NOTE Bishop et al.: Effects of harvest methods on sustainability of a bay scallop fishery 



715 



hand-harvesting of bay scallops (see Burgess 

 and Bianchi 3 ), hand-harvesting yielded six 

 times the bay scallop harvest obtained per 

 unit of time by dredging, while reducing del- 

 eterious environmental effects. Hand-harves- 

 tikng did not result in uprooting of seagrass 

 or displacing juvenile bay scallops, whereas 

 dredging caused significant damage to sea- 

 grass. Ten minutes of dredging resulted in an 

 average dry weight loss of 200 g of seagrass 

 per plot — 9 % of the estimated biomass of sea- 

 grass present prior to harvest. Despite this siz- 

 able removal of seagrass biomass, a persistent 

 impact of dredging on seagrass biomass was 

 not detected one month later. To the contrary, 

 a 39% increase in seagrass biomass was seen 

 across the dredged plots that was not repli- 

 cated in the control plots. This result indicated 

 that dredging had only a short-term negative 

 impact on seagrass shoots (the necessary pro- 

 duction of new leaves) and instead appeared 

 to stimulate new production during the winter 

 period that was more than sufficient to replace 

 dredging damage. 



Despite the rapid recovery of seagrass from 

 dredging injury, a sustained negative impact 

 of dredging on the density of juvenile bay 

 scallops within plots was detected over the one-month 

 period of our study. In contrast to the small increases 

 in juvenile scallop density that occurred in hand-har- 

 vested and control plots over the course of the study, 

 mean density of juveniles in dredged plots declined from 

 1.37 ±0.33 (1 SE) to 0.89 ±0.23 per 0.5 m 2 . This 40% 

 reduction in juvenile scallops in dredged plots cannot be 

 explained by the bycatch alone. Whereas total bycatch 

 of juveniles was, on average, two scallops per dredged 

 plot, the average reduction in the density of juvenile 

 bay scallops was 0.5 per 0.5-m 2 quadrat or 200 per 

 200-m 2 plot. 



Instead, the reduction in density of juvenile scallops 

 in dredged plots is best explained by their migration 



Before 



After 



Time 



Figure 2 



Mean (±1 SE) dry weight of seagrass per 0.25-m 2 quadrat in con- 

 trol (undisturbed), hand-harvested, and dredged plots immedi- 

 ately before and one month after the 10-minute treatments were 

 applied. n=15. 



after dredging injury to seagrass habitat into adjacent 

 undisturbed control and hand-harvested plots. Abun- 

 dances of juvenile bay scallops in hand-harvested and 

 control plots increased over the one month of our study 

 by an amount more than sufficient to compensate for 

 losses of juveniles from dredged plots. These increases 

 in abundances in control and hand-harvested plots can- 

 not be attributed to the settlement of new recruits: fall 

 recruitment of juvenile scallops to seagrass beds is 

 typically completed by the end of December (Peterson 

 et al., 1989), spring spawning does not commence until 

 March (Peterson and Summerson, 1992), and scallops 

 spawned during our experiment could not possibly have 

 grown fast enough over one month to reach a size re- 



