Abstract.- The effect of benthic 

 dredging on coastal fisheries has 

 been of concern for several decades, 

 but little work quantifying direct 

 population impacts has been pub- 

 lished. Modeling approaches have 

 been used extensively to assess ef- 

 fects of power plant entrainment on 

 fishery stocks. Several important dif- 

 ferences between power plant and 

 dredge operations prevent direct ap- 

 plication of these models to dredge 

 problems: Entrainment by dredges 

 is short-term, has a moving intake, 

 and affects all age-classes of the 

 population. We present an equiva- 

 lent adult loss model of impacts to 

 the Washington coast 'Dungeness 

 crab Cancer magister Dana fishery 

 from dredging of a navigation chan- 

 nel in Grays Harbor, Washington. 

 The model is driven by empirical 

 population data to account for spatial 

 and temporal variation in abundance 

 and age-class structure. Results 

 show that impacts are quite sensitive 

 to the type of dredge used and the 

 season in which dredging occurs. 

 Contrary to initial expectations, the 

 -I- age-group loss was unimportant 

 relative to losses from older age- 

 classes. Despite many limitations, 

 the model has proven useful for 

 focusing impact assessment work, as 

 a basis for scheduling construction to 

 reduce impacts, and as a basis for 

 scaling mitigation projects. 



Predicting effects of 



dredging on a crab population: 



An equivalent adult loss approach 



Thomas C. Wainwright 



David A. Armstrong 



Paul A. Dinnel 



Jose M. Orensanz 



Katherine A. McGraw 



School of Fisheries, WH-10 



University of Washington, Seattle, Washington 98195 



Manuscript accepted 17 January 1992. 

 Fishery Bulletin, U.S. 90:171-182 (1992). 



The effect of dredging on marine or- 

 ganisms has been an issue of environ- 

 mental concern for several decades. 

 Most studies on the impact of dredg- 

 ing and disposal of dredged material 

 are concerned with changes in in- 

 faunal species assemblages and com- 

 munity characteristics, and generally 

 measure effects by pre- and post- 

 dredging comparisons. Very little 

 work has been done on the direct ef- 

 fects of entrainment on populations 

 of mobile epibenthic invertebrates or 

 demersal fish, in part because such 

 species are difficult to quantify. The 

 reviews by Morton (1977) and Poiner 

 and Kennedy (1984) indicate a strong 

 research emphasis on habitat modifi- 

 cation (by either dredging or disposal 

 of sediments) and water column ef- 

 fects (turbidity, release of chemical 

 pollutants) during dredging opera- 

 tions. Water column effects were also 

 the focus of a workshop on anadro- 

 mous fish and dredging (Simenstad 

 1990). Virtually no published works 

 report on direct population losses 

 due to entrainment or burial during 

 dredging, except Stevens (1981) and 

 Armstrong et al. (1982). There are 

 few predictive models of dredging 

 impacts other than that of Bella and 

 Williamson (1980), who developed a 

 model of dredging effects in Coos 

 Bay, Oregon. Their model focused 

 on water chemistry and sediments, 

 but also gave some consideration to 



broad categories of animals. 



In sharp contrast, power plant en- 

 trainment and impingement of fish 

 has generated a large quantitative 

 modeling literature (e.g. van Winkle 

 1977). Among the methods used in 

 power plant assessments, the "equiv- 

 alent adult loss" (Horst 1975, Good- 

 year 1977) and "production fore- 

 gone" (Rago 1984) approaches are 

 transferable to dredging operations, 

 if sufficient biological and operational 

 data are available. There are, how- 

 ever, several noteworthy differences 

 between power plant and dredging 

 operations which require different 

 considerations in their analyses. 

 Firstly, power plant water intakes 

 operate continuously at a fixed loca- 

 tion, while dredging operations are 

 generally short-term, with a moving 

 intake. This means that continuous, 

 equilibrium approaches (e.g., MacCall 

 et al. 1982) are not appropriate for 

 dredging. Secondly, mobile benthic 

 invertebrate populations are char- 

 acterized by spatial aggregations and 

 seasonal shifts in distribution which 

 must be taken into account in esti- 

 mating entrainment by a moving 

 dredge. Finally, power plant entrain- 

 ment is usually restricted to a single 

 age-class (larvae or early juveniles), 

 whereas dredging removes all age- 

 classes present in the dredged habi- 

 tat, but may kill age-classes at dif- 

 ferent rates. 



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