Date of Award

12-2003

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Oceanography

Advisor

Leslie Watling

Second Committee Member

Joseph Kelley

Third Committee Member

Sara Lindsay

Abstract

Despite the widespread occurrence of trawl fisheries on mud-bottoms, there is limited knowledge concerning the effects of trawling induced disturbance on these habitats and their resident macrofaunal communities. I investigated the cumulative impacts of seasonal commercial shrimp trawling on infaunal habitat and macrofaunal community structure on two mud-bottom fishing grounds in the Gulf of Maine from June 2000 - December 2001. One fishing ground located near the Outer Pumpkin Ledges (Pumpkin) experienced trawling activity during the 2000-2001 fishing season. In contrast, the other fishing ground near Monhegan Island was not trawled during the same period because shrimp abundances were low. Consequently, impacts of trawling reported for the Pumpkin fishing ground are representative of both past and more recent short- term effects of trawling while results reported for the Monhegan are indicative of longer- term, cumulative effects only. To further examine short-term effects on trawling on mud bottom habitat structure, a Before-After Control-Impact (BACI) experiment was carried out at a different location (Thrumcap). Images of infaunal habitat structure obtained by sediment x-radiography showed no evidence of changes in overall structure (as measured by relative sediment density) related to commercial or experimental (BACI) shrimp trawling disturbance; however, excess 2 ' 0 ~ b activity profiles suggest that trawling may affect sediment mixing regimes. Macrofaunal communities on the two fishing grounds exhibited different responses to shrimp trawling disturbance which I attribute to disparities in levels of fishing activity during the 2000-2001 shrimp season. Multivariate community analysis showed that the Pumpkin fishing ground displayed significant differences in macrofaunal community structure compared to adjacent untrawled areas. Abundances of opportunistic polychaete families were higher in the trawled areas while disturbance-sensitive taxa, such as bivalves, were more abundant in the untrawled area. Similar patterns in taxa abundance were not observed at Monhegan. Results from mud bottom fishing grounds suggest that seasonal shrimp trawling disturbance produced at least short-term changes in infaunal community structure, but did not appear to result in long-term cumulative changes. Resilience to trawling disturbance may be due in part to high levels of biological sediment disturbance from high densities of large surface-dwelling megafauna such as lobsters, fishes, and brittle stars. These animals rework sediments to a depth of 16- 17 cm by burrowing, pit-digging and possibly foraging. Sediment reworking by these benthic megafauna creates disturbance that appears to maintain macrofaunal communities in a perpetually low successional state, thereby potentially minimizing trawling impacts.

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