Hannah et al.: Effects of shrimp trawling on macroinvertebrate abundance and diversity 
37 
trawl fisheries is also low in comparison to that of trawl 
fisheries in other areas, such as the Gulf of Mexico and 
New England (NRC, 2002). Effort in the ocean shrimp 
trawl fishery has also been reduced recently by a vessel 
buyback program that has reduced fleet size. Macro- 
invertebrate populations in the HT sites at Nehalem 
Bank may have recovered somewhat from the impacts 
of the heaviest trawling during earlier years (Fig. 2 A). 
However, recovery is not likely to be a significant factor 
for Halipteris spp., which are believed to be very long- 
lived (Wilson et al., 2002). The deep mud habitats being 
trawled for ocean shrimp have low levels of natural dis- 
turbance and are considered some of the most vulner- 
able to the negative effects of physical disturbance and 
are expected to have long recovery times (Jones, 1992; 
Kaiser, 1998; Tuck et al., 1998; Kaiser et al,. 2002; 
Dernie et al., 2003). Over time, additional surveys of 
the Nehalem Bank sites should provide insight into 
both recovery time and the condition of fully recovered 
mud-seafloor habitats (recently closed sites 1A and 2A) 
as well as continued changes in areas that experience 
additional trawling (e.g., sites IB and 2B). 
Our data indicate that the effect of the ocean shrimp 
fishery on the complexity of mud-seafloor habitats near 
Nehalem Bank may be a mixture of physical and long- 
term ecological effects. The increased density of hagfish 
burrows at HT sites was unexpected. This finding con- 
flicts with most other trawl-impacts studies that evalu- 
ated biogenic features in soft-bottom habitats (Auster 
et al., 1996; Simpson and Watling, 2005; Stone et al., 
2005). One possible explanation for this difference is 
that through fishery removal of hagfish predators and 
competitors, or from supplementation of hagfish food 
resources with discards, the ocean shrimp fishery is 
creating conditions that favor the growth and survival 
of hagfish. This hypothesis is in general agreement with 
the findings from other studies that have shown ben- 
efits for scavenger populations as a result of trawling 
(Groenewold and Fonds, 2000; Rumohr and Kujawski, 
2000), and is consistent with what little is known about 
hagfish food habits (Martini et al., 1997). It is unknown 
how increases in hagfish populations may have in turn 
influenced other aspects of the benthic ecology of the 
ocean shrimp grounds; however, it is possible that this 
ecological effect could generate changes that are equiva- 
lent to, or greater than, the direct physical effects of 
trawling. If fishery discards are stimulating hagfish 
population growth, it is possible that this effect will 
diminish over time because fish bycatch in the ocean 
shrimp fishery has been reduced roughly 66-86% (in 
terms of weight) by the requirement (since 2002) that 
vessels use approved bycatch reduction devices (Hannah 
and Jones, 2007). 
To adequately understand and manage the ecosystem 
effects of trawl fisheries will require a great deal of 
information, well beyond basic information on removals 
and the physical effects of trawl footropes and doors. To 
prevent long-term detrimental effects from trawling, 
information on recovery times of macroinvertebrate 
populations is critical. The data developed here provide 
an opportunity for follow-up studies to better under- 
stand the recovery of macrobenthos and other changes 
in habitat after the cessation of trawl impacts at the 
Nehalem Bank closed area (Fig. 1). They also provide a 
starting point for understanding the effects of the ocean 
shrimp trawl fishery on the ecosystem. 
Acknowledgments 
The skipper and crew of the FV MissYvonne provided 
critical expertise in successfully deploying the ROV 
from their vessel at Nehalem Bank. M. Schuiteman 
assisted with the ROV field work. This research was 
funded, in part, by National Oceanic and Atmospheric 
Administration award NA06NMF4070244 under the 
Interjurisdictional Fisheries Act. 
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