mechanisms of West Coast Dungeness crabs for 22 years, most recently exploring how crab larvae are transported in the plankton by ocean currents during the spring upwelling in the ocean current when winds come from the north.

"This model says the population would be constant if it wasn't constantly perturbed by environmental changes from year to year," Botsford says. "It points us toward the environmental influences but doesn't identify them. More research is needed to identify the actual impacts of winds, currents and El Nino conditions."

Hastings says the next step is focusing on these explicit environmental influences. Such demanding questions require complicated calculations, but recent advances in computing power available should give the researchers the tools to answer them. Their current model takes five days, round-the-clock, of powerful computer time.

Higgins and Hastings expect the mathematical results for this system to influence efforts to understand other systems, such as the stock market.

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