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Entrainment by Hydraulic Dredges - A Review of Potential Impacts

K. J. Reine / D. G. Clarke / R. M. Engler

Effects of entrainment by hydraulic dredging operations on aquatic organisms have been and continue to be an issue of environmental concern. Clearly, assessment of actual impacts due to entrainment pose severe technical challenges. Studies to date illustrate the difficulties in determining precise estimates of absolute entrainment rates and have seldom been able to determine population-level consequences with any degree of confidence. Placing mortalities due to entrainment by dredges into a fishery population dynamic framework, such that dredging can be viewed within the context of other anthropogenic and natural sources of mortality for a resource, would enhance the ability to balance needs for resource protection and dredging project schedules. The persistent lack of quantitative data characterizing effects of entrainment hampers efforts to resolve the issue as it applies to environmental windows. Much of the available evidence suggests that entrainment is not a significant problem for many species of fish and shellfish in many bodies of water that require periodic dredging. Also, many dredging restrictions are based upon limited scientific data, suggesting that well-designed studies and new assessment tools are needed to remove entrainment issues from the realm of subjectivity. In the same regard, however, certain types of dredging operations appear to pose sufficient risk to sensitive resources such that continued application of restrictions is justified. Notably, these exceptions include the conduct of dredging operations in narrow constricted river channels, particularly where sturgeon may be present in appreciable numbers or in channels seasonally occupied by sea turtles or other protected species. Additional studies are definitely warranted to establish the necessity of entrainment-related environmental windows for these resources of concern.