Laboratory Assessment of Potential Impacts to Dungeness Crabs from Disposal of Dredged Material from the Columbia River

Abstract

Executive SummaryDredging of the Columbia River navigation channel has raised concerns about dredging-related impacts on Dungeness crabs (Cancer magister) in the estuary, mouth of the estuary, and nearshore ocean areas adjacent to the Columbia River. The Portland District, U.S. Army Corps of Engineers engaged the Marine Sciences Laboratory (MSL) of the U.S. Department of Energy's Pacific Northwest National Laboratory to review the state of knowledge and conduct studies concerning impacts on Dungeness crabs resulting from disposal during the Columbia River Channel Improvement Project and annual maintenance dredging in the mouth of the Columbia River. The present study concerns potential effects on Dungeness crabs from dredged material disposal specific to the mouth of the Columbia River.A phased approach to evaluating these effects is being used, the objectives of which are to synthesize that which is known about disposal effects on Dungeness crabs (Phase I, completed previously), to quantify the effects of burial and surge currents (Phase II, this study), and estimate population-level effects (Phase III, proposed future study). The initial step in Phase I included development of a conceptual model to synthesize knowledge about crab biology and physical processes occurring during disposal, and to identify the potential mechanisms by which crabs may be injured. Phase I also included numerical modeling of the disposal process using the Short-Term Fate (STFATE) dredged material disposal model developed by the Corps' Engineer Research and Development Center in Vicksburg, Mississippi, which provided information on the magnitude of vertical and horizontal forces and burial depth expected to be encountered by crabs during a variety of disposal scenarios. The results of the STFATE modeling were evaluated together with information on crab biology to identify areas of greatest crab vulnerability. The Phase I report concluded that crabs were more likely to be vulnerable to injury by burial or by being tumbled during the horizontal surge current generated during the dynamic collapse of the dredged material plume upon contact with the bottom. Crabs should be less vulnerable to the compression (vertical) forces exerted during a disposal event. Recommendations for future studies from the Phase I study included the simulation of burial depths and surge currents generated during a short-duration disposal event in shallow water, with experiments designed to include aspects of crab behavior that could change their vulnerability to a disposal event. These Phase I recommendations provided the background and direction for the present study.This report describes laboratory experiments conducted as part of the Phase II disposal effects assessment. Two types of experiments were performed to isolate the effects from the two components of the disposal event posing the most risk to crabs; burial from deposition of the dredged material on the bottom, and injury caused by tumbling in the horizontal surge current. Burial experiments focused on...

Cover letter RE “Video observations of the acute effects of dredged sediment deposition on mobile epifauna”

“…Results indicate burial up to about 10 cm was not detrimental for most crab sizes tested if they were unconstrained by experimental conditions. The sediment accumulation levels at BVLs thus were below depths of concern documented by Chang and Levings [25] and Vavrinec et al [26].…”

“…Thus one should expect a disruptive effect of these sediment disposal events on epifaunal organism status and behavior. However, the Dungeness crab was found resilient to simulated dredge activity conditions by Vavrinec et al [26] who used flume experiments to examine crab responses to a discharge of sediment-laden water, and concluded behavioral responses by crab could minimize burial risk at velocities up to 3.2 m/s. Our studies of acoustically tagged Dungeness crab likewise indicate high survival and mobility after sediment deposition events [29].…”

“…Post impact video from both upper and lower cameras revealed sediment deposition levels were moderate (maximum of 4 cm). Previous laboratory experiments examined the survival of crabs as functions of burial depth and time, the primary concerns being the maximum depth a crab could reestablish respiratory currents to the overlaying water or from which they could extricate themselves [25, 26, 37]. Results indicate burial up to about 10 cm was not detrimental for most crab sizes tested if they were unconstrained by experimental conditions.…”

“…At the MCR, individual deposition events average around 4,200 m 3 and were expected to result in a footprint of approximately 1.8 × 10 5 m 2 and sediment depth range up to 9 cm along the centerline [23]. Sediment deposition exceeding this depth could cause mortalities to benthic organisms such as the Dungeness crab [25, 26]. These model results derived from scaling calculations could only be approximated in the experimental test facilities used by Johnson and Fong [20] and Vavrinec et al [26], and thus in situ experiments and observations were required for confirmation.…”

Lethal and sublethal effects of simulated dredged sediment deposition on overwintering blue crabs Callinectes sapidus

“…Instead, at high disposal overburdens, buried crabs may be unable to maintain respiratory currents that bring oxygenated waters to the gills and subsequently suffer mortality (Chang & Levings 1978, Pearson et al 2006. In laboratory experiments, Dungeness crab mortality occurred at deeper burial depths, with nuanced responses dependent on crab size and gender (Chang & Levings 1978, Vavrinec et al 2007.…”