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Fishery Bulletin 103(4) 



tained by sieves (see Irlandi et al., 1999 for growth 

 rates). Scallops colonizing hand-harvested and control 

 plots were of the right size and of sufficient abundance 

 to be those missing from dredged plots. The migration 

 appears to have included active swimming because tidal 

 currents were perpendicular to the direction of scallop 

 movement. 



Although juvenile scallops are largely sessile, our in- 

 terpretation that juveniles migrate in response to dredg- 

 ing is consistent with field and laboratory observations 

 of juvenile bay scallop behavior. During seasonal slough- 

 ing of eelgrass blades, juvenile bay scallops break away 



2.4 



Dredged 



Hand-harvested 



Control 



Before 



Time 



After 



Figure 3 



Mean (±1 SE) number of juvenile bay scallops (s40 mm in 

 height) per 0.5-m 2 quadrat in control (undisturbed), hand 

 vested, and dredged plots immediately before and one month 

 the 10-minute treatments were applied. ;i = 15. 



shell 

 -har- 

 after 



and re-establish byssal attachments to seagrass blades 

 (Thayer et al., 1975). Mesocosm observations confirm 

 that juveniles are capable of swimming distances of at 

 least several meters when displaced (Bishop, personal 

 observ. ). Thus, our experimental restriction on dredging 

 to small areas may have facilitated relocation of scallops 

 to adjacent, undisturbed habitat, where they remained 

 one month later even after seagrass had regrown in the 

 dredged plots. In the case of the commercial fishery, 

 however, juvenile scallops emigrating from disturbed 

 habitat over the extensive fished areas would be far less 

 likely to encounter undisturbed seagrass habitat for re- 

 attachment. Indeed, transport to unfavorable 

 unvegetated habitat where predation risk is 

 enhanced would likely inflate mortality. 



In our study, juvenile scallops lost from the 

 dredged plots came primarily from the small- 

 est size classes. Small juvenile scallops are 

 more susceptible to benthic predators that 

 forage within seagrass beds than larger ju- 

 veniles (Pohle et al., 1991). Because the for- 

 aging efficiency of some predators increases 

 with decreasing biomass of seagrass (Prescott, 

 1990), a decrease in seagrass biomass, even 

 for a period of weeks, would likely increase 

 predation on juvenile scallops. Thus, small ju- 

 veniles probably are increasing their chances 

 of survival by emigrating away from depleted 

 and into denser seagrass. Larger juveniles, in 

 contrast, experience a partial size refuge from 

 predators (e.g., Pohle et al., 1991), and thus 

 have less incentive to emigrate. 



This study considered the impact of only a 

 single bay scallop-harvesting event on sea- 

 grass biomass and abundance of juvenile bay 

 scallops within small experimental plots. 

 Fishing disturbances are, however, typically 

 chronic, occurring multiple times within a 

 given season, and over large spatial scales. 



