The study, sponsored by the Portland, Ore., District of the U.S. Army Corps of Engineers, sought to address the impact of regular maintenance dredging and a river deepening project on Dungeness crab. The Columbia River, which serves as a waterway for commerce from the West and Midwest, must be dredged yearly to keep the river deep enough for safe navigation. Dredging vessels ply the river from the ocean inland each summer, sucking up huge amounts of sediment and the marine life living in it.
The Corps also plans to deepen the channel from 40 to 43 feet in the stretch from Portland to about three miles from the ocean, which will involve dredging. The Corps is increasing the authorized depth of the river so larger ships can call at ports on the Columbia. Currently, large ships carrying grain and other cargo cannot be fully loaded because there is not enough water for them to return to the ocean safely.
In looking at crab entrainment, or how many crab are trapped by dredges, PNNL researchers compared entrainment data and levels of salinity in the river, finding that lower levels of salinity meant less crabs. As salinity increased, so did crab abundance. "By timing dredging to coincide with lower levels of salinity, the Corps may be able to reduce the impact of dredging to crabs," said Walt Pearson of PNNL's Marine Sciences Laboratory. Several factors determine salinity, including location in the estuary, tides and river flow. These variables influence salinity daily, monthly, seasonally and yearly.
As a result of these findings, the Corps no longer considers dredging a threat to crab in some areas in the upper part of the Columbia River estuary because salinity levels are so low. Areas in the middle of the estuary vary in crab abundance because salinity levels vary.
PNNL researchers will refine their salinity-entrainment model before using it to further investigate salinity in the Columbia River and what it may mean for crab.
In addition, researchers used entrainment data from a summer 2002 study and a modified dredge impact model to make predictions about the number of crab that would be lost to entrainment during the deepening project. Applying a harvest rate, they estimated that the total crab that would be lost to crab fisheries would be equivalent to about one percent of one year's landings.