variability of dispersal capability among species, seems certain to generate variation in community composition at large spatial scales. The extent of this variability, its relation to whether particular sites are larval sources or sinks, and biodiversity as a consequence of local interaction remain major topics in metapopulation dynamics and conservation biology. Marine, nearshore conditions provide an appropriate environment to investigate the interplay between connected populations in open systems.

Continental Shelves and Slopes

"The canneries themselves fought the war by getting the limit taken off fish and catching them all ... It was the same noble impulse that stripped the forests of the West and right now is pumping water out of California's earth faster than it can rain back in."

Steinbeck (1954)

Continental shelves represent the great interface between the continents and open oceans. Directly and indirectly impacted by the natural and human effects that operate on land margins, the shelves are the only "open ocean" most people will ever see or know—and yet the shelves are separated by boundary currents from most of the sea. The concept of shelf waters representing "the ocean" is reinforced by the popularization since the 1950s of "food from the sea," resulting in shelves being portrayed primarily as sites of most of the major world fisheries, located on fishing banks, in upwelling zones, and on broad shallow platforms.

Shelf waters have suffered habitat alteration and changes in biodiversity due to overfishing and the extensive physical damage caused by the deployment of mobile fishing gear (Graham, 1955; Hutchings, 1990). Regional "hot spots" include dramatic shifts in community structure in northeast American shelf fish communities (NOAA, 1992) and the Eastern Bering Sea shelf, where a critical environmental concern is the decline in abundance of certain marine mammal and seabird populations (Alverson, 1992; Pascual and Adkison, 1994), with the Steller sea lion being particularly threatened. Increasing stresses in offshore systems from inshore pollution are of escalating concern but remain poorly understood. Other stresses arise from oil spilled in the sea during tanker transport and operations (NRC, 1985), and to a lesser extent, from oil and gas exploration on the "outer continental shelf," the subject of extensive environmental impact studies since the 1960s (NRC, 1985; Boesch and Rabalais, 1987).

The seemingly distant and more immune slope waters are no longer far away and no longer immune. Deep-water fisheries—and their attendant physical effects (e.g., habitat alteration due to dredging and trawling)—have entered slope waters (Robertson and Grimes, 1983; Polovina and Ralston, 1986). Deep oil and



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement