Wainwright et al.: Effects of dredging on a crab population 



179 



30O T 



15 T 



Unodjuslcd Loss 



.DM^- 



n n 



M. DM. 



C H P C H P C H P 



Jun-Scp Ocl-Dcc Jon-Wot 



fl 



tquivalenl Adull Loss 



CHP Clip CMP CHP 



Apr-May Jun-Sep Ocl-Oec Jon- 



Uat 



Figure 7 



Entrainment rates of Dungeness crab by season and dredge 

 type for the Inner Harbor, as in Figure 6. 



1983 1984 



1985 1986 1987 1988 Mean 



800 T 



" 600 



400 



5-200 



Scenorio 2 



I 



L3 'i 



1983 1984 1985 1986 



987 1988 Meon 



Figure 8 



Total estimated age 2+ equivalent losses for hypothetical 

 dredging in six years for two project scenarios, (upper) Full 

 use of confined disposal; (lower) limited confined disposal. Age- 

 classes as in Figure 5. 



impacts, with 1983 construction resulting in impacts 

 nearly three times the average for the other years. This 

 is apparently because 1983 followed two years of 

 strong settlement, as evidenced by the high abundance 

 of both age 1 + and > 1 -i- crab in that year (Fig. 5; see 

 also Gunderson et al. 1990). This emphasizes the im- 

 portance of population monitoring during construction 

 to accurately assess impacts. 



Discussion 



Gear and season comparisons made with DIM provided 

 several results which were subsequently used to sched- 

 ule construction gear, season, and location combina- 

 tions so as to reduce crab losses. As expected, the clam- 

 shell dredge (which moves slowly and does little 

 mechanical damage to organisms) had insignificant im- 

 pact in all seasons and areas. Comparing pipeline and 

 hopper dredge effects, our initial impression was that, 

 with confined disposal (resulting in 100% loss of all age- 

 classes), the pipeline dredge would cause extremely 

 high losses relative to the hopper dredge. This is true 



when one considers the imadjusted losses (Figs. 6A and 

 7A). However, when viewed on an equivalent adult loss 

 basis (Figs. 6B and 7B), the pipeline dredge loss rate 

 is only 10-50% higher than that of the hopper dredge. 

 The equivalent adult loss viewpoint was also important 

 in seasonal comparisons, especially in the Inner Har- 

 bor (Fig. 7) where unadjusted loss was highest in 

 spring, but equivalent adult loss peaked in fall. 



During any modeling endeavor in applied ecology, 

 certain decisions must be made to limit the scope and 

 applicability of the model. Many decisions are made 

 simply on the basis of information or time available, 

 while others reflect the biases and experiences of the 

 authors. One of the major decisions in this project was 

 the choice between predicting short-term losses via the 

 equivalent adult loss approach, or accounting for poten- 

 tial longer-term losses due to reduction of the local 

 reproductive stock via "production foregone" (Rago 

 1984) techniques. For local, short-term entrainment to 

 have longer-term population effects requires a strong 

 influence of current stock size on future recruitment. 



