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Fishery Bulletin 106(4) 
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Sampling date 
Figure 6 
Trends in the abundance of four noncolonial species characteristic of site 17 in closed area 
II (CA-II). Each black square represents a single photographic transect, and solid lines 
connect the means from each sampling date. The shaded area indicates the time period 
before bottom fishing was restricted in CA-II. 
tom-fishing effort was statistically significant at shal- 
low sites. Average species richness was greater at deep 
sites than at shallow sites (S d = 22 and S shallow = 13), 
but these two depths did not differ in terms of mean 
values of Simpson’s index of diversity (1— A' d =0.77 and 
1— A' s/iaWoUJ =0.76). The highest values of Simpson’s index 
at shallow sites were recorded in areas with 50-100 
hours of scallop dredging per year. At greater levels of 
bottom-fishing disturbance, a decline in Simpson’s index 
was observed. At deep sites, Simpson’s index varied 
relatively little with disturbance, although its peak 
value occurred in an area with an intermediate amount 
of disturbance (i.e., three days of dredging per year). 
Trends in Simpson’s index were not significant when 
modeled with linear and polynomial regression. 
Discussion 
Colonial epifauna 
Because sessile, colonial organisms cannot avoid the 
path of mobile fishing gear nor can they quickly immi- 
grate into recently disturbed areas, they may be more 
adversely affected by bottom fishing than motile spe- 
cies. All six taxa of colonial epifauna examined in this 
study were affected by bottom fishing, although the 
magnitude and direction of this effect often depended 
on depth and varied between years. Because the effect 
of disturbance on noncolonial species composition was 
fairly subtle, our study indicates a greater sensitivity 
of colonial epifauna to disturbance by mobile fishing 
gear. Similarly, studies conducted in the Gulf of Alaska 
and Irish Sea show that most motile organisms are less 
severely affected by chronic and experimental trawling 
than are anthozoans, sponges, bryozoans, hydroids, tubi- 
colous polychaetes, and barnacles (Freese et al., 1999; 
Bradshaw et ah, 2002). Nevertheless, sessile colonial 
epifauna exhibit variable morphological forms and pos- 
sess diverse life history characteristics that can result in 
different responses to bottom fishing. Colonial organisms 
whose physical structure can be described as branching, 
sheet-forming, or mound-forming generally grow slowly 
but are better competitors for space on the seafloor than 
encrusting and stoloniferous epifauna (Hughes, 1989). 
Because of their slow growth patterns, organisms with 
branching, sheet-forming, and mound-forming structures 
are typically adapted to stable environments where they 
are less likely to be detached from their substrate by 
physical forces associated with natural disturbances. 
Anthropogenic disturbance causes formerly stable envi- 
ronments to become unstable, thus leaving colonial epi- 
fauna with a branching structure maladapted to their 
new environment. 
On Georges Bank, sponges appear to be the colonial 
epifaunal taxa most negatively affected by bottom fish- 
