- Crabs were generally distributed from the mouth of the estuary to river mile 17; the farthest upstream distribution was to Grays Point (rm 20) during both winters.
- No crabs were collected in Youngs and Cathlamet Bays or in intertidal areas of Baker Bay.
- Overall, crab densities on the bar were significantly less than densities upstream from the bar. Densities on the bar were greatest in spring and summer; early instar crabs were relatively abundant on the bar during this time. Densities of crabs on the bar were significantly greater during the second year of the study than during the first year. During June and July 1985, when large numbers of 0+ age crabs were present, densities tended to be higher on the northern portion of the bar.
- In areas upstream from the bar, densities were not significantly different between the 2 years. Generally there were no significant seasonal differences among upstream densities, except during the first year when densities were significantly greater during fall.
- In the estuary there was considerable variation in crab densities among individual stations and also within individual stations at different times.
- Replicate sampling at Stations 5 and 6 indicated that crabs followed a contagious distribution.
- Large changes in monthly densities indicated movements of crabs within the estuary and/or between the ocean and estuary.
- Densities of Dungeness crabs, particularly size classes II and III, were greatest in channel areas of Baker Bay (Ilwaco and Chinook Channels) during fall, spring, and summer. Densities tended to be least during February-May, with a slight increase in March for Ilwaco Channel. Densities of Dungeness crabs in the Flavel area (Station 10, second year) were relatively high from March through July.
- Bottom salinity and temperature were poor predictors of crab densities for all size classes combined and for individual size classes.
- Incidence of the egg predator Carcinonemertes errans averaged 6% in the estuary and 79% in the ocean.
Because the Columbia River estuary is extensively populated by Dungeness crabs, it is important to minimize the effects of dredging. Dredging in Chinook Channel should be done during winter and early spring, i.e., late February through early May, when crab densities are low. In Ilwaco Channel, late January through early May, with the exception of March, is the best period for dredging. Further studies need to be done to assess dredging mortality of early instar crabs on the bar. Also a study is needed to estimate the proportion of the early instar population impacted by dredging. In the interim, if practical, the dredging schedule for the bar and adjoining ocean portion of the navigational channel should be adjusted to reduce dredging time in June and July; perhaps dredging could be started earlier.