Hannah et al. : Effects of shrimp trawling on macroinvertebrate abundance and diversity 
31 
124°45'0"W 
124°30'0"W 
46°0‘0"N- 
45°45'0"N- 
Figure 1 
Map of the study area at Nehalem Bank, Oregon, showing the four 
study sites (1A, IB, 2A, and 2B) where the effects of ocean shrimp 
( Pandalus jordani) trawling on macrobenthos and seafloor habitat 
were evaluated. The Nehalem Bank trawl closure is also outlined. 
initial survey of the condition of two sites within this 
closed area to monitor changes over time (sites 1A and 
2A, Fig. 1). We also surveyed two nearby sites that 
remain open to ocean shrimp trawling as control sites 
(sites IB and 2B, Fig. 1). 
In the absence of true control sites that have not been 
trawled, researchers studying trawl effects on habitat 
have resorted to comparing habitat condition across 
a gradient of trawling intensity (Thrush et al., 1998; 
Kaiser et al., 2000). The mud-habitat sites within the 
Nehalem Bank trawl closure area (sites 1A and 2A, Fig. 
1) have not been as heavily trawled for ocean shrimp as 
have the two nearby mud seafloor sites that are closer 
to the center of the ocean shrimp trawl grounds (sites 
IB and 2B, Fig. 1). 
A second objective of this study was to compare the 
condition of the seafloor habitat and macrobenthos for 
the four sites to determine if current differences may be 
related to the amount of prior trawling. The nature of 
differences between these four sites, if related to trawl- 
ing history, could be useful in developing hypotheses 
regarding expected changes within the closed areas 
over time. 
Materials and methods 
Study area and field surveys 
Nehalem Bank is a 74.6 km 2 rocky outcrop composed 
of sandstone and siltstone located off northern Oregon, 
between the 150- and 200-m depth contours (Lanier, 
2006). Our study sites comprised four square sections 
of mud habitat (1850 m wide) in the vicinity of Nehalem 
Bank, two within and two outside the area recently 
closed to trawling (Fig. 1). Our study design was chosen 
primarily to provide a detailed baseline survey of the 
limited areas of soft-bottom habitat that are enclosed 
within the Nehalem Bank trawl closure area (Fig. 1) so 
that the process of habitat recovery from physical trawl 
impacts can be examined in future studies. Accord- 
ingly, our design was much more spatially restricted 
than would be optimal for a study aimed primarily at 
quantifying the effects of trawling history on habitat 
and associated fauna. However, the contrast in trawl- 
ing history between the eastern and western pairs of 
sites allowed for an analysis of how trawling may have 
impacted these sites. 
